Draft of the 'Discourse Concerning Light and Colors'Isaac Newtonc.16,465 wordsNewton ProjectBrighton2011Newton Project, Sussex University
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c. early 1675, c. 16,675 words.MS Add. 3970.3, ff. 501r-517r, Cambridge University Library, Cambridge, UKUKCambridgeCambridge University LibraryPortsmouth CollectionMS Add. 3970.3, ff. 501r-517rc. early 1675EnglandEnglishHolographUnknown HandScienceOpticsCorrespondenceTranscribed and tagged by Linda CrossUpdated to Newton V3.0 (TEI P5 Schema) by Michael HawkinsInline images embedded and minor code corrections by Michael HawkinsProofread by Margarita Fernandez-ChasProofed by Robert IliffeReviewed by John Young
499501
SrSir
I suppose you understand that all transparent substances (as Glass water, Air &c) when made very thin by being blown into bubbles or otherwise formed into plates, doe exhibit various colours according to their various thinnesse, although at a greater thicknesse they appeare very cleare & colourlesse. In my former discourse about yethe constitution of light, I ommited these colours, because they seemed of a more difficult consideration, & were not necessary for establishment of the Doctrine wchwhich I propounded. But because they may conduce to further discoveries for completing that Theory, especially as to yethe constitution of yethe parts of naturall bodies on wchwhich their colours or transparency depend, I have now sent you an account of them. To render this discourse short & distinct, I have first described the principall of my Observations, & then considered & made use of them. The Observations are these:
Obs: 1. Compressing two Prisms hard together that their sides (wchwhichby chance were a very little convex) might somewhere touch one another, I found the place in wchwhich they touched to become absolutely transparent as if they had there been one continued piece of Glasse. For when yethe light fell so obliquely on yethe Air wchwhich in other places was between them, as to be all reflected; it seemed in that place of contact it wasto be wholly transmitted: in so much that when looked upon it appeared like a black or dark spot by reason ytthatnolittle or no sensiblelight was reflected from thence as from other places; & when looked through it seemed (as it were,) a hole in that Air whatich was formed into a thin plate by being compressed between yethe glasses. And through this hole Objects that were beyond might be seen distinctly wchwhich could not at all be seen through other parts of yethe glasses where the Air was interjacent. Although yethe glasses were a little convex yet this transparent spot was of a considerable breadth, wchwhich breadth seemed principally to proceed from the yeilding inwards of the glasses by parts of the glasses by reason of their mutuall pressure For by pressing them very hard together it would become much broader then otherwise.
Obs: 2. When the plate of Air, by turning the Prisms about their common Axis became so little inclined by yethe incident rays, that some of them began to be transmitted, there arose in it many slender arcs of colours wchwhich at first were shaped almost like a Conchoid as you see them here delineated. And by continuing the motion of the Prisms, these arcs increased & bended more & more about the said transparent spot, till they were completed into circles or rings incompassing it, & afterwards continually 2 grew more & more contracted.
These parcs at their first appearance were of a violet & blew colour, & between them were white arcs of circles wchwhichpresently (by continuing yethe motion of yethe Prisms) became a little tinged in their inward limbs with red & yellow, & to their outward limbs yethe blew was adjacent. So ytthat the order of these colours from the centrall dark spot, was at that time White, Blew, vViolet; bBlack ; Red, Orange, Yellow, White, Blew, Violet; &c. But the Yellow & Red were much fainter then yethebBlew & vViolet.
The motion of the Prisms about their Axis being continued these colours contracted more & more, shrinking towards the whitenesse on either side of it untill they totally vanished into it. And then the circles in those parts appeared black & white without any other colours intermixed. But by further incliningmovingyethe Prisms about, the colours again emerged out of the whitenesse, the violet & blew at its ––– inward limb, & at its outward limb the red & yellow. So that now their order from the centrall spot was White, Yellow, Red; Black; Violet, Blew, White, Yellow, Red, &c. contrary to what it was before.
Obs: 3 When the rings or some parts of them appeared onlly black & white, they were very distinct & well defined, & the blacknesse seemed as intense as that of yethe centrall spot. Also in yethevicine placesborders of yethe ringswhere yethe colours began to emerge out of yethe whitenesse, they were pretty distinct, wchwhich made them visible to a very great multitude. I have sometimes numbered above thirty successions (recconing every black & white ring for one succession,) & seen more of them wchwhich by reason of their smallnesse I could not number. But in other positions of yethe Prisms at wchwhich the rings appeared of many colours, I could not distinguish above 8 or 9 of them, & the exteriour of those too were rvery confused & dilute.
In these two Observations to see the colourrings distinct, & without anyother colour but black & white, I found it very necessary that Ito hold my eye at a good distance from them. For by approaching nearer, although in the same inclination of my eye to yethe plane of yethe rings, yet there emerged a blewish colour out of yethe white, wchwhich by dilating it selfe more & more into the black rendered the circles lesse distinct, & left yethe white a little tinged with red & yellow. I found also that by looking through a slit or oblong hole wchwhich was narrower then the pupill of my eye & held close to it parallel to the Prisms, I could see the circles much distincter & visible to a far greater number then otherwise.
Obs: 4. To observe more nicely the order of the colours wchwhich arose out of the white circles as yethe rays became lesse & lesse inclined to the plate of Air; I took two object glasses the one a plano-convex for a fourteen foot Telescope, & the other a large double convex for one of about fifty foot, & upon this laying the other with its plane side downwards, I pressed them slowly together to make the colours successively5005023sively emerge in the middle of the circles & then slowly lifted the upper glasse from yethe lower to make them successively vanish again in the same place, where being of a considerable breadth, I could more easily discerne them. And by this means I observed their succession & quantity to be as followeth.
Next to yethe pellucid centrall spot made by the contact of the glasses, succededed Violet, Blew, White, Yellow, & Red. The violet &Blew wereas so very little in quantity that I could not discerne themitin the circles made by the Prismes, nor could I well distinguish any violet in it,, but yethe Yellow & Red were pretty copious & seemed about as much in extent as the white, & foure or five times more then the Blew & Violet. The next circuit or order of colours immediately incompassing these was vViolet Blew, gGreen, Yellow, & Red. And these were all of them copious & vivid excepting the green wchwhich was very little in quantity & seemed much more faint & dilute then yethe other colours. Of yethe other four the Violet was least & the blew lesse then yethe yellow or red. The third circuit or order was also Purple, Blew, Green, Yellow, & Red in wchwhich the purple seemed a little more reddish then the Violet in the former circuit, & yethe Green was much more conspicuous, being as brisque & lively copious as any of yethe other colours aexcept the Yellow, but the Red began to be a little faded inclining very much to purple. After these succeeded in yethe fourth circuit Green & Red. The gGreen was very copious & lively, inclining on one side to blew & on yethe other to yellow. But in this 4th circuit there was neither violet blew nor yellow, & the Red was very imperfect & dirty. Also the succeeding colours became more & more imperfect & dilute till after three or four more revolutions they ended in perfect whitenesse.
Obs: 5. To determin the intervall of the glasses or thicknesse of the interjacent Air by wchwhich each colour was produced, I measured the diameters of yethe first six rings at the most lucid part of their Orbits, & squaring them I found their squares to be in Arithmeticall progression of yethe odd numbers 1, 3, 5, 7, 9, 11. And thereforesince one of the glasses was plane & the other sphericall, their intervalls at those rings must be in the same progression. I measured also the diameters of the dark or faint rings between the more lucid colours & found their squares to be in Arithmeticall progression of the eaven numbers 2, 4, 6, 8, 10, 12. And it being very nice & difficult to take these measures exactly I repeated them divers times at divers parts of the glasses, that by their agreement I might be confirmed in them. And the same Method I used in determining some others of the following Observations.
Obs: 6. The diameter of the sixt ring at the most lucid part of its orbit was parts of an inch, & the diameter of the Sphere 4 on wchwhich the double convex object glass was ground was 102 foot, as I found by measuring it; & consequently the thicknesse of the Air, or Aereall intervall of the glasses at that ring was of an inch. For as the diameter of the said sphere (102 foot or 1224 inches) is to yethesemidiameter of the ring so very nearely is that semi-diameter to yethesaid distanceintervall of the glasses. Now by the preceedent Observation the eleventh part of this distanceintervall () is the thicknesse of the Air at that interval part of the first ring where themost lucid of the colours the citrine Yellow would be mostmost vivid were it nor mixed with other colours in the white of this ring, & this doubled gives the difference of their intervalls its thicknesses at the citrine Yellow in all the other rings. viz or to use a round number, the eighty thousandth part of an inch.
Obs: 7. These dimensions were taken when my eye was placed perpendicularly over the glasses in or neare the axis of the rRings But when I viewed them obliquely they became bigger, continually swelling as I removed my eye further from their Axis. And partly by measuring the diameter of the same circle at severall obliquities of my eye, partly by other meanes, as also by making use of the two Prisms for very great obliquities, I found its diameter, & consequently the thicknesse of yethe Air at its perimeter in all those obliquities to be very nearly in the proportions expressed in this TableAngle of Incidence on yethe Air.Angle of Refraction into yethe Air.Diameter of yethe Ring.Thicknesse of yethe Air.00gr. 00min00gr. 00g min.10.10.6. 26.10. 00...12. 45.20. 00...18. 49.30. 00...24. 30.40. 00..13.29. 37.50. 00...33. 58.60. 00.14.20.35. 47.65. 00...37. 19.70. 00...38. 33.75. 00..37.39. 2780. 00...40. 085. 00.29..40. 1190. 00.35..In the two first Collumns are expressed the obliquities of the incident & emergent rays to the Plate of Air, that is their angles of incidence & refraction. In the third Collumn yethe diameter of any coloured Ring at those obliquities is exprest in parts of wchwhichten constitute th at diameter when the rays are perpendicular. And in the 4th Collumn the thicknesse of the Air at yethe circumference of that Ring is exprest in parts of wchwhichalso 10 constitute that thicknesse when the rays are perpendicular.Obs. 8.5035Obs: 8. The dark spot in the middle of the Rings increased also by yethe obliquation of the eye; although almost insensibly. But if instead of the Object-glasses the Prisms were made use of, its increase was more manifest when viewed so obliquely ytthatno colours appeared about it. It was least when the rays were incident most obliquely on yethe interjacent Air, & as yethe obliquity decreased it increased more & more untill yethe coloured rings appeared, & then decreased again, but not so much as it increased before. And hence it is evident that the transparency was not onely at yethe absolute contact of yethe glasses but also where they had some little intervall. I have sometimes observedyethe diameter of that spot to be between half & two fift parts of the diameter of the exterior circumference of the Red in the first circuit or revolution of colours when viewed almost perpendicularly; whereas when viewed obliquely it hath wholly vanished & become opake & white like yethe other parts of yethe glasse. Whence it may be collected ytthat the glasses did then scarcely or not at all touch one another, & that their intervall at the perimeter of that spot when viewed perpendicularly was about a 5t or a 6t part of their intervall at the circumference of the said RedObs: 9. By looking through the two contiguous Object-glasses, I found that the interjacent Air exhibited Rings of colours as well by transmitting light as by reflecting it. The centrall spot was now white, & from it the order of the colours were Yellowish Red; Black; Violet, Blew, White, Yellow, Red; Violet, Blew, Green, Yellow, Red, &c But these colours were very faint & dilute unlesse when the light was trajected very obliquely through the glasses. For by that meanes they became pretty vivid. Onely the first Yellowish Red, like yetheViolet & blew in the 4th Observation was so little & faint as scarcely to be discerned. Compareing the coloured Rings made by reflexion, wthwith these made by transmission of the light; I found that White was opposite to Black, Red to Blew, Yellow to Violet, & Green to a compound of Red & Violet. That is, those parts of the glasse were black when looked at through wchwhich when looked upon appeared white, & on yethe contrary. And so those wchwhich in one case exhibited blew did in the other case exhibit Red. And the like of the other colours.Obs: 10. Wetting the Object glasses a little at their edges, the water crept in slowly between them, & the circles thereby became lesse & the colours more faint: in so much that as yethe water crept along one half of them at wchwhich it first arrived would appeare broken of from yethe other half & contracted into a lesse Room. By measuring them I found the proportion of their diameters to the diameters of the like circles made by Air to be about seven to eight, & consequently the intervalls of the glasses at like circles caused by these two Mediums water & air are as about three to four. Perhaps it may be6 a generall Rule that if any other Medium more or lesses dense ynwater be compressed between the glasses, their intervall at the Rings caused thereby will be to their intervall caused by the interjacent Air, as the sines are wchwhich measure the refraction made out of that Medium into AirObs: 11. When the water was between the glasses if I pressed the upper glasse variously at its edges to make the Rings move nimbly from one place to another, a little bright spot would –––– immediately follow the center of them, wchwhich upon creeping in of the ambient water into that place would presently vanish. It's appearance was such as interjacent Air would have caused, & it exhibited the same colours. But it was not Air, for where any aereall bubbles were in the water they would not vanish. The reflexion must rather have been caused by a subtiler Medium wchwhich could recede through the Glasse at yethe creeping in of the water.Obs: 12. These Observations were made in the open Air. But further to examin the effects of coloured light falling on yetheGlasses, I darkened the Room & viewed them by reflexion of the colours of a Prism cast on a sheet of white paper my eye being so placed ytthat I could see yethe coloured paper by reflexion in yethe glasses as in a looking glass.. And by this meanes the Rings became distincter & visible to a far greater number then in the open Air. I have sometimes seen more then 20 of them, whereas in yethe open Air I could not discerne above 8 or 9.Obs: 13. Appointing an Assistant to move the Prism to & fro about its Axis, that all itsthe colours might successively fall on theat plart of the paper wchwhich I saw by reflexion from that part of the glasses where yethe circles appeared& beso ytthat all yethe colours might be successively reflected from the circles to my eye whilst I held it immoveable; I found the circles wchwhich the Red light made to be manifestly biger then those wchwhich were made by the Blew & Violet. And it was very pleasant to see them gradually swell or contract accordingly as the colour of the light was changed. The interval of yethe Glasses at any of the rings when they were made by the utmost red light was to their intervall at yethe same Ring when made by the utmost Violet, greater then 3 to 2 & lesse then 13 to 8. By the most of my Observations it was as 9 to 14 to 9. And this proportion seemed very nearely the same in all obliquities of my eye; unlesse when two Prisms were made use of instead of the Object-glasses. For then at a certain greatgreat obliquity of yethe rays to yethe plate of air between yethe Prismsof my eye the Rings made by the severall colours seemed equall, & at a greatergreaterobliquity those made by the Violet would be greater then the same Rings made by the Red.Obs 14. While the Prism was turned about uniformlyits axiswthwith an eaven motion The Contraction or dilatation of a Ring made by all the severall colours of the Prism successively reflected from the Object-Glasses, was swiftest in the Red, slowest in yethe Violet, & in intermediate colours it had intermediate degrees of celerity. Comparing the extent or breadth of yetherings or variation of the Air's thicknesswchwhich each colour obteined or moved V moved through over & took up by this contraction or dilatation I50247 I found that the Blew was sensibly more extended then the Violet, yethe Yellow then yethe Blew, & the Red then the Yellow. And to make a juster estimation of their proportions I observed that yetheextent or breadth of the Red squared that is the variation of yethethickness of the air where all the red by contraction & dilatation appeared was almost double to that of the Violet, & that yethe light was of a middle colour between BlewYellow & Green at that intervall of the Glasses wchwhich was an Arithmeticall mean between the two extreamesintervalls whicherereflected the whole ofyethe colours began & ended: contrary to what happens in the colours made by the refraction of a Prism, where the Red is most contracted, the Violet most expanded, & in yethe middst of themall the colours is the confine of Green & Blew.Obs: 15. These Rings were not of various colours like those in the open Air but appeared all over of that Prismatique colour onely wthwithwchwhich they were illuminated. And by projecting the Prismatique colours immediately upon yethe glasses, I found ytthat the light wchwhich fell on yethe dark spaces wchwhich were between the coloured Rings, was transmitted through yethe Glasses wthwithout any variation of colour. For on a white paper placed behind, it would paint Rings of the same colour wthwith those wchwhich were reflected, & of the bignesse of their intermediate spaces. And from hence the origine of these Rings is manifest, namely that yethe aereall intervall of the glasses, according to its various thicknesse, is disposed in some places to reflect, & in others to transmit the light of any colour, & in the same place to reflect one colour where it transmits another.Obs: 16. The squares of the diameters of these Rings made by any Prismatique colour were in Arithmeticall Progression as in yethe 5t Observation. And yethe diameter of yethe sixt circle when made by yethecitrine Yellow & viewed almost perpendicularly, was about parts of an inch, agreable to yethe 6t Observation. The precedent oObservations were made wthwith a rarer thin Medium terminated by a denser, such as was Air or water ––– comprest between two Glasses. In those that follow are set down the appearances of a denser Medium thin'd within a rarer, such as are pPlates of Moscovy Glasse, Bubbles of Water, & some other thin substances terminated as all sides wthwith AirObs: 17. If a Bubble be blown wthwith water, first made tenacious by dissolving a little soape in it, tis a common observation that after a while it will appeare tinged wthwith a great variety of colours. To defend these bubbles from being agitated wthwith by the externall Air (whereby their colours are irregularly moved one among another, so ytthat no accurate observation can be made of them,) as soon as I had blown any of them I covered it with a cleare glasse, & by that meanes its colours emerged in a very regular order like so many concentrick rings incompassing the top of the bBubble. And 8 as yethe bubble grew thinner by the continuall subsiding of yethe water these rings dilated slowly & overspread the whole bubble descending in order to yethe bottom of it where they vanished successively. In the meane while after all the colours were emerged at the top, there grew in the center of the Rings a small round black spot like that in the first Observation wchwhich continually dilated it selfe till it became sometimes more then or of an inch in breadth before the Bubble brake. At first I thought there had been no light reflected from the water in that place, but observing it more curiously I saw within it severall smaller round spots wchwhich appeared much blacker & darker then the rest, whereby I knew that there was some reflexion of at the other places wchwhich were not so dark as thatose spots. And by further tryall I found ytthat I could see the images of some things (as of a candle or the Sun) very faintly reflected not onely from the great black spot but also from the little darker spots wchwhich were within it.Besides the afforesaid coloured Rings there would often appeare small spots of colours ascending & descending up & down yethe sides of the Bubble, by reason of some inequalities in the subsiding of the water. And sometimes small black spots generated at the sides of would ascend up to yethe larger black spot at yethetop of the Bubble & unite with it.Obs: 18. Because the colours of these Bubbles were more extended & lively then those of Air thind between towo Glasses, & so more easy to be distinguished: I shall here give you a further description of thereir order as they were observed in viewing them by reflexion of yethe Skies when of a white colour, whilst a black substance was placed behind the bubble. And they were these; Red, Blew; Red, Blew; Red, Blew; Red, Green; Red, Yellow, Green, Blew, Purple; Red, Yellow, Green, Blew, Violet; Red, Yellow, White, Blew, Black.The three first successions of Red & Blew were very dilute & dirty, especially the first where the Red seemed in a manner to be white Amongst these there was scarcely any other colour sensible, onely the Blews (& principally the second blew) inclined a little to Green.The fourth Red was also dilute & dirty but not so much as yetheformer three; After that succeeded little or no Yellow, but a copious Green wchwhich at first was inclined a little to yellow, & then became a pretty brisque & good Willow gGreen, & afterwards changed to a blewish colour; but there succeeded neither Blew nor violet.The fift Red at first wasinclined very much inclined to purple & afterwards became more bright & brisque but yet not very pure. This was succeeded with a very bright & intense Yellow, wchwhich was but little in quantity, & soon changed to Green: But that Green was copious& something more pure deep & lively then the former Green. After that50359 that followed an excellent Blew of a bright Sky colour, & then a Purple wchwhich was lesse in quantity then the Blew & much inclined to Red.The sixt Red was at first of a very faire & lively Scarlet, & soon after of a brighter colour being very pure & brisque & the best of all the Reds. Then after a lively Orange followed an intense bright & copious Yellow, which was also the best of all the Yellows; & this changed first to a Greenish Yellow, & then to a Greenish Blew; but the Green between the Yellow & Blew was very little & dilute, seeming rather a Greenish White then a Green. The Blew wchwhich succeeded became very good & of a fair Bright Sky colour, but yet something inferior to the former Blew, And the Violet was intense & deepe wthwith little or no rednesse in it And lesse in quantity then yethe BlewIn the last Red appeared a tincture of Scarlet next the RedViolet, wchwhich soon changed to a brighter colour inclining to an OranOrange; And yethe Yellow wchwhich followed was at first pretty good & lively but afterwards it grew more & more dilute untill by degrees it ended in perfect whitenesse. And this whitenesse, if the water was very tenacious & well tempered, would slowly spread & dilate it selfe over the greatest part of the bubble, continually growing paler at the top, where at length it would crack in many places, & those cracks as they dilated would appeare of a pretty good, but yet obscure & dark sky colour; the white between the Blew spots diminishing, untill it resembled the threds of an irregular net=work, & soon after vanished & letft all the upper part of the Bubble of the said dark Blew colour. And this colour after the afforesaid manner dilated it self downwards untill sometimes it hath overspread the whole Bubble. In the meane-while at the top, wchwhich was of a darker Blew then the bottom & appeared also full of many round blew spots something darker then the rest, there would emerge one or more very black spots & wthwith those other spots of an intenser blacknesse, wchwhich I mentioned in yethe former Observation; &And these continually dilated themselves untill the Bubble brake.If the water was not very tenacious the Black spots would break forth in the white without any sensible intervention of the blew. And sometimes they would brekak forth within the precedent Yellow, or Red, or perhaps within the Blew of yethe second order before the intermediate colours had time to display themselves.By this description you may perceive how great an affinity these colours have wthwith those of Air described in the 4th Observation, although set down in a contrary order by reason that they begin to appeare when the Bubble is the thickest, & are most conveniently recconed from the lowest & thickest part of the Bubble upwards.Obs: 19. Viewing at severall oblique positions of my eye the BubblesRings of colours emerging on yethe top of the Bubble, I found that they 10 were sensibly dilated by increasing the obliquity, but yet not so much by far as those made by thinned Air in the 7th Observation. For there they distendedwere dilated so much as, when viewed most obliquely, to arrive at a part of the Plate more then twelve times thicker then that where they appeared when viewed perpendicularly, whereas in this case the thicknesse of the water at wchwhich they arrived when viewed most obliquely, was to that thicknesse wchwhich exhibited them by perpendicular rays something lesse then 8 to 5. By the best of my oObservations it was between 15 & to 10; an increase about 24 times lesse then in in the other case.Sometimes the Bubble would become of an uniform thicknesse all over except at yethe top of it neare the black spot, as I knew because it would exhibit the same appeareance of colours in all ––– positions of the eye: And then the colours wchwhich were seen at its apparent circumference by the obliquest rays would be different from those that were seen in other places by rays lesse oblique to it. And divers Spectators might see yethe same part of it differing colours by viewing it at very differing obliquities. Now observing how much the colours at the same place of the Bubble or at divers places of equall thicknesse were varied by the severall obliquities of the rays; by assistance of the 4th 14th 16th & 18th Observations, as they are hereafter explained, I collected the thicknesse of the water requisite to exhibit any one & the same colour at severall obliquities to be very nearely in the proportion expressed in this Table.Incidence on yethe Water.Refraction into yethe Water.Thicknesse of yethe water.00degr.. 00min.00degr.. 00min.10.15. 00.11. 11..30. 00.22. 1..45. 00.32. 2..60. 00.40. 30.13.75. 00.46. 25..90. 00.48. 35..In the first two Collumns are expressed yethe obliquities of the rays to the superficies of yethe water, that is their angles of incidence & refraction. Where I suppose that yethe sines wchwhich measure them are in round numbers as 3 to 4, though probably the dissolution of soape in the water may a little alter its refractive virtue. In the third collumn the thicknesses of yethe Bubble at wchwhich any one colour is exhibited in those severall obliquities, is exprest an parts of which ten constitute that thicknesse when the rays are perpendicular.I have sometimes observed of the colours wchwhich arise out of on polished Steel when heating it, or on Belmstall & some other Metalline substances when melted & poured on the ground where504611 where it may cook in the open Air, that they have, like these of water-bubbles, been a little changed by viewing them at divers obliquities, & particulary that a deep blew of violet when viewed very obliquely hath been changed to a deep Red. But yethe changes of these colours are not so great & sensible as of those made by water. For the Scoria or vitrified part of yethe Metall wchwhich most Metalls when mheated or melted do continually protrude to their surface where& which by covering themmetalls in forme of a thin glassy skin, causes these colours, is much denser then water; and I find ytthatyethe change made by the obliquation of the eye is least in colours of yethe densest thin substances.Obs: 20. As in the 9th Observation so here the Bubble by transmitted light appeared of a contrary colour to that wchwhich it exhited by reflexion. Thus when the Bubble being looked on by the light of the clouds reflected from it, seemed Yellow or Red at its apparent circumference, if yethe clouds at yethe same time were or very suddenlyimmediately after were viewed through it, the colour at its circumference would be blew or Purple or Blew. And on the contrary when by reflected light it appeared Blew, it would appeare Red by transmitted light.Obs: 21. By wetting very thin Plates of Moscovy-glasse whose thinnesse made the like colours appeare, the colours became more faint; especially by wetting yethe Plates on that side opposite to yethe eye; but I could not perceive any variation of their species. So ytthatthen the thicknesse of a plate requisite to produce any colour depends onely on yethe density of the Plate & not on that of yethe ambient Medium. And hence by the 10th& 16th Observations may be known the thicknesse ofwhich Bubbles of Water or thin plates of Moscovy-glasse, or of other substances, wchwhich they have at any colour produced by them.Obs: 22. A thin transparent body wchwhich is denser then its ambient Medium, exhibits more brisque & vivid colours then that wchwhich is so much rarer; as I have particularly observed in Air & Glasse. For blowing Glasse very thin at a Lamp Furnace, those plates incompassed with Air did exhibit colours much more thenvivid then those of Air made thin between two glasses.Obs: 23. Comparing the quantity of light reflected from the severall Rings, I found it was most copious from the first or inmost, & in the exterior Rings became gradually lesse & lesse. Also the whitenesse of the first Ring was stronger then that reflected from those parts of the thinned Medium wchwhich were without the Rings; as I could manifestly perceive by viewing at thea distance the Rings made by the two Object glasses; Or by compareing two bubbles of water blown at distant times in the first of wchwhich the whitenesse appeared wchwhich succeded all the colours, & in the other the whitenesse wchwhich preceded them in yethe otherall .Obs: 24. When the two Object-Glasses were layd upon one another so as to make the Rings of Colours appeare, though with my naked eye I 12 could not discerne above 8 or 9 of those rings yet by viewing them through a Prism I have seen a far greater multitude, insomuch that I could sometimes number more then fourty besides many others that were so very small & close together that I could not keep my eye so steddy on them severally as to number them. But by their extent I have sometimes estimated them to be more then a hundred. And I beleive yethe experiment may be improved to yethediscovery of far greater numbers. For they seem to be really unlimited, though visible onely so far as they can be separated by the refraction, as I shall hereafter explaine.But it was but one side of these rings, namely that towards wchwhich the refraction was made, wchwhich by that refraction was rendered distinct, & the other side became more confused then towhen viewed byyethe naked eye, insomuch that I there I could not discerne above one or two, & sometimes none of those rings, of wchwhich I could discerne 8 or 9 wthwithmy naked eye. And their segments or arcs wchwhich on the other side ––– appeared so numerous, did not for the most part exceeded noted not the third part of a circle. If the refraction was very great or the Prisms very distant from the Object-glasses, the middle part of those arcs fbecame also confused so as to disappeare & constitute an eaven whitenesse, whilst on either side their ends of those arcs, as also the whole arcs furthest from the center became distincter then before, appearing in yethe forme you see them here designed.The arcs when they seemed distinctest were onely white & black successively without any other colours intermixed. But in other places there appeared colours whose order was inverted by the refraction in such manner that if I first held the Prism very neare the Object-Glasses & then gradually removed it further of towards my eye, the colours of yethe 2d 3d 4th & following Rings shrunk towards the white that emerged between them, untill they wholly vanished into it at the middle of the arcs, & afterwards emerged again in a contrary order. But at the ends of the arcs they retained their order unchanged.I have sometimes so layd one Object-glasse upon tanhe other that to the naked eye they have all over seemed uniformly white wthwithout the least appearance of any of yethe coloured rings; & yet by viewing them through a Prism great multitudes of those rings have discovered themselves. And in like manner Plates of Moscovy-Glasse & bubbles of Glasse blown at a Lamp-Furnace wchwhich were not so thin as to exhibit any colours to yethe naked eye, have through yethe Prism exhibited a great variety of them ranged irregularly up & down in the form of waves. And so bubbles of water before they began to exhibit their colours to yethe naked eye of a by stander have appeared through a Prism, girded about with many parallel & horizontal Rings; to produce wchwhich effect, it was necessary to hold the Prism parallel or very nearely parallel to yethe Horizon, & to dispose it so that the rays might be refracted upwards.
Haveing505 50713The second Part Remarks upon yethe foregoing Observations.Having given my Observations of these colours, before I make use of them to unfold the causes of the colours of naturall bodies, it is convenient that by the simplest of them, such as are the 2d, 3d, 4th, 9th, 12th, 18th 20th & 24th, I first explaine the more compounded., such as are yethe 2d, 3d, 4th, 9th, 12t, 18th, 20th & 24th.And first to show how the colours in the 4th & 18th Observations are produced, let there be taken in any right line the lengths YZ, YA, & YH in proportion as 4, 9, & 142 5 & 8 & between ZA & ZH eleven meane proportionalls, of wchwhich let ZB be the second, ZC the third, ZD the fift, ZE the seventh, & ZF the ninth, & ZG the tenth. And at the points A, B, C, D, E, F, G, H let perpendiculars Aα, Bβ &c be erected, by whose intervalls the extent of the severall colours set underneath against them, is to be represented. Then divide the line Aα in such proportion as the numbers 1, 2, 3; 5, 6, 7; 9, 10, 11; &c. set at yethe points of division denote. And through those divisions from Y draw lines 1I, 2K, 3L; 5M, 6N, 7O; &cNow if A2 be supposed to represent the thicknesse of any thin transparent body at which the outmost Violet is most copiously reflected in the first Ring or series of colours, then by the 13th Observation HK will represent its thicknesse at wchwhich the outmost Red is most copiously reflected in the same series. Also by the 5t & 16 Observations A6 & HN will denote the thicknesses at wchwhich those extreme colours are most copiously –––– reflected in the second series, & A10. HQ the thicknesses at wchwhich they are most copiously reflected in yethe third series & so on. And the thicknesse at which any of the intermediate colours are reflected most copiously, will according to yethe 14 Observation be defined by the distance of the line AH from the intermediate parts of the lines 2K, 6N, 10Q, &c against wchwhichthe names of those colours are written below.But further to define the latitude of these colours in each Ring or series, let A2 designe the least thicknesse & A3 the greatest thicknesse at wchwhich the extreame PuViolet in the first series is reflected & let HI & HL designe the like limits for the extreame Red, & let the14 the intermediate colours be limited by the intermediate parts of the lines 1I & 3L against wchwhich the names of those colours are written. And in the second series let those limits be the lines 5M & 7O; & so on: But yet wthwith this caution that the reflexions be supposed strongest at the intermediate spaces 2K, 6N, 10Q &c & from thence to decrease gradually towards these limits 1I, 3L; 5M, 7O; &c on either side; where you must not conceive them to be precisely limited but to decay indefinitely. And whereas I have assigned the same latitude to every series, I did it because although the colours in the first series seem to be a little broader then the rest by reason of a stronger reflexion there, yet that inequality is so insensible as scarcely to be determined by observation.Now according to this ODescription conceiving that the rays in wchwhichseverall colours inhere are by turnes reflected at the spaces iIL3, 5MO7, 9PR11 &c:, & transmitted at yethe spaces AHI1, 3LM5, 7OP9 &c: it is easy to know what colour must in the open Air be exhibited at any thicknesse of a transparent thin bodyBody. For if a Ruler be applyed parallel to AH at that distance from it by wchwhich the thicknesse of the Body is represented, the alternate spaces 1IL3, 5MO7 &c wchwhich it crosseth will denote the reflected originall colours of which the colour exhibited in the open Air is compounded. Thus if the constitution of the Green in the third series of colours be desired; apply the Ruler as you see at πρσ, φ, And by its passing through some of the Blew at π, & yellow at σ, as well as through the green at ρ, you mady conclude that the green exhibited at that thicknesse of the Body is principally constituted of originall green, but not without a mixture of some bBlew & Yellow.By this meanes you may know how the colours from the center of the Rings outwards ought to succede in order as they were described in yethe 4th & 18th Observations. For if you move the Ruler gradually from AH through all distances, having past over the first space wchwhich denotes little or no reflexion to be made by thinnest substances, it will first arrive at 1 the Violet, & then very quickly at the blew & Green wchwhich together wthwithytthat Violet compound blew, & then at yethe Yellow & Red by whose further addition that Blew is converted into whitenesse, wchhich whitenesse continues during the transit of the edge of the ruler from I to 3, & after that by the successive deficience of its component colours turnes first to compound Yellow & then to Red, & last of all the Red ceaseth at L. Then begin the colours of the second series, wchwhich succeed in order during the transit of the edge of the ruler from 5 & Obetween 5 & O, & are more lively then before because more expanded & severed. And for the same reason instead of the former white there intercedes between yethe blew & Yellow a mixture of Orange, Yellow, Green, Blew & Indico, all wchwhichtogether ought to exhibit a dilute & imperfect Green. So the colours of the506815 the third series all succeed in order; first the Violet wchwhich a little interferes with the Red of the second order & is thereby inclined to a reddish Purple; then the Blew & Green wchwhich are lesse mixed with other colours & consequently more lively then before, especially the Green: Then follows the Yellow some of which towards the Green is distinct & good, but that part of it towards the succeding Red, as also that Red is mixed wthwith the vViolet & Blew of the fourth series, whereby various degrees of Red very much inclining to Purple are compounded. This Violet & Blew wchwhich should succeed this Red being mixed with & hidden in it, there succeeds a Green, And this at first is much inclined to Blew, but soon becomes a good Green, the onely unmixt & lively colour in this fourth series. For as it verges towards the Yellow it begins to interfere wthwith the colours of the fift series, by whose mixture the succeeding Yellow & Red are very much diluted & made dirty, especially the Yellow wchwhich being the weaker colour is scarce able to shew it selfe. After this the severall series interfere more & more & their colours become more & more intermixed, till after thre or four more revolutions (inwchwhichyethe Red & Blew ––– predominate by turnes) all sorts of colours are in all places pretty equally blended & compound an eaven whitenesse.And since by the 15th Observation the rays indued wthwith one collour are transmitted where those of another colour are reflected, the reason of the colours made by the transmitted light in the 9th & 20th Observations is also from hence evident.If not onely the order & species of these colours, be desiredbut allso the precise thicknesse of the Plate or thin Body at wchwhich they are exhibited, be desired in parts of an inch, that may be also performedobteined by assistance of the 6t or 16th Observation. For according to those Observations the thicknesse of the thinned Air wchwhichbetween two Glasses exhibited the most luminous part of the sixt ringOrange or Bright Red of the sixt order, was parts of an inch. Now suppose this thicknesse be represented by G7 & the eleventh part of it Gλ will be about of an inch. And so Gμ, Gν, Gξ, Gο will be , , , & . And this being known it is easy to determin what thicknesse of Air is represented by Gφ or by any other distance of the Ruler from AH.But further since by the 10th Observation the thicknesse of Air iswas to the thicknesse of water wchwhich between the same Glasses exhibited the same colour, as 4 to 3; & by the 21th Observation yethe colours of thin Bodies are not varied by varying the ambient Medium;:the thicknesse of a Bubble of water exhibiting any colour will be of the thicknesse of Air produceing the same colour. And so according to the same 10th & 21th Observations the thicknesse of a Plate of Glasse whose refraction is measured by the proportion of the sines 16 31 to 20 may be of the thicknesse of Air produceing the same colours. And the like of other Mediums. On these grounds I have composed the following Table wherein the thicknesse of Air, water, & glasse at wchwhicheach colour is most intense & specific, is expressed in parts of an inch divided into ten hundred thousand parts equall parts.ThethicknesseofIn particles of pellucid bodies lesse thenAir ––WaterGlasseIn those ofBlack2, or lesse.The Colours Of the 1st orderBlew2.White4.Yellow865Orange9.Red.10.Violet129.Indico9.Blew11.Of the 2d OrderGreen1612.Yellow.Orange.Bright Red201513.Scarlet16.Purple2314Indico2418.Blew19.Of the 3d orderGreen.Yellow2219.Red3120.Blewish Red25.Blewish Green3627.Yellow Green.4th OrderYellowish Green.Red4433.5t OrderGreenish Blew38.Red4337.6t OrderGreenish Blew6448.Red53.7th Order.Grenish Blew5850.Red or white8463.Now if this Table be compared with the 3d Scheme pag. 13 you will there see the constitution of each colour as to its ingredients or the originall colours of wchwhich it is compounded, & thence be enabled to judg of its intensenesse or imperfection, wchwhich may suffice in explication of the 4th & 18thObservations, unlesse it be further desired to delineate the manner how the colours appeare when yethe two Object-glasses are layd upon one another. To doe wchwhich let there be described a large Arc of a circles & a streight50917 streight line wchwhich may touch that Arc, & parallel to that Tangent severall occult lines at such distances from it as the numbers set against the severall colours in yethe Table denote. For the Arc & its Tangent will represent colours the superficies of the glasses terminating yethe interjacent Air, & the places where the occult lines cut the Arc will show at what distances from yethecenter or point of contact each colour is reflected.There are also other uses of this Table: For by its assistance yethe Thicknesse of the Bubble in yethe 19th Observation was determined by the colours wchwhich it exhibited. And so yethe bignesse of yethe parts of naturall bodies may be conjectured at by their colours, as shall be hereafter shown. Also if two or more very thin Plates be layd one upon another so as to compose one Plate equalling them all in thicknesse, the resulting colour may be hereby determined. For instance, Mr Hook in his Micrographia observes that a faint yellow Plate of Moscovy-Glasse layd upon a blew one constituted a very deep purple. The Yellow of yethe first order is a faint one, & the thicknesse of the Plate exhibiting it, according to yethe Table, is to wchwhich add the thicknesse exhibiting Blew of yethe second order, & yethe summe will be , wchwhichismost nearely approaches the thicknesse exhibiting yethe Purple of yethe third order.To explain in yethe next place the circumstances of the 2d & 3dObservations, that is, how the coloursrings of colours may, (by turning yethe Prisms about their common Axis yethe contrary way to that expressed in those Observations,)maybe converted into white & black Rings, & afterwards into rings of colours again the colours of each ring lying now in an inverted order: it must be remembered that those rings of colours are dilated by the obliquation of the rays to yethe Air wchwhich intercedes yetheGlasses, & that according to yethe Table in the 7th Observation their dilatation or recession from their common centerincrease of their diameter is most manifest & speedy when they are obliquest. Now the rays of Yellow being more refracted by the first superficies of the said Air wchwhich intercedes thethen those of red are thereby made more oblique to yethe second superficies at wchwhich they are reflected to propduce the coloured Rings, & consequently the circle of Yellow circle in each Ring will be more dilated then yetheRed; & yethe excesse of its dilatation will be so much the greater by how much the greater is the obliquity of the rays, untill at last it become of equall extent with the Red of the same Ring. And for the same reason the Green blew & violet will be also so much dilated by the still greater obliquity of their rays as to become all very nearely of equall extent with the Red, that is, equally distant from the center of the Rings. And then all the colours of the same seriesring must be coincident, & by their mixture exhibit a white Ring. And these white Rings must have black or dark Rings between them because they doe not spread & interfere with one another as before. 18 And for that reason also they must become distincter & visible to far greater numbers. But yet yethe Violet being obliquest will be something more dilated in proportion to its extent then the other colours, & so very apt to appeare at the exterior verges of the white.Afterwards by a greater obliquity of the rays, the Violet & Blew become sensibly more dilated then yethe Red & Yellow & so being further removed from yethe center of the Rings, the colours must emerge out of the white in an order contrary to ytthatwchwhich they had before, the Violet & Blew at the exterior limbs of each ring, & yethe Red & Yellow at yethe interiorr. And the Violet by reason of the greatest obliquity of its rays being in proportion most of all expanded will soonest appeare at yethe exterior limb of each white Ring & become more conspicuous then the rest. And the severall series of rings of colours belonging to the several rings will by their unfolding & spreading will begin again to interfere, & thereby render the Rings lesse distinct, & not visible to so great numbers.If instead of the Prisms the Object-Glasses be made use of; the Rings wchwhich they exhibit become not white & distinct by the obliquity of the eye by reason that the rays in their passage through that Air wchwhich indtercedes yethe Glasses are very nearely parallel to themselves when those lines in wchwhich they were first incident on the Glasses, & consequently thosethe rays indued with severall colours are not inclined one more then another to that Air, as it happens in the Prisms.There is yet another circumstance of these experiments to be considered, & that is why the black & white Rings wchwhich when viewed at distance appeare distinct, should not onely become confused by viewing them neare at hand, but also yeild a vViolet colour at both the edges of every white Ring. And the reason is that yethe rays wchwhich enter the eye at severall parts of yethe pupill, have severall obliquities to yethegGlasses, & those wchwhich are most oblique, if considered apart, would represent the Ringsbigger then those wchwhich are the least oblique. Whence the breadth of the perimeter of every white white Ring is expanded outwards by the obliquest rays, & inwards by the least oblique. And this expansion is so much the greater by how much the greater is the difference of the obliquity; that is, by how much the Pupill is wider, or the Eye nearer to the Glasses. And the breadth of the Violet must be most expanded, because the rays apt to excite a sensation of that colour are most oblique to the second or further superficies of the thind Air at wchwhich they are reflected, & have also the greatest variation of obliquity, wchwhich makes that colour soonest emerge out of the edges of the white. And the breadth of every Ring is thus augmented, the dark intervalls must be diminished untill the neighbouring Rings become continuous, the & are blended, the exterior first, & then those nearer center, so that they can no longer be distinguished apart but seem to constitute an eaven & uniform whitenesse.Amongst50851019Amongst all the Observations there is none accompanied with so add circumstances as the 24th. Of those yethe principall are that in thin Plates ofwchwhich to yethe naked eye seem of an eaven & uniforme trans-parentwhitenesse without any terminations of shadows, the refraction of a Prism should make the Rings of colours appeare, whereas it usually makes Objects appeare coloured onely there where they are terminated with shaddows or have parts unequally luminous; & that it should make those Rings exceedingly distinct & white, although it usually renders Objects confused & coloured. The cause of these things you will understand by considering that all the Rings of colours are actuallyreally in the Plate when viewed with the naked eye, although by reason of the great breadth of their circumferences they so much interfere & are blended together that they constitute seem to constitute an eaven whitenesse. But when the rays passe through the Prism to the eye the orbits of the severall colours in every Ring are refracted some more then others according to their degrees of refrangibility: by wchwhich meanes the colours on one side of the Ring (that is on one side of its center) become more unfolded & dilated & those on the other side more complicated and contracted. And where by a due refraction they are so much contracted ytthat the severall Rings become narrower then to interfere with one another, they must appeare distinct & also white if the constituent colours be so much contracted as to be wholly coincident. But on the other side where the orbit of every Ring is made broader by the further unfolding of its colours, it must interfere more with other Rings then before & so become lesse distinct.To explaine this a little further suppose the concentrik circles AZV & BZX represent the red & Violet of any order wchwhich together wthwith the intermediate colours constitute any one of these Rings. Now these being viewed through a Prism the violet circle BCX will by a greater refraction be further translated from its place then the Red AZV & so approach nearer to it on that side towards wchwhich the refractions are made. For instance, if the Red be translated to av the vViolet may be translated to bx so as to approach nearer to it at x then before, & if yethe Red be further translated to aV the vViolet may be so much further translated to bX as to convene with it at X, & if the Red be yet further translated to αγ the Violet may be still so much further translated to βξ as to passe beyond it at ξ & convene with it at e & f. And this being understood not onely of the Red & Violet but of all the other intermediate colours, & also of every revolution of those colours, you will easily perceive how those of the same revolution or 20 order by their nearenesse at cdxv & &γξ, & their coincidence at cdXV, e, & f ought to constitute pretty distinct arcs of circles, especially at cdXV or at e &f, & that they will appeare severall at cdxv,& at cdXVexhibit whitenesse by their coincidence & again appeare severall at γξ, but yet in a contrary order to that which they had before & still retaine beyond e & f. But on the other side at ab, ab, or αβ these colours must become much more confused by being dilated & spread so as to interfere with those of the other orders. And the same confusion will happen at γξ between e & f if the refraction be very great, or the Prism very distant from the Object-Glasses: in wchwhich case no parts of the Ring will be seen save onely two little aArcs at e & f whose distance from one another will be augmented by removing the Prism still further from the Object-Glasses. And these little Arcs must be distinctest & whitest at their Middle, & at their ends where they begin to grow confused they must be coloured. And the colours at one end of every Arc must be in a contrary order to those at the other end, by reason that they crosse in the intermediate white. Namely their ends wchwhich verge towards γξ will be red & yellow on that side next the center, & blew & violet on the other side. But their other ends wchwhich verge from γξ will on the contrary be blew & violet on that side towards the center & on the other side red & yellow.For confirmation of all this I need alledg no more than that it is Mathematically demonstrable from my former Principles. But yet I shall add that thay which please to take the paines, may by the Testimony of their senses be assured that these explications are not Hypotheticall but infalibly true & genuineNow as all these things follow from yethe properties of light of a mathematical way of reasoning so the truth of them may be manifested by experiments. For in a dark Room by viewing these Rings through a Prism by reflexion of the severall Prismatique colour wchwhich an Astsistant causes to move to & fro upon a wall or paper from whence they are reflected, whilst the spectators eye, the Prism and the Object glasses (as in the 123th Observation) are placed steddy: the position of the circles made successively by the severall colours will be found such in respect of one another as I have described atin the figuresabxv, or abXV, or αβξγ. And by the same method yethe truth of yethe explications of other Observations is asmay be examined.By what hath been said, the like Phænomena of waterbBubbles of water & thin Plates of Glasse may be understood. But in small fragments of those Plates, there is this further observable, that ifwhen lying flat upon a Table beare turned about their centers whilst they are viewed through a Prism they will in some postures exhibit waves of various colours &: the some of them willdo exhibit these waves in one or two onely; but the most of them dodo in all positions exhibit & that for thin the most part appeare almost all 50951121 the Glasseplates. The reason is that the superficies of such Plates are not eaven but have many cavities & swellings wchwhich how shallow soever do a little vary the thicknesse of the Plate: For at the severall sides of those cavities for the reasons newly described there ought to be produced waves in severall postures of the Prism. Now though it be but some very small & narrow parts of the Glasse by which these waves for yethe most part are caused, yet they may seem to extend themselves over the whole Glasse, because from the narrowest of those parts there are colours of severall orders that is of rings several rings confusedly reflected, wchwhich by refraction of the Prism are unfolded &separated &according to their degrees of refraction, dispersed to severall places so as to constitute so many severall waves as there were divers orders of the colours promiscuously reflected from that part of the Glasse.These are the principall Phænomena of thin Plates or Bubbles whose explications depend on those properties of light twhatich I have heretofore dielivered. And these you see do necessarily follow from them & agree with them even to their very least circumstances; & not onely so, but do very much tend to their proof. Thus by the 24th Observation it appeares that the rays of severall colours made as well by thin Plates or Bubbles as by the refractions of a Prism have severall degrees of refrangibility whereby those of each order wchwhich at their reflexion from the Plate or Bubble are intermixed with those of the other ortders, are separated from them by refraction, & associated together so as to become visible by themselves like Arcs of Circles. For if the rays were all alike refrangible, tis impossible that the whitenesse wchwhich to the naked sense appeares uniforme should by refraction have its parts transposed & ranged into those black & white Arcs.It appeares also that the unequall refractions of difform rays proceed not from any contingent irregularityies: Such as are Veines an uneaven polish, or fortuitous position of the pores of Glasse; unequall & casual motions in the Air or Æther; the spreading, breaking, or dividing the same ray into many diverging parts; or the like. For admitting any such irregularities, it would be impossible for refractions to render those Rings so very distinct & well defined as they doe in the 24th Observation. It is necessary therefore that every ray have its proper & constant degree of refrangibility connate with it, according to wchwhich its refraction is ever justly & regularly performed; & that severall rays have severall of those degrees.And what is said of their rRefrangibility may be also understood of their rReflecxtibility; that is of their dispositions to be reflected 22 some at a greater & others at a lesse thicknessesse of thin Plates or Bubbles namely that those dispositions are also connate with the rays & immutable; as may appeare by the 13th 14th & 15th Observations compared with the 4th &18th.By the precedent Observations it appeares also that whitenesse is a dissimilar mixture of all colours, & that light ia a mixture of rays indued with all those colours. For considering the multitude of the Rings of colours in the 3d, 12t, & 24th Observations, it is manifest that although in the 4th & 18th Observations there appeare no more then 8 or 9 of those Rings, yet there are really a far greater number wchwhich so much interfere with one & mingle with one another as after those 8 or 9 revolutions to dilute one another wholly & constitute an eaven & uni sensibly uniforme whitenesse. And consequently that whitenesse must be allowed a mixture of all colours, & the light wchwhich conveys it to the eye must be a mixture of th rays indued with all those colours. This I beleive hath seemed the most Paradoxicall of all my assertions, & met with the most universall & obstinate Prejudice;.bBut to me it appeares as infallibly true & certaine, as it can seem extravagant to others. For hitherto I never tryed any way to mix all colours by which I could not in some degree or other produce whitenesse, & yet I have made as many tryalls as I could excogitate ways of mixing colours; of wchwhich I may take occasion to discourse hereafter.But further by the 24th Observation it appeares that there is a constant relation between cColours & rRefrangibility, the most refrangible rays being Violet, the least refrangible Red, & those of intermediate colours having proportionally intermediate degrees of refrangibility. And by the 13th, 14th, & 15th Observations compared wthwith the 4thor 18th, there appeares to be the same constant relation between cColour & rReflectxibility, the Violet being on equall termesin like circumstances reflected at least thicknesses of any thin Plate or Bubble, yethe Red at greatest thicknesses, & the intermediate colours at intermediate thicknesses. Whence it follows that the colorifique dispositions of rays are also connate with them & immutable, & by consequence that all the productions & appearances of colours in the world are derived not from any Physicall change caused in light by refraction or reflexion, but onely from the various mixtures or separations of rays by vertue of their different rRefrangibility or Reflexibility. And in this respect it is that the Science of colours becomes a Speculation as truly Mathematical as any other part of Optiques.more proper for Mathematicians then naturalists. & deserves rather to be esteemed Mathematicall then Physicall, as I told you in my former letter, & may hereafter explane more fully.51051223The third pPart Conclusions drawn from yethe foregoing Observations.I am now come to theanotherlast part of this designe wchwhichis to consider how the Phænomena of thin transparent Plates stand related to those of all other naturall bodies. Of these bodies I have already told you that they appeare of divers colours accordingly as they are originally disposed to reflect the rays indued withthose colours most copiously the rays originally disposed indued with those colours. But their constitutions whereby they reflect some rays more copiously then others remaines to be inquired after. Anddiscovered, & these I shall endeavour to manifest in the following Propositions.Prop: 1. Those superficies of transparent bodies reflect the greatest quantity of light wchwhich have the greatest refracting power; that is, wchwhichintercede Mediums that differ most in their refractive densities: And in the confines of equally denserefracting Mediums there is no reflexion.The Analogy between reflexion & refraction will appeare by considering that when light passeth obliquely out of one Medium into another which refracts from the Perpendicular, the greater is the difference of their refractive density the lesse obliquity is requisite to cause a totall reflexion.; becauseFor as the sines are wchwhich measure the refraction, so is the sine of incidence at wchwhich the totall reflexion begins; to yethe Radius of yethe Circle, & consequently that incidence is least where there is the greatest difference of the sines. Thus in the passing of light out of water into Air where the refraction is measured by the ratio of the sines 3 to 4 the totall reflexion begins when the angle of incidence is about 48degr 35min. In passing out of Glasse into Air where the refraction is measured by the ratio of yethe sines 20 to 31 the totall reflexion begins when the angle of incidence is 40degr 10min. And so in passing out of the Christall or more strongly refracting Mediums into Air, there is still a lesse obliquity requisite to cause a totall reflexion. Superficies therefore wchwhich refract most do soonest reflect all the light, which is incident on them, & so must be allowed most strongly reflecxtive.But the truth of this Proposition will further appeare by observing that in the superficies interceding any two of thesetransparent Mediums (such as are Air, Water, or any other liquors, oyle Common Glasse, Chriystall, & Metalline GlassesIsland glass, white transparent Arsnick, Diamonds &c,) the reflexion is stronger or weaker accordingly as the superficies hath a greater or lesse refracting power. Thus when other Mediums are contiguous to Air; the reflexion is stronger in the superficies of Glasse then of Water, still stronger in the superficies of Chriystall & strongest in the superficies of Metalline Glasse. So in the confine of Water & common Glasse theFor Iin the confine of Air & common glass yethe reflexion is stronger then in the confine of air & water & still stronger in the confine of aAir & cCrystal & stronger in the confine of Air & a Diamond. If any of these solids be immer & such like transparent solids be immerged in water or oyle or in spirit of wine or spirit of Turpentine its beco reflexion becomes much weaker then before, & still weaker if they be immer24ged in that denser yethe heavy liquor of well rectified oyle of Vitriol wchwhichdiffers less from them in density If water be distinguished into two parts by any imaginary surface; the reflereflexion is very weake, but yet stronger then in the confine of water & oyle or almost any other two liquors, & still stronger in the confine of water & Chriystall or Metalline Glasse: accordingly as those Mediums differ more or lesse in density. Soreflexion in thate confine of those two parts is none at all: in the confine of water & ice tis very little, in that of water & oyle tis something greater, in that of water & glass still greater & in that of water & crystal or other denser substances still greater, accordingly as those mediums differ more or less in density. Hence in the confine of common Glasse & Chriystall there ought to be a weake reflexion & a stronger reflexion in the confine of common & Metalline Glasse tho I have not yet tried this: But in the confine of two Glasses of equall density, there is not any sensible reflexiōonas was shown in yethe 1st Observation. And the same may be understood of the superficies interceding two chriystalls or liquors or any other substances in wchwhich no refraction is caused. Whence it comes to passe thatSo then the reason why uniforme pellucid Mediums (such as water glass or crystal) have no sensible reflexion but in their externall superficies where they are adjacent to other Mediums of a different density, is because all their contiguous parts have one & the same degree of density.Prop 2. The least parts of almost all naturall bodies are in some measure transparent. And the opacity of those bodies ariseth from the multitude of reflexions caused in their internall transparent parts.That this is so has been observed by others & will easily be granted by them that have been conversant with Microscopes. And it may also be tryed by applying any substance to a hole through wchwhich some light is immitted into a dark Room. For how opake soever that substance may seem in the open Air, it will by that meanes appeare very manifestly transparent if it be of a sufficient thinnesse. Only white Metalline Bodies must be excepted wchwhich by reason of their excessive density seem to reflect almost all the light incident on their first superficies, unless by solution in menstruums they be reduced into very small particles, & then they become transparent.Prop: 3. Between the parts of opake or coloured bodies are many intersticesspaces, either empty or replenished with Mediums of other densities: as water between the tinging corpuscles wherewith any liquor is impregnated; Air between the aqueous globules that constitute Clouds or mistsmists; & for yethe most part spaces voyd of both Air & Water but yet perhaps replenished wthwith some Subtiler Mediumin some measure not wholy void of all substance. between the parts of naturallhard bodies.The truth of this is evinced by the two precedent Propositions For by the second Proposition there are many reflexions frommade by the internall parts of Bodies, wchwhich, by the 1st Proposition would not happen if the parts of those bodies were cōontinued without any such interstices between them, because reflexions are caused onely in superficies wchwhich intercede Mediums of a differing density. by Prop 1But further that this discontinuity of parts is the principall cause of the opacity of Bodies will appeare by considering that opake substances become transparent by filling their pores with any substance51325 substance of equall or almost equall density with their parts. Thus Paper dipped in water or oyle, the Oculus Mundi stone steeped in water, Linnen cloth oyled or varnished, & many other substances soaked in such Liquors as will intimately pervade their little pores become by that meanes more transparent then otherwise. So on the contrary the most transparent substances may by evacuating their pores or separating their parts be rendered sufficiently opake, as salts or wet paper or the Oculus mundi stone by being dried, horn by being scraped, Glasse by being reduced to pouder or otherwise flawed, turpentine by being stirred about with water till they more imperfectly & water by being formed into many small bubbles either alone in the forme of froth, or by shaking it together either with Oyle of Turpentine or wthwith some wthwith other convenient Liquor wthwithwchwhich it will not perfectly incorporate.& Horne by being scraped.And To the increase of the opacity of these bodies it conduces something that by the 23rd Observation the reflexions of very thin transparent substances are considerably stronger then those made by the same substances of a greater thicknesse. [And to the reflexion of solid bodies it may further add, that the interstices of their parts are voyd of Air. For that for the most part they are so is reasonable to beleive considering the ineptitude wchwhich Air hath to pervade small cavities as appeares by the ascension of water in slender glasse pipes, paper, cloth & other such like substances whose pores are found too small to be replenished wthwith Air, & yet large enough to admit water; & by the difficulty wherewith Air pervades the pores of a Bladder through wchwhich water finds ready passage. And according to yethe 11th Observation the cavities thus voyd of Air will cause the same kind of effects as to reflexion wchwhich those do that are replenished with it, but yet something more manifestly because the Medium in relation to refractions is rarest when most empty of Air as Mr Hook hath proved in his Micrographia. In wchwhich Book he hath also largely discoursed of this & the precedent Proposition, & delivered many other very excellent things concerning the colours of thin Plates & other naturall Bodies wchwhich I have not scrupled to make use of so far as they were for my purpose.]Prop: 4. The parts of bodies & their interstices must not be lesse then of some definite bignesse to render them opake & coloured. For the opakest bodies of their parts be subtily divided (as Metalls by being dissolved in Acid Menstruums, &c) become perfectly transparent. And you may also remember that in the 8th Observation there was no sensible reflexion at the superficies of the Object-glasses where they were very neare one another though they did not absolutely touch. And in the 17th Observation the reflexion of the Water-bubble 26 where it became thinnest, was almost insensible so as to cause the apparitions of very black spots to appear on yethe top of the bubble by the want of reflected light.On these grounds I conceive it is that water, salte, glasse, stones & such like substances are transparent. For upon divers Considerations they seem to be as porousfull of pores or interstices between their parts as other bodies are, but yet their parts & parts& interstices to be too small to cause any opacity reflexions in their common surfaces.Prop: 5. The transparent parts of bodies according to their severall sizes must reflect rays of one colour & transmit those of another, on the same grounds that thin Plates or Bubbles doe reflect or transmit those rays. And this I take to be the ground of all their colours.For if a thind or plated Body wchwhich being of an eaven thicknesse appeares all over of one uniforme colour should be slit into threds or broken into fragments of the same thicknesse with the Plate,. I see no reason why a heap of those threds or fragments should not constitute a masse or pouder of yethe same colour wchwhich the Plate exhibited before it was broken. And the parts of all naturall bodies being like so –– many fragments of a Plate must on the same grounds exhibit the same colours.Now that they doe so will further appeare by the affinity of their Properties. As that the infusion of Nephritic WoodLignum Nephriticum & many other substances reflect one colour & transmit another like thin bodies in the 19th & 20th Observations. That the colours of Silks, Cloths, & other substances wchwhich water or Oyle can intimately penetrate become more faint & obscure by being im̄mmerged in those liquors, & recover their vigor again by being dryed much after the manner declared of thin Bodies in the 10th & 21th Observations. And that some of those coloured pouders wchwhich Painters use may have their colours a little changed by being very elaborately & finely ground. Where I see not what can be justly pretended for those changes besides the breaking of their parts into lesse parts by that contrition after the same manner that the colour of a thin Plate is changed by varying its thicknesse. For which reason also it is that yethemanycoloured Flowers of Plants & vegetables by being bruised usually become more transparent then before, or at least in some degree or other change their colours. Nor is it much lesse to my purpose that by mixing divers liquors very odd & remarqueable productions & changes of colours may be effected, of wchwhich no cause can be more obvious & naturallrationall then that51251427ytthat the saline corpuscles of one liquor do variously act upon or unite with the tinging corpluscles of another, so as to make them swell or shrink (whereby not onely their bulk but their density also may be changed,) or to divide them into smaller corpuscles, or make many of them associate into one cluster. For we see how apt those saline Menstruums are to penetrate & dissolve substances to wchwhich they are applyed & some of them to precipitate what others dissolve. In like manner if we consider the various Phænomema of the Atmosphære, we may observe that when vapors are first raised, they hinder not the transparency of the Air, being divided into parts too small to cause any reflexion in their superficies. But when in order to compose drops of rain they begin to coalesce & constitute globules of all intermediate sizes, those globules when they become of a convenient size to reflect some colours & transmit others, may constitute clouds of various colours according to their sizes. And I see not what can be rationally conceived inin so transparent a substance as water for the production of these colours besides the various sizes of its fluid & globular parcells, [wchwhich seem to affect a globular figure most; but yet perhaps not without some irregularityinstability in the figure smallest of them by reason that those are most easily agitated by heat or any trembling motions in yethe Air.]Prop: 6. The parts of Bodies on wchwhich their colours depend are denser then the Medium wchwhich pervades their interstices.This will appeare by considering that the colour of a Body depends not onely on the rays wchwhich are incident perpendicularly on its parts, but on those also wchwhich are incident at all other angles. Andthat according to yethe 7th Observation a very little variation of obliquity will change yethe reflected colour where the thin body or small particle is rarer then the ambient Medium, insomuch that such a small particle will at diversly oblique incidences reflect all sorts of colours in so great a variety that the colour resulting from them all confusedly reflected from a heap of such particles must rather be a white or dirty grey then any other colour, or at leabest it must be some but a very imperfect & dirty colour. Whereas if the thin Body or small particle be much denser then the ambient Medium, the colours according to the 19th Observation are so little changed of the variation of obliquity, that the rays wchwhich are reflected least obliquely may predominate over the rest so much as to cause a heap of such particles to appear very intensely of their colour. It conduces also something to the confirmation of this Proposition that according 28 to the 22th Observation, the colours exhibited by the denser thin body within the rarer are more brisque then those exhibited by therrarerthe rarerwithin the denser.Prop: 7. The bignesse of the component parts of Naturall Bodies may be conjectured by their colours.For since the parts of these bodies by Prop: 5 doe most probably exhibit the same colours with a Plate of equall thicknesse, provided they have the same refractive density; & since their parts seem for yethe most part to have much the same density with water or glasse, as by many circumstances is obvious to collect: to determin the sizes of those parts you need onely have recourse to the precedent Tables in wchwhich the thicknesse of Water or Glasse exhibiting any colour is expressed. Thus if it be desired to know the diameter of a corpuscle wchwhich being of equall density with glasse shall reflect Green of the third order; the number shows it to be about parts of an inch.The greatest difficulty is here to know of what order the colour of any Body is. And for this end we must have recourse to the 4th & 18th Observations from whence may be collected these particularsScarlets & other Reds, Oranges & Yellows if they be pure & intense are most probably of the second Order. Those of the first & third order also may be pretty good, onely the yellow of the first order is faint & the Orange & Red of the third order hathvetooa great a mixture of Violet & Blew.There may be good Greens of the fourth order but the purest are of the third. And of this order the Green of all Vegetables seemes to be, partly by reason of the intensenesse of their colours, & partly because when they wither some of them turne to a greenish Yellow, & others to a more perfect Yellow or Orange or perhaps to Red, passing first through all the affore said intermediate colours. Which changes seem to be effected by the exhaling of the moisture wchwhich may leave the tinging corpuscles more dense, & something aumented by the accretion of the oyly & earthy part of that moisture. Now the Green without doubt is of the same order with those colours into wchwhich it changeth because the changes are graduall, & those colours though usually not very purefull yet for the most part are often too purefull & lively to be of the fourth order.Blews & Purples may be either of the second or third order But the best are of the third. Thus the colour of violets seems to be of that order becaus their Syrup by acid liquors turnes red and51529 and by urinous & alcalizate turne green. For since it is of the nature of acids to dissolve or attenuate, & of Alcalies toto precipitate or incrassate, if the purple colour of yethe Syrup was of the second order, as acid liquor by attenuating its tinging corpuscles would change it to a red of the first order, & an Alcaly by incrassating them would change it to a green of the second order; wchwhich red & Green, especially the Green, seem too imperfect to be the colours produced by these changes. But if the said purple be supposed of the third order, its change to red of the second & green of the third may without any inconvenience be allowedIf there be found any body of a deeper & lesse reddish purple then that of Violets its colour most probably is of the second order. But yet there being no body commonly known whose colour is constantly more deep then theirs, I have made use of their name to denote the deepest & least reddish Purples, such as manifestly transcent their colour in purity.The Blew of the first Order though very faint & little may possibly be the colour of some substances. And particularly the Azure colour of the skys seems to be of this order. For all vapors when they begin to condense & coalesce into small parcells become first of that bignesse whereby such an Azure must be reflected before they can constitute clouds of other colours. And so this being the first colour wchwhich vapors begin to reflect, it ought to be the colour of the finest & most transparent Skys in wchwhich vapors are not arrived to that grossnesse requisite to reflect other colours as we find it is by experience.Whitenesse if it be intense is eitherthe most intense & luminous is that in the first order of colours. of wchwhich sort perhaps is the colour of white LeadFor of this sort I take the colour of white metals to be; or else it is a mixture of those succeeding the third or fourth order, such as is the colour of paper linnen, & most white substances. If corpuscles of various sizes exhibiting the colours of the second & third order be mixed, they should rather constitute an imperfect whitenesse or Grey of wchwhich I have already spoken. But yet it seems not impossible for them to exhibit an intense whitenesse if they be disposed to transmit all the light wchwhich they reflect not, & do not retain & stifle somemuch of it For thus I told you that Froth at a distance hath appeared very white & yet neare at hand the severall Bubbles of wchwhich it was constituted were seen tinged all over with rings of colours of the four or five first orders.Lastly for the production of Black the corpuscles must be lesse then any of those wchwhich exhibit colours. For at all greater sizes there is too much light reflected to constitute this colour. But if they be 30 supposed a little lesse then is requisite to reflect the white & very faint blew of the first order they will according to the 4th, 8th, 17th, & 18th Observations reflect so very little light as to appeare intensely black, & yet may perhaps varioublly refract it to & fro with in themselves so long untill it happen to be stifled & lost, by wchwhich means they will appear black in all positions of the eye without any transparency. And from hence may be understood why Fire & the more subtile dissolver Putrefaction turne substances to black; why small quantities of black substances impart their colour very freely & intensely to other substances to wchwhich they are applied; x why Glasse ground very elaborately with sand on a Copper plate till it be well polished makes the sand together with what is worne of from the glasse & Copper become very black; why black substances do soonest of all others become hot in yethe sun's light & for burne, (wchwhich effect may proceed partly from yethe multitude of refractions in a little room, & partly from the easy commotion of so very small corpuscles;) & why blacks are usually a little inclined to a blewish colour. For that they are so may be seen by illuminating white paper by reflexionlight reflected from black substances, wchwhichFor the paper will usually appeare of a blewish white. And the reason is that Black borders on the obscure Blew described inof yethe first order. described in the 18th Observation, the larger& therefore thecorpuscles of black substances are most apt to reflect that colourreflects more rays of that colour then of any other.In these descriptions I have been the more particular because I hopeit is not imposible but that Microscopes willmay at length be improved to the discovery of the corpusclesparticles of Bodies on wchwhich their colours depend if they are not already in some measure arrived to that degree of perfection. For if those Instruments are or can be so far improved as with sufficient distinctnesse to represent Objects five or six times hundred times bigger then at a foot distance they appear to orour naked eyes, I should hope that we might be apble to discover some of the greatest of those corpuscles. And by one that would magnify thre or four thousand times perhaps they might all be discovered but those which produce blacknesse. In the meane while I see nothing materiall thatin this discourse that rationally may be doubted of excepting this pPosition, That transparent corpuscles of the same thicknesse & density with a Plate, do exhibit the same colour. And this I would have understood not without some latitude, as well because those corpuscles may be of irregular figures, & many rays must be obliquely incident on them, & so have a shorter way through them then the length of their diameters, as because the straitnesse of the Medium pent in on all sides wchwhich in such corpuscles may a little after its motions or other qualities on wchwhich the reflexion depends. But yet I cannot much suspect the last because I have observed of some small Plates of Moscovy-Glasse wchwhich were of an eaven thicknesse, that through a Microscope they have appeared of the same colour at their edges & corners where the included Medium51631 Medium was terminated, wchwhich they appeared of in other places. However it will add much to orour satisfaction if those corpuscles can be discovered with Microscopes, wchwhich if we shall at length attain to I feare it will be the utmost improvement of this Sense. For it seems impossible to see the more secret & noble works of Nature within those corpuscles by reason of their transparency.This may suffice concerning the constitution of naturall bodies on wchwhich their colours depend. But for further understanding the nature of reflexions I shall add these two following Propositions.Prop: 8. The cause of Reflexion is not the impinging of Light on the solid par or impervious parts of bodies, as is commonly supposed. beleived.This will appear by the following Considerations. First that in the passage of light out of Glasse into Air there is a reflexion is as strong orstrongerthenas in its passage out of aAir into Glasse or rather a little stronger & by many degrees stronger thenstronger then in its passage out of Glass into wWater. And it seems not probable that Air should have more reflecting parts then water of Glass. But if that should possibly be supposed yet it will availe nothing: for the reflexion is as strong or stronger thwhen the Air is drawn away from the Glass, (suppose in yethe Air-Pump invented by Mr Boyle,) as when it is adjacent to it. Secondly if light in its passage out of Glass into Air be incident more obliquely then at an angle of 40 or 41 degrees it is wholly reflected, if lesse obliquely it is in great measure transmitted. Now it is not to be imagined that light at one degree of obliquity should meet with pores enew in the Air to transmit the greater part of it, & at another degree of obliquity should meet with nothing but parts to reflect it wholly: especially considering that in its passage out of Air into Glass, how oblique soever be its incidence, it finds pores enew in the Glass to transmit the greatest part of it. If any man suppose that it is not reflected by the Air but by the outmost superficiall parts of the Glasse, there is still the same difficulty: besides that such a supposition is unintelligible, & will also appear to be false by applying water behind some part of yethe Glasse instead of Air. For so in a convenient obliquity of the rays suppose of 45 or 46 degrees at wchwhich they are all reflected where the Air is adjacent to the Glass, they shall be in great measure transmitted where the water is adjacent to it: wchwhich argues that their reflexion or transmission depends on the constitution of the Air & water behind the glass & not on their parts striking of yethe rays upon the parts of the glass. Thirdly if the colours made by a Prism placed at the entrance of a beam of light into a darkened Room be successively cast on a second Prism placed at a great distance from the former, in such manner that they are all alike incident upon it the second Prism may be so inclined 31 to the incident rays that those wchwhich are of a blew colour shall be all reflected by it, & yet those of a red colour pretty copioulsly transmitted. Now if the reflexion be caused by the parts of Air or Glass I would ask why at the same obliquity of incidence the bBlew should wholly impinge on those parts so as to be all reflected, & yet the red find pores enew to be in great measure transmitted Fourthly where two Glasses touch one another there is no sensible reflexion as was declared in the 1st Observation; & yet I see no reason why the rays shoudshould not impinge on the parts of Glass when contiguous to another Glasse as much as when contiguous to Air. Fiftly when the top of a water-Bubble (in the 17th Observation) by the continuall subsiding & exhaleing of the water grew very thin there was such a little & almost insensible quantity of light reflected from it that it appeared intensely black, whereas round about that black spot where the water was thicker the reflexion was so strong as to make the water seem very white. Nor is it onely at the least thicknesse of thin Plates or Bubbles that there is no manifest reflexion but at many other thicknesses continually greater & greater. For in the 15th Observation the rays of the same colour were by turnes transmitted at one thicknesse & reflected at another thicknesse for an indeterminate number of successions. And yet in the superficies of the thinned Body where it is of any one thicknesse there are as many parts for the rays to impinge on, as where it is of any other thicknesse. LastlyLastly if reflexion were caused by the parts of reflecting bodies it would be impossible for thin Plates or Bubbles at the same place to reflect the rays of one colour & transmit those of another as they do according to the 13 & 15 Observations. For it is not to be imagined that at one place the rays wchwhich for instance exhibit a blew colour should have the fortune to dash upon the parts & those wchwhichexhibit a Red to hit upon the pores of the Body; & then at another place where the body is either a little thicker or a little thinner, that on the contrary the bBlew should hit upon its pores & the Red upon its parts. Lastly were yethe rays of light reflected by impinging on the solid parts of bodies, their reflexions from polished bodies could not be so regular as they are. For ✝Prop. 9. Tis most probbable that the rays wchwhich impinge on the solid parts of any body, are not reflected but shifted & lost in the body.This is consentaneous to the precedent Proposition & will further appear by considering that if all the rays should be reflected which impinge on the internall parts of clear water or crystall, those substances should rather have a cloudy colour then so very clear transparency51533 transparency. And further there would be no Principle of the obscobscurity or blacknesse wchwhich some bodies have in all positions of the eye. For to produce this effectFor if all or almost all the light after incident on those bodies did after many reflexions & refractions come out again it would make those bodies as lucid as other bodies are are, & therefore to make them look black it is necessary that many rays be retainedstoptretained & lost in thethembodyies, & it seems not probable that any rays can be retained stopt & retainedstifled in itthemwchwhich do not impinge on itstheir partsIf you now ask how rays are reflected without impinging on the parts of a body, & how those wchwhich impinge on its parts may be stopped & stifled, it requires an Hypothesis to explaine it by., the description of wchwhich is besides my designe. And so the manner how severall rays are unequally refrangible & reflecxtible, & originally indued with severallpowers of striking yethe sense wthwith several colours remaines to be explained Hypothetically: But I shall content my self with haveing sheown that de facto the rays of light are really indued with those properties. And the inventing of such an Hypothesis is no part of my designe I undertoo>tk only to discover yethe properties of light so far Ias I could derive them from experiments; & therefore content my self wthwith having shewn that yethe rays of light are endued wthwith such propertiesthe prop those properties.