Draft of the 'Discourse concerning Light and Colors'Isaac Newtonc.15,173 wordsNewton ProjectBrighton2011Newton Project, Sussex University
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c. early 1675, c. 16,292 words, 2 pp.2 pp.MS Add. 3970.3, ff. 549r-567v, Cambridge University Library, Cambridge, UKUKCambridgeCambridge University LibraryPortsmouth CollectionMS Add. 3970.3, ff. 549r-567vc. early 1675EnglandEnglishJohn WickinsUnknown HandIsaac NewtonScienceOpticsCorrespondenceTranscribed and tagged by Linda CrossTagging reviewed by John YoungUpdated to Newton V3.0 (TEI P5 Schema) by Michael HawkinsInline images embedded and minor code corrections by Michael HawkinsProofread text and code by Margarita Fernandez-ChasProofed by Robert IliffeTranscription audited by Michael Hawkins
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I suppose you understand ytthat all transparent substances (as Glasse, water, Air &c) when made very thinne by being blown into bubbles or otherwise formed into plates, doe exhibite various colours according to their various thinnesse, although at a greater thicknesse they appear very clear & colourlesse. In my former discourse about yethe constitution of light, I omitted those colours, because they seemed of a more difficult consideration, & were not necessary for stablishmtment of yethe Doctrine which 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 yethe 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 somwhere touch one another, I found yethe place in wchwhich they touched to become * *Note that there is some little light reflected from yethethose parts of this black spot where yethe glasses by reason of their convexity & some little uneavenness of their surfaces do not come to absolute contact. For by viewing yethe sun by reflexion from this spot, not only yethe verges of it became lucid, but divers lucid veins & specks appeared into yethe midst of yethe thickness. But yet some parts of yethe spot seemed still as black as before I take. absolutely transparent, as if they had been there one continued peice of Glasse. for when yethelight fell so obliquely on yethe Air, which in other placesplaces was between them, as to be all reflected; in that place of contact it wasseemed wholy transmitted: in so much that when looked upon it apeared like a black or dark spot by reason ytthat no sensible light was reflected frōomthence as frōom other places; & when looked through it seemed as it were, a hole in yetthat Air that was formed into a thinner plate by being compressed between yetthat glasses. And through this hole objects that were beyond might be seen distinctly wchwhich could not at all be seen through other parts of yethegGlasses, where yethe Air was interjacent. Although yethe glasses were a little convex yet this transparent spot was considerable breadth, wchwhich breeadth seemed principally proceed frōomyethe yeilding inwards of yethe parts of yethe glasses reason of their mutual pressure. For by pressing them hard together it would become much broader then other. 2 Obs: 2. When yethe plate of Air, by turning the Prisms about their com̄mmon Axis became so little inclined to yethe incident rays, that some of ymthem began to be transmitted; there arose in it many slender Arcs of coloures wchwhich at first were shaped almost like the conchoid as you see them here delineated. And by yethe continuing the motion of yethe Prisms, those Arcs increased & bended more & more about yethe said transparent spot, till they were completed into circles or rings incompassing it, & afterwards continually grew more & more contracted.
These Arcs at their first appeareance were of a bviolet & blew colour, & between them were white Arcs of circles wchwhich presently became a little tinged in their inward limbs wthwith red & yellow, & to their outward limbs yethe blew was adjacent. So ytthatyethe order of those colours frōomyethe Centrall dark spot, was at that time white, Blew, Violet; DarknesseBlack; Red, Orang, yellow, White Blew, Violet, &c. But the yellow & Red were much fainter then yethe Blew & Violet.
The motion of yethe Prisms about their Axis being continued those colours contracted more & more, shrinking towards yethe whitenesse on either side of it, untill they totally vanished into it. And then yethe circles in those parts appeared black & white wthwithout any other colours intermixed. But by further mmovingyethe Prisms about, the colours again emerged out of yethewhitenesse, the Violet & blew at its inward limb, & at its outward limb the red & yellow. So that now their order frōomyethecentrall spot was white, Yellow, Red, Black, Violet, Blew ywhite, yellow, Red, &c. contrary to what it was before.
Obs: 3. When the rings at some parts of them appeared only black & white, they were very distinct & wel-defined & the blacknesse seemed as intense as that of yethe Centrall Spot. Also in yethe placesBorders of these rings where yethe colours began to emerge out of yethewhitenesse, 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 smalnesse I could not number. But in other Positions of yethe Prisms at wchwhichyetheappeared of many colours, I could not distinguish above of them, & yethe exteriour of those too were dilute.
In these two Observations to see yetheColoursringsany other colour but black & white, I found 56455023 that I held my eye at a good distance frōom them. For by approaching nearer, although in yethe same inclination of my eye, yet there ged a blewish colour out of yethe white, which by deilating it selfe more & more into the black rendered yethe circles lesse distinct & left ytethewhite a little tinged wthwith red & yellow. I found also ytthatby looking through a slit or oblong hole wchwhich was narrower then the pupill of my eye & held close to it paralell to yethePrisms, I could see yethe 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 yethe white circles as yethe rays became lesse & lesse inlclined to yethe plate of Air; I took two Objectglasses the one of a plano-convex for a fourteen foot Telescope, & yethe other a large double convex for one of fifty foot, & upon this laying yethe other wthwith its plane side downwards, I pressed ymthem slowly together to make the colours successively emerge in yethe middle of yethe Circles & ynthenslowly lifted the upper glasse frōomyethe lower to make them successively vanish again in yethe same place, where being of a considerable bredth, I could more easily discern them. And by this meanes I observed their succession & quantity to be as followeth.
Next to yethe pellucid centrall pot made by yethe contact of yethe glasses succeeded Violet, Blew, White, Yellow, & Red. The Violet & blew were soe very little in quantity that I could not discerne them in yethe circles made by yethe Prisms, but yethe yellow & Red were pretty copious, & seemed about as much in extent as the white, & 4 or 5 times more then yethe blew & Violet. The next circuit or order of colours imediately incompassing these was Violet, Blew, Green, yellow, & Red. And these were all of them copious & vivid excepting yethe green wchwhich was very little in quantity & seemed much more faint & dilute then yethe other colours. Of yethe other four the Violet was least & yethe blew lesse then yethe yellow or Red. The third circuit or order was also Purple, Blew, Green, yellow, & Red, in which the Purple seemed more redish then yethe Violet in yethe former circuit, & yethe green was much more conspicuous, being as bris & copious as any of yethe other colours except yethe yellow, but yethe Red began to be a little faded inclining very much to purple. After these succeded Green & Red. The Green was very copious & lively inclining on yethe side to blew & yethe other to yetheyellow. But in this 4th circuit was neither Violet, blew, nor yellow & yethe red was very imꝑperfect & dirty. Also the succeeding colours became more & more imꝑperfect & dilute after three or four more revo Revolutions they ended in ꝑperfect whitenesse.
Obs: 5. To determine the intervall of yethe glasses, or thicknesse of yethe interjacent Air by which each colour was produced, I measured yethe diameters of yethe first six rings at yethemost 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 since one of yethe glasses was plane & yethe other Sphericall their intervalls at those rings must be inofyethe same proportiongression. I measured also the diameters of yethe dark or faint rings between the more lucid colours & found their squares to be in Arithmeticall progression of yethe eaven numbers 2, 4, 6, 8, 10, 12. And it being very nice & difficult to take those measures exactly I repeated them diverse times at divers parts of yethe glasses, that by their agreement I might be confirmed in them. And the same Method I used in determining some others of yethe following observations.
Obs: 6. The diameter of yethe first ring at yethe most lucid part of its orbit was parts of an inch & yethe diameter of yethe Sphere on which yethe double convex object glasse was ground was 102 foot, as I found by measuring it; & consequently yethethicknesse of yethe Air, or Aereall intervall of yethe glasses at ytthatring was of an inch. For as yethe diameter of yethe said sphere (102 foot or 1224 inches) is to yethe semi diameter of yethe ring so very nearly is ytthat semi diameter to yethe said distance of yethe glasses. Now by yethe precedent observationsyethe eleventh part of this distance () is yethe thicknesse of yethe Air at ytthat part of yethe first ring where yethe yellow would be most vivid were it not mixed wthwithother colours in yethewhite, & this doubled gives yethe difference of its thicknesses at yethe yellow in all yethe other rings. viz or to use a round number yethe eighty thousandth part of an inch.
Obs: 7. Those dimensions were taken when my eye was placed perpendicularly over yethe glasses in or near yethe Axis of yetheRings, but when I viewed ymthem -->oblily they became bigger continually swelling as I removed my eye further frōom their Axis. And partly by measuring yethe diameter of yethe same Circle at severall obliquities of my eye, partly as also by making use of yethe two Prisms fortios, I found its diameter, & consequently yethe thicknesse at its Perimeter in all those obliquities to be Proportions expressed in this Table. 5655518Incidence on yethe Air.Refraction into yethe Air.Diameter of yethe RingThicknesse of yethe Air.gr. min.gr. min00. 00.00. 00.10.10.6. 26.10. 00.10.10.12. 45.20. 00.10.10.18. 49.30. 00.10.11.24. 30.40. 00.11.13.29. 37.50. 00.12.15.33. 58.60. 00.14.20.35. 47.65. 00.15.23.37. 19.70. 00.16.28.38. 33.75. 00.19.37.39. 27.80. 00.2252.40. 0.85. 00.2984.40. 11.90. 00.35122.In the two first Columns are expressed the obliquities of yethe rayes to yethepPlate of Air, that is their angles of inciden& refraction. In the third Column yethe diameter, of any coloured Ring in those obliquities, is exprest in parts of wchwhich ten constitute that diameter when the rayes are perpendicular. And in yethe fourth Column the thicknesse of yetheAir at yethe Circūumference of that Ring is exprest in parts of which also 10 constitute that thicknesse when yethe rayes are perpendicular.Obs. 8. The dark Spot in yethe middle of yethe Rings increased also by yethe obliquation of yethe eye although almost in sensibly But if in stead of yethe Object-glasses yethe Prisms were made use of, its increase was more manifest when viewed soe obliquely ytthat no colours appeared about it. It was least when the rays were incident most obliquely on yethe interjacent Air & increased more & more untill yethe coloured rings appeared, & then decreased again but not soe much as it increased before. And hence itis evident ytthatyethe transparency was not only at yethe absolute contacof yethe glasses but also where they had some little intervall. I have sometimes observed the diameter of ytthat spot to be between halfe & two fift parts of yethe diameter of yethe exterior circumference of the Red in yethe first circuit or revolution of colours when viewed almost perpendicularly, wheras when viewed oblily it hath wholy vanish'd & become opake & white like yethe other parts of yethe glasse. Whence it may be collected that the glasses did then scearclyscarcely or not at all touch one another, & that their intervall at yethe perimeter of that Spot when viewed perpendicularly was about a 5th or 6th part of their intervall at yethecircumference of yethe said Red.Obs: 9. By looking through yethe two contiguous o)bject-glasses, I yethe interjacent Air exhibited Rings of colours as well by reflecting it. The centrall Spot The centrall Spot was now white, & frōom it yethe order of yethe 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 yetheglasses, For by that means they became pretty vivid. Onely yethe first yellowish-Red, like yethe blew in yethe 4th Observation was soe little & faint as scearcely, to be discerned: comparing yethe coloured Rings made by reflexion, wthwith those made by transmission of yethe light I found ytthat white was opposite to black, Red to blew, yellow to Violet & green to a compound of Red & Violet. That is, those parts of yethe glasse were black when looked through, wchwhichwhen looked upon appeared white & on yethe contrary. And so those wchwhich in one case exhibited blew, did in yethe other case exbiteexhibit Red. And the like of yethe other colours.Obs: 10. Wetting the object-glasses a little at their edges the water crept in slowly between ymthem & yethe circles thereby became lesse & yethe colours more faint: in so much that as yethewater crept along, one halfe of them at wchwhich it first arrived would appear broken off frōomyethe other halfe & contracted into a lesse rRoom. By measuring them I found the proportion of their diameters to yethe diameters of yethe like circles made by Air to be about seavenseven to eight, & consequently yethe intervalls or the glasses at like circles caused by those two mediums water & Air are as about three to four. Perhaps it may be a generall Rule that if any other medium more or lesse dense then water be compressed between yethe glasses, their intervall at the Rings caused thereby will be to their intervall caused by interjacent Air, as the sines are wchwhich measure the refraction made out of that Medium into Air.Obs: 11. When the water was between the glasses If I pressed the upper glasse variously at its edges to make yethe Rings move nimbly from one place to another, a little bright Spot would immediately follow yethe center of them, wchwhich upon creeping in of the ambient water into that place would presently vanish; Its 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 yethe water they would not vanish. The reflexion must rather have been caused by a sensiblewchwhich could recede through the glasse at WaterObs: 12 These observations were made in ther to examine yethe effects of coloured light darkned yethe Room & viewed ymthem by reflecxteincast on a sheet of white paper. And by this & visible to a far another number 566552 I have sometimes seen more then 20 of them, wheras in yethe open Air I could not discerne above 8 or 9.Obs: 13. Appointing an Assistant to move the Prism to & frōomabout its Axis, that all its colours might successively fall on yethe same place of yethe paper & be reflected from yethe circles to my eye whilst I held it immoveable; I found the circles which yethe red light made to be manifestly bigger then those which were made by yethe Blew & Violet. And it was very pleasant to see them gradually swell or contract accordingly as yethe colour of yethe light was changed. The intervall of yethe Glasses at any of yetheRings when they were made by yethe utmost red light was to their intervall at yethe same Ring when made by yethe utmost Violet, greater then 3 to 2 & lesse then 13 to 8. By the most of my Observations it was as 9 to 14. And this proportion seemed very nearly the same in all obliquities of my eye; unlesse when two Prisms were made use of instead of yetheyObject-glasses. For then at a certain great obliquity the Rings made by yethe severall colours seemed equall, & at a greater obliquity those made by yetheViolet would be greater then yethe same Rings made by yethe Red.Obs: 14. While the Prism was turned about uniformly, The contraction or dilatation of a Ring made by all yethe severall colours of yethe Prism successively reflected frōomyethe Object glasses, was swiftest in yethe Red, slowest in yethe Violet & in intermediate colours it had intermediate degrees of celerity. comparing the extent wchwhich each colour obteined by this contraction or dilatation I found that the blew was sensibly more extended then the vViolet, the yellow then yethe Blew, & the Red then the yellow. And to make a juster estimation of their proportions I observed that the extent of yethe Red was almost double to that of yethe Violet, & that yethe light was of a middle colour between yellow & Green at that intervall of yethe Glasses, wchwhich was an Arithmeticall mean between yethetwo extreames: contrary to what happens in the colours made by the refraction of a Prism, where the rRed is most contracted, the Violet most expanded, & in yethe midst of them is yethe confine of Green & Blew.Obs: 15. These Rings were not of various colours like those in yethe open Air but appeared all over of that Prismati colours only wthwithwchwhich they were illuminated. And by projecting the Prismati colours im̄mmediately upon yethe glasses I found that the light fell on yethe dark Spaces wchwhich were between the coloured Rings as transmitted through the Glasses wthwithout any variation of colou on a white paper placed behind, it would paint Rings of yethe colour wthwith those which were reflected & of the bignesse. And from hence And frōom hence yethe origine of those Rings is manifest, namely that yetheAereall intervall of the glasses, according to its various thicknesse, is disposed in some places, to reflect, & in others to transmitt yethelight of any colour, & in yethe same place to reflect one colour where it transmitts another.Obs: 16. The squares of the diameters of these Rings made by any Prismati colour were in Arithmeticall progression as in yethe 5t Observation. And the diameter of yethe 6t circle when made by yethe yellow & viewed almost perpendicularly was about parts of an inch, agreeable to yethe 6t Observation.The precedent Observations were made wthwith a rarer thin Medium terminated by a denser, such as was Air or water compressed betwixt two Glasses. In those that follow are set down the appearances of a denser medium thin'd wthwithin a rarer, such as are plates of Moscovy-glasse, Bubbles of water, & some othersotherthin substances terminated on all sides wthwith Air.Obs: 17. If a Bubble be blown wthwith water, first made tenacious by dissolving a little soap in it tis a com̄mmon observation that after a while it will appear tinged wthwith a great variety of colours. To defend these Bubbles frōom being agitated by the externall Air (wherby their colours are irregularly moved one among another, so ytthat no acurate observation can be made of them,) as soon as I had blown any of them I covered it wthwith a clear glasse, & by that means its colours emerged in a very regular order like soe many concentrick Rings incompassing the top of the Bubbles And as yethe Bubble grew thinner by yethe 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 mean time while after all the colours were emerged at yethe top, there grew in yethe center of yethe Rings a small round black Spot, like that in yethe first observation, wchwhich continually dilated it selfe till it became sometimes more then or of an inch in breadth before the Bubble broak. At first I thought there had been noe light reflected frōomyethe water in that place, but observing it more curiously I saw wthwithin it severall smaller round spots wchwhich appeared much blacker & darker then yetherest, whereby I know that there was some reflection at the other places wchwhich were not soe dark as those Spots. And by further tryall I found that I could see the images of of a candle or yethe Sun) very faintly reflected great black spot but also frōom the little wthwithin it.Besides the aforesaid coloured Rings pear small Spots of colours ascending yethe sides of the Bubble 5675539 subsiding of the water. And sometimes small black Spots generated at yethe sides would ascend up to yethe larger black Spot at yethe top of the Bubble & unite wthwith it.Obs: 18. Because the colours of these Bubbles were more extended & lively then those of Air thin'd between two Glasses, & so more easie to be distinguished. I shall now give you a further description of their order as they were observed in viewing them by reflexion of the skies when of a white colour whilst a black substance was placed behind them 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, specially the first where the Red seemed in a manner to be white; Amongst those there was scarcely any other colour sensible, inonely yethe Blewes (& principally the second,) inclined a little to Green.The fourth Red was also dilute & dirty but not so much as yethe former 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 bris and good willow Green, & afterwards changed to a blewish colour; but there succeeded neither Blew nor Violet.The fift Red at first was very much inclined to purple & afterwards became more bright & brisk but yet not very pure. This was succeeded wthwith 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 that followed an excellent Blew, of a bright sky colour, & then a purplewchwhich was lesse in quantity then the Blew & much inclined to Red.SThe sixt Red was at first of a very fair & lively scarlet, & soon after of a brighter colour being very pure & brisk & the best of all the Reds. Then after a lively Orange followed an intense bright & copious yellow, which was also yethe best of all yethe yellows; & this changed first to a greenish yellow, &then to a greenish Blew; But yethe green between the yellow & blew was very little & dilute, seeming rather a greenish white then a green. The blew wchwhich succeeded became very & of a fair bright sky colour, but yet something inferior to yethe former Blew, And the Violet was intense & wchwhich little or no rednesse in it, And lesse in quantity the Blew.In the lastIn the last Red appeared a tincture of Scarlet next the Violet, wchwhich soon changed to a brighter colour inclining to an Orange; And yethe yellow wchwhich followed was at first pretty good & lively, but afterwards it grew more & more dilute untill by degrees it ended in ꝑperfect whitenesse. And this whitenesse if the water was very tenacious & well tempered; would slowly Spread & dilate it selfe over yethe greatest part of the Bubble, continually growing paler at yethe top, where at length it would crack in many places, & those cracks as they dilated would appear 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 nett-work & soon after vanished & left all yethe upper part of yethe Bubble of yethe said dark blew colour. And this colour after the aforesaid manner dilated it selfe downwards untill sometimes it hath overspread the whole Bubble. In the mean while at yethe top wchwhich was of a darker Blew then the bottom & appeared also of many round blew Spots something darker then the rest, there would emerge one or more very darkblackSpots & wthwithin those other Spots of an intenser blacknesse, wchwhich I mentioned in yethe former observation And these continually dilated them selves untill yethe Bubble broak.If the water was not very tenacious the Black spots would break forth in the white wthwithout any sensible intervention of the blew: And sometimes they would break forth wthwithin yethe precedent yellow, or Red, or perhaps wthwithin 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 yethe 4th Observation although set down in a contrary order by reason that they begin to appear when the Bubble is thickest, & are most conveniently reckoned from the lowest & thickest part of the Bubble upwards.Obs: 19. Viewing at severall oblique positions of my eye the Rings of colours emerging on yethe top of the Bubble; I found that they were sensibly dilated by increasing the obliquity but yet not so much by far as those made by thin'd Air in yethe 7th Observation. For there they distended viewed most oblily to arrive at a part 12 times thicker then that where they perpendicularly, whereas in this case the at wchwhich they arriv'd when viewed most oblily which exhibited ymthem by perpendicular Rayes 568554 By the best of my Observations it was between 15 and 15to 10; an increase about 24 times lesse then in the other case.Sometimes the Bubble would become of an uniform thicknesse all over except at yethe top of it near yethe black Spot; as I knew because it would exhibit yethe same appearance of colours in all positions of yethe eye: And then the colours wchwhichwere seen at its apparent circumference by yethe obliquest rays would be different frōom those ytthat were seen in other places by rayes lesse oblique to it. And divers spectators might see yethe same part of it of differing colours by viewing it at very differing obliquities. Now observing how much yethecolours at yethe same place of yethe Bubble or at divers places of equall thicknesse were varied by yethe severall obliquities of yethe rayes; by assistance of yethe 4th, 14th, 16th & 18th observations as they are hereafter explained, I collected the thicknesse of yethe water requisite to exhibit any one oryethe same colour as severall obliquities to be very nearly inyethe proportion expressed in this Table.Incidence on the water.Refraction into the water.Thicknesse of the water.degr. mindegr min.00. 00.00. 001015. 00.11. 11.10.30. 00.22. 1.10.45. 00.32. 2.11.60. 00.40. 30.13.75. 00.46. 25.14.90. 00.48. 35.15.In the two first columns are experessed yethe obliquities of yethe rayes to yethe 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 three to four, though probably yethe dissolution of soape in yethe water may a little alter its refractive Virtue. In the third column the thicknesses of yethe Bubble at wchwhich any one colours is exhibited in those severall obliquities, is exprest in parts of which ten constitute that thicknesse when the rays are perpendicular.I have sometimes observed of the colours wchwhich arise on polished steel by heating it, or on Bell-metall & some other me substances when melted & poured on the ground where cool in yethe open Air, that they have like those of water water-bubbles, been a little changed by viewing them at divers obliquities, & particularly that a deep Blew or Violet when viewed very oblily hath been changed to a deep Red. But yethe Changes of these colours are not so sensible as of those made by water For the coria or vitrified part of yethe metall wchwhich most metalls when heated or melted continually protrude to their surface where by covering them in form of a thin glassy skin it causes these colours, is much denser then water; and I find that yethechange made by the obliquation of yethe 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 exhibited by reflexion. Thus when the Bubble being looked on by yethe light of yethe clouds reflected frōom it, seemed Red at its apparent circumference, if the clouds at yethe same time or very suddenly were viewed through it, the colour at its circumference would be blew. And on the contrary when by reflected light it appeared blew, it would appear Red by transmitted light.Obs: 21. By wetting Plates of Muscovy-glasse whose thinnesse made the like colours appear, 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 that yethe thicknesse of a Plate requisite to produce any colour depends only on the density of yethe Plate & not of yethe ambient medium. And hence by yethe 10th & 16th observations may be known the thicknesse of Bubbles of water or Plates of Moscovy-glasse, or of any 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 brisk & 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 encompassed wthwith Air did exhibit colours much more vivid then those of Air made thin between two glasses.Obs: 23. Comparing the quantity of light reflected frōom the severall Rings, I found it was most copious from the first or inmost, & in yethe exterior Rings became gradually lesse & lesse. Also the whitenesse of the first Ring wasstronger then that reflected frōom those parts of the were wthwithout yethe Rings; as I could manifestat distance yethe Rings made by the two colourscomparing two Bubbles of water blown the first of wchwhich the whitenesse appeared lours, & the whitenesse wchwhich preceded 5551Obs: 24. When the two object glasses were layd upon one another so as to make the Ringes of colours appear, though wthwith my naked eye I could not discern above 8 or 9 of those Ringes yet by viewing them through a Prism I have seen a far greater multitude, in so much that I could 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 the discovery of far greater numbers. For they seem to be really unlimited, throough visible onely so far as they can be separated by yethe refraction, as I shall hereafter explain.But it was but one side of the Rings, namely that towards wchwhichyethe refraction was made , wchwhich by that refraction was rendered distinct, & yethe other side became more confused then to yethe naked eye, In so much ytthat there I could not discerne above one or two, & sometimes none of those Rings, of wchwhichI could discern 8 or 9 wthwith my naked eye. And their segments or arcs wchwhich on the other side appeared so numerous, for yethemost part exceeded not yethe third part of a circle. If the refraction was very great or yethe Prisms very distant from yetheObject-glasses, the middle part of those arcs became also confused so as to disappear & constitute an even whitenenesse, whilst on either side their ends, as also the whole arcs furthest from yethe center became distincter then before, appearing in the form you see them here designed.The arcs where they seemed distinctest were only white & black successively wthwithout any other colours intermixed. But in other places there appeared colours whose order was inverted by yethe refraction in such manner that if I first held the Prism very near the Object glasses & then gradually removed it farther of towards my eye, the colours of yethe 2d, 3d, 4th & following Rings shrunk towards yethewhite that emerged between ymthem untill they wholy vanished into it at yethe middle of yethe Arcs, & afterwards emerged again in a contrary order. But at yethe ends of the arcs they retained their order unchanged.I have sometimes so layd one Object-glasse upon the 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 which were not soe thin as to exhibit any colours to yethe naked eye, have through the Prism exhibited a great variety of them ranged irregularly up & down in the form of wWaves. And so bubbles of water before they began to exhibit their colours to the naked eye of a by stander have appeared through a Prism, girded about wthwith many parallell & horizontal Rings: to produce wchwhich effect, it was necessary to hold yethe Prism parallel or very nearly parallel to yethe Horizon, & to dispose it so that the rayes might be refracted upwards.Having given my Observations of these colours before I make use of them to unfold yethe causes of yethe colours of naturall bodies, it is convenient that by yethe simplest of them, I first explain the more compounded, such as are yethe 2d, 3d, 4th, 59th, 12th, 18th, 20th, & 24th.And first to show how yethe colours in yethe 4th & 18th Observations are produced, let there be taken in any right line yethe lengths YZ, YA, & YH in proportion as 4, 9, & 14 & between ZA & ZH eleven mean proportionalls, of wchwhich let ZB be yethe second, ZC the third, ZD the fift, ZE yethe seventh, ZF yethe 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 agtagainstymthem, 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 the point of division denote. And through those divisions from Y draw lines 1J, 2K, 3L; 5M, 6N, 7O; &cNow if A2 be supposed to represent the thicknesse of any thin transparent body at which the utmost violet is most copiously reflected in yethe first Ring or series of colours, then by the 13th Observasent its thicknesse at wchwhich the utmost Red 556 in the same series. Also by the 5t & 16th Observations HN will denote the thicknesses at wchwhich those extream colours are most copiously reflected reflected in yethe second series, & so on. And the thicknesse at which any of the intermediate colours are reflected most copiously will according to yethe 14th Observation, be defined by the intermediate parts of the lines 2K, 6N, &c agtagainstwhich the names of those colours are written below.But faurther to define the latitude of these colours in each Ring or series let A1 design the least thicknesse & A3 the greatest thicknesse at wchwhich the extream Violet in the first series is reflected, & let HJ & HL design the like limits for the extream Red, & the intermediate colours be limited by the intermediate parts of yethe lines 1J & 3L; against wchwhich the names of those colours are written. And in the second series let those limits of the lines 5M & 7O; & so on: But yet wthwith this caution that the reflexions be supposed strongest at the intermediate spaces 2K, 6N, 10R &c & to decrease gradually towards these limits 1J,3L; 5M, 7O; &c on either side, where you must not conceive ymthem to be precisely limited but to decay indefinitely. And wheras I have designassigned the same latitude to every series, I did it because although the colours in yethe first series seem to be a little broader then yethe rest by reason of a stronger reflexion there, yet that inequality is so insensible as scarcely to dbe determined by observation.Now according to this Description conceiving that the rayes in ytthat severall colours in here are by turnes reflected at the spaces 1J, L3, 5MO7, 9PR11, &c; & transmitted at the Spaces AHJ1, 3LM5, 7OP9 &c; it is easie to know what colour must in the open Air be exhibited at any thicknesse of a transparent thin bBody. For if a Ruler be applyed parallel to AH, at that distance frōom it by which the distance thicknesse of yethebBody is represented, the alternate spaces 1JL3, 5MO7 &c. which it crosseth will denote yethe reflected originall colours of which the colour exhibited in the open Air is compounded: Thus if the constitution of yethe Green in the third series of colours be desired; apply the Ruler as you see at π, ρσ, φ, And by its passing through some off the Blew at π, & yellow at σ, as well as through yethegreen ρ, you may conclude that green exhibited at that thicknesse of yethe body is principally constituted of originall green, but not wthwithout a mixture of some blew & yellow.By this meanes you may know how yethe colours frōom the water of the Rings outwards ought to succeed in order as they were described in the fourth & 18th observations. For if For if you move the Ruler gradually frōom AH through all distances, having past over the first space wchwhich denotes litthle or no reflexion to be made by thinnest substances, it will first arrive at yethe Violet & then very quickly at the Blew & green wchwhichtogether wthwith that Violet compound Blew, & then at yethe yellow & Red by whose further addition that Blew is converted into whitenesse, which whitenesse continues during the transit frōom J to 3, & after that by yethe 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 yethe second series, wchwhich succeed in order between 5 & O, & are more lively then before because more expanded & severed And for the same reason instead of yethe former white thereintercedes between yethe blew & yellow a mixture of Orange, Yellow, Green, Blew & Indico, all wchwhich together ought to exhibit a dilute & imperfect Green. So the colours of the third series all succeed in order; first the Violet wchwhich a little interferes wthwith the Red of yethe second order & is thereby inclined to a reddish Purple; then theey blew & green wtchwhich are lesse mixed wthwith other colours & consequently more lively then before; especially the Green: Then followes yethe yellow some of wchwhich towards yethe Green is distinct & good, but that part of it towards yethe succeeding Red, as also that Red is mixed wthwith the Violet & Blew of the fourth series whereby various degrees of Red very much inclining to Purple are compounded. The Violet & Blew wchwhich should succeed this Red being mixed wthwith & hidden in it, there succeeds a Green, aAnd this at first is much inclined to blew, but soon becomes a good Green, the only unmixt & lively colour in this fourth series. For as it verges towards yethe yellow its begins to interfere wthwithyethe colours of yethe fift series, by whose mixture yethe succeeding yellow & Red are very much diluted & made dirty, especially the yellow wchwhich being the weaker colour is scarce able to show it selfe. After this the severall series interfere more & more & their colours become more & more intermixed till after three or four more revolutions (in wchwhichyethe Red & blew predominate by turnes) all sorts of colours are in all places pretty equally blended & compound one eaven whitenesse.And since by yethe 15th Observation the rayes indued with one colour are transmitted where those of another clolour reason of yethe colours made by yethe transmitted Observations is also frōom hence evident.If not only yethe order & Species of thesethe precise thicknesse of yethe Plate of thinare exhibited, be defined in parts of an performed by performed by assistance of yethe 6t or 6th5575532 For according to those observations the thicknesse of yethe thin Air wchwhich between two glasses exhibited the Orange as bright red of yethe 6th order was parts of an inch. Now suppose this thicknesse to be represented by Gτ & yethe eleventh part of it Gλ as will be about of an inch. And so Gμ, Gν, Gζ, Gο, will be , , , & . And this being known it is easie to determine what thicknesse of Air is represented by Gφ or any other distance of yethe Ruler frōom AH.But further since by yethe 10th observation the thicknesse of Air was to the thicknesse of water wchwhich between the same glasses exhibited the same colour, as 4 to 3; & by yethe 21th observation yethe colours of thin Bodies are not varied by varying yetheambient Medium; the thicknesse of a Bubble of water exhibiting any colour will be of the thicknesse of Air producing 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 yetheseries sines 31 to 20 may be of the thicknesse of Air producing the same colours. And the like of other Mediums. On these grounds I have composed the following Table wherin the thicknesse of Air, water, & glasse at wchwhich each colour is most intense & specific, is expressed in parts of an inch divided into ten hundred thousand equall parts.The thicknesse ofAirwaterGlasse_______________________________________________________________Black211, or lesseThe coloursBlew221.of yethefirstwhite543.orderYellow865.Orange965.Red.1076_______________________________________________________________Violet1297.Indico1398.Of theBlew14119.secondGreen161210.orderYellow171311.Orange191412.Bright Red201513.Scarlet211613._______________________________________________________________Purple2317.Indico241815.Of theBlew251916.third orderGreen272017.Yellow292219.Red312320.Blewish Red332521.______________________________________________________________________________________________________________________________Blewish Green362723.4thoOrderGreen372824.Yellowish green392925.Red.443328._______________________________________________________________5t OrderGreenish blew3832.Red574337._______________________________________________________________6t OrderGreenish blew644841.Red5345._______________________________________________________________7th OrderGreenish blew775850.Red or white846354._______________________________________________________________Now if this Table be compared wthwith the third Scheme 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 judge of its intensenesse or imperfection; wchwhich may suffice in explication of yethe fourth & 18thObservationsunlesse it be further desired to delineate the manner how the colours appear when the two Object-glasses are layd upon one another. To doe wchwhich let there be described a large Arc of a Circle, & a streight lineswchwhich may touch that Arc, & parallel to that Tangent severall occult lines at such distances frōom it as the numbers set agtagainstyethe severall colours in yethe Table denote. For yethe Arc & its Tangent will represent the suꝑperficies of yethe glasses terminating yethe interjacent Air, & the places where the occult lines cut the Arc will show at what distances frōom the center or point of contact each colour is reflected.There are also other uses of this Table: For biy its assistance yethe thicknesse of yethe Bubble in yethe 19th Observation was determined by yethe colours wchwhich it exhibited. And so yethebignesse 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 Moscovy 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 yethe plate exhibiting it according to the Table, is 5 to wchwhich add 9the thicknesse exhibiting Blew of yethe second order, & yethe summe nearly approaching 14 the thicknesse the third order.To explain in the next place the 2d & 3d Observations, that is, how ysthese colours Prisms about their com̄mmon Axis yethe558 that expressed in those Observations) may be converted into white & black Rings, & afterwards into colours again in an inverted order: it must be remembred that those colours are dilated by yethe obliquation of yethe rayes to yethe Air wchwhich intercedes yethe Glasses, & that according to yethe Table in yethe 7th Observation their dilatation or recession frōom their com̄mmon center is most manifest & speedy when they are obliquest. Now yethe rays of yellow being more refracted by yethe first suꝑperficies ofat the said Air then those of Red, are therby made more obli to the second suꝑperficies at wchwhich they are reflected to produce the coloured Rings, & consequently yethe yellow in each Ring will be more dilated then the Red; & the excesse of its dilatation will be so much the greater by how much yethe greater is yethe obliquity of yethe rayes, untill at last it become of equall extent wthwithyethe Red of yethe same Ring. And for yethe same reason the Green, blew & Violet will be also so much dilated by yethe still greater obliquity of their rayes as to become all very nearly of equall extent wthwithyethe Red, that is, equally distant frōomyethe center of the Rings. And then all yethe colours of yethe same series 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 wthwithone another as before. 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 then the other colours, & so very apt to appear at yethe exterior verges of yethe white.Afterwards by a greater obliquity of yethe rays, the Violet & Blew become sensibly more dilated then yethe Red & yYellow, & so being further removed frōomyethe center of yethe Rings, the colours must emerge out of yethe white in an order contrary to ytthat which they had before, yethe Violet & Blew at yethe exterior limbs, & yethe Red & yellow at yethe interior. And yethe Violet by reason of yethe greatest obliquity of its rayes being in proportion most of all expanded will soonest appear at yethe exterior limb of each White Ring & become more conspicuous then the rest. And yethe severall series of colours by their unfolding & spreading will begin again to interfere, & thereby render the Rings lesse distinct & not visible to soe great numbers.If instead of yethe Prisms the objectglasses be made use of the Rings which they exhibit become not white & distinct by yethe obliquity of yethe eye, by reason ytthatyethe rayes in theirpassagepPassage through that Air wchwhich intercedesdyethe glasses are very nearly parallel to themselves when first incident on the glasses, & consequently those indued wthwith 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 wchwhichwhen viewed at distance appear distinct, should not only becom confused by viewing them near at hand, but also yeild a Violet colour at both yethe edges of every white Ring. And yethe reason is that yethe rayes wchwhich enter yethe eye at severall parts of yethe Pupill have severall obliquities to yethe glasses, & those wchwhich are most obli, if considered apart, would represent the Rings bigger then those wchwhich are yethe least oblique. Whenceyethe breadth of the Perimeter of every white Ring is expanded outwards by yethe obliquest rays & inwards by yethe least oblique. And this expansion is so much the greater by how much the greater is yethe difference of yethe obliquity that is, by how much the Pupill is wider, or yethe eye nearer to yethe Glasses. And the breadth of the Violet must be most expanded, because the rayes apt to excite a sensation of that colour are most oblique to yethe second or further suꝑperficies of the thin'd Air at wchwhich they are reflected, & have also yethe greatest variation of obliquity, wchwhich makes that colour soonest emerge out of yethe edges of yethe white. And as the breadth of every Ring is thus augmented, the dark intervalls must be diminished, untill the neighbouring Rings become continuous, & are blended, the exterior first, & then those nearer the center, so that they can no longer be distinguished apart but seem to constitute an eaven & uniform whitenesse.Amongst all yethe Observations theire is none accompanied wthwithso odd circūumstances as yethe 24th. Of those yethe Principall are that in thin Plates wchwhich to the naked eye seem of an eaven & uniform transparent whitenesse the refraction of a Prism should make the Rings of colours appear; wheras it usually makes Objects appear coloured onely where they are terminated wthwith shadows, 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 wthwith the although by reason of the great breadth of rency they so much interfere & are blended they seem to constitute an eaven whitenesse rayes passe through the Prism to the eye severall colours in every Ring are 5593 then others according to their degree of refrangibility wchwhich meanes the colours one one side of the Ring become more unfolded & dilated & on the other side more complicated & contracted. And where by a due refraction they are so much contracted that the severall Rings become narrower then to interfere wthwith one another, they must appear distinct & also white if the constituent colors be so much contracted as to be wholy coincident. But on the other side where every Ring is made broader by the further unfolding of its colours, it must interfere more wthwith other Rings then before & so become lesse distinctTo explain this a little further, suppose the concentrick circles AB and CD 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 BC will by a greater refraction be further translated from its place then the Red AD & so approach nearer to it on that side towards wchwhich the refractions are made. For instance, if the Red be translated to ad the Violet may be translated to bc so as to approach nearer to it at c then before, & if the Red be further translated to ad the Violet may be still so much further translated to bc as to convene wthwith it at c, & if the Red be yet further translated to αδ, the Violet may be still so much further translated to βγ as to pass beyond it at γ & convene wthwith it at e & f. And this being understood not only of the Red & Violet but of all the other intermediate colours, & also of every revolution of those colours, you will easily perceive how these of the same revolution or order by their nearnesse atcd & δγ & their coincidence at cd, e, & f ought to constitute pretty distinct arcs of circles, especially at cd or at e & f & that they will appear severall at atcd, at cd exhibit whitenesse by their coincidence, & again appear severall at δγ, but yet in a contrary order to that which they had before & still retain beyond e and f. But on the other side at ab, ab, αβ these colours must become much more confused by by being dilated & spread soe as to interfere wthwith those of other orders. And the same confusion will happen at δγ between e & f if the refraction be very great, or the Prism very distant from the Object-Glasses. And these little Arcs must be distinctest & whitest at their middle, & at their ends, where in which case no parts of the Ring will be seen save only two little Arcs 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 the those at yethe other end, by reason that they crosse in the intermediate white. Namely their ends wchwhich verge towards δγ will be red & yellow on that side next yethe center & blew & Violet on the other side. But their other ends which verge from δγwill on yethe contrary be blew & Violet on that side towards the center & on the other side red & yellow.For confirmation of all this I need alledge no more ynthanthat it is mathematically demonstrable from my former Principles. But yet I shall add that they which please to take the pains, may by the Testimony of their senses be assured that these explications are not Hypotheticall but infallibly true & genuine. For in a dark Room by viewing these Rings through a Prism by reflextion of the severall Prismati colours wchwhich an Assistant causes to move to & fro upon a wall or paper from whence they are reflected, whilst the spectators eye, the Prism & Object-glasses (as in the 13 observation) are placed steddy: the position of yethe circles made successively by the severall colours will be found such in respect of one another as I have described at abcd, or abcd or αβγδ. And by the same Method, the truth of yetheexplicationsofthe other Observations, is to be examined. By what hath been said, the the like Bubbles & thin Plates of Glasse may be small fragments of those Plates there is servable, that if they lying flatt 556560 be turned about their centers whilst they are viewed through a Prism: some of them exhibit waves in one or two positions only, but the most of them do in all positions exhibit those waves & that for yethe most part appearing, almost all over the Glasse. The reason is that yethe suꝑperficies of such Plates are not eaven but have many cavities & swellings wchwhich how shallow soever do a little vary the thicknesse of the Plate: And by the severall sides osf those cavities there must be produced waves in severall postures of yethe Prism. Now though it be but some very small and narrow parts of yethe Glasse by which these waves for the 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 confusedly reflected wchwhich by refraction of the Prism are unfolded & dispersed to severall places so as to constitute so many severall waves as there were divers orders of yethe colours promiscuously reflected from that part of the Glasse.These are the Principall Phænomena of thin Plates or Bubbles whose explications depend on the Properties of light that I have heretofore delivered: And these you see doe neccessarily follow from them & agree wthwith them even to their very least circumstances; & not onely so, but doe very much tend to their proof. Thus by the 24th Observation it appears that the rayes of severall colours made as well by thin Plates or Bubbles as by yetherefractions of a Prism have severall degrees of refrangibility, whereby those of each order which at their reflexion from the Plate or Bubble are intermixed wthwiththose of other orders, are seperated from them by refraction, & associated together so as to become visible by themselves like Arcs of cCircles. For if the rayes were all alike refrangible, 'tis impossible that the whitenesse which to the naked sense appears uniform should by refraction have its parts transposed & ranged into those black & white Arcs.It appears also that the unequall refractions difformrayes proceed not from any contingent irregularities. Such as are V, an uneaven polish, or for fortuitous position of the pores of the Glasse; unequall motions in yethe Air or Æther.spreading spreading, breaking or dividing the same ray into many diverging parts; or yethe 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 24thoObservation. It is necessary therefore that every ray have its proper & constant degree of refrangibility connate wthwith it, according to wchwhich its refraction is ever justly & regularly performed; & that severall rayes have severall of those degrees.And what is said of their rRefrangibility may be also understood of their Reflexibility, that is of their dispositions to be reflected some at a greater & others at a lesse thicknesse of thin Plates or Bubbles, namely ytthatthose dispositions are also connate wthwith the rayes & im̄mmutable,as may appear by the 13th 14th & 15th Observations compared wthwith the 4th & 18th.By the precedent Observations it appears also that whitenesse is a dissimilar mixture of all colours, & that light is a mixture of rayes endewed wthwith all those colours. For considering the multitude of yethe Rings of colours in yethe 3d, 12t, & 24thObservations, it is manifest that although in the 4th & 18th Observations there appear noe more than 8 or 9 of those Rings, yet there are really a far greater number wchwhich soe much interfere & mingle wthwithonone another as after those 8 or 9 revolutions to dilute one another wholy & constitute an eaven & sensibly uniforme whitenesse. And consequently that whitenesse must be allowed a mixture of all colours, & the light which conveys it to the eye must be a mixture of rayes endewed wthwith all those colours.This I beleive hath seemed the most paradoxical of all my assertions, & met with the most universall & obstinate prejudice; But to me it appears as infallibly true & certain, as it can seem extravagent to others. For hitherto I never tried any way toto mix all colours by which I could not in some degree or other produce whitenesse & yet I have made as manytrialls as I could excogitate ways of mixing colours which I may take occasion to discourse But further by the 24th Observation it is a constant relation between colours & refrangible rayes being Violet, the least of intermediate colours having proportionally 5575614refrangibility. And by the 13th, 14th, & 15th Observations compared wthwith the 4th or 18th, there appears to be yethe same constant relation between colour & reflextibility, the Violet being on equall termes reflected at least thicknesse of any thin Plate or Bubble, yetherRed at greatest thicknesse, & the intermediate colours at intermediate thicknesses. Whence ait followes that yethecolorifi dispositions of rayes are also connate wthwith them & im̄mmutable, & by consequence that all yethe productions & appearances of colours in the world are derived not from any Physicall change caused in light by refraction or reflexion but only from yethe various mixtures or seperations of rayes by virtue of their different Refrangibility or Reflextibility. And in this respect it is that yethe science of colours becomes a speculation more proper for Mathematicians then naturalists, & deserves rather to be esteemed Mathematicall then Physicall, as I told you in my former letter, & may hereafter explain more fully.I am now come to yethe last part of this design wchwhichis to consider how yethe Phænomena of thin transparent Plates stand related to those of all other naturall bodies. Of these bodies I have already told you that they appear of divers colours accordingly as they are disposed to reflect most copious by the rayes endewed wthwith those colours. But their constitutions wherby they reflect some more rayes more copiously then others, remaines to be enquired after. And this I shall endeavour in the following Propositions.Prop.1. Those superficies reflect yethe greatest quantity of light wchwhich have yethe greatest refracting power, that is, which intercede mediums that differ most in their refracting densities. And in the confines of equally dense mediums there is noe reflexton.The analogy between reflexion & refraction will appear by considering that when light passeth oblily out of one Medium into another which refracts from the perpendicular, the greater is the difference of their density yethe lesse obliquity is requisite to cause a totall reflexion; because as the sines are which measure the refraction, so is yethe sine of incidence at which the totall reflexion begins, to yethe radius ofyethe circle, & consequently that incidence is least where there where there is the greatest difference of the sines. Thus in yethepassing 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 sbout 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 yethe angle of incidence is 40degr. 10min. And so in passing out of Chriystall or more strongly refracting Mediums into Air, there is still a lesse obliquity requisite to cause a totall reflexion. Superficies therfore wchwhich refract most do soonest reflect all the light which is incident on them, & so must be allowed most strongly reflective.But the truth of this Proposition will further appear by observing that in yethe superficies interceding any two of those Mediums, Air of water or other liquors, com̄mmon Glasse, Chrystall, & Metalline Glasses, the reflexion is stronger or weaker accordingly as yethe superficies hath a greater or lesse refracting power. Thus when other Mediums are contiguous to Air; the reflexion is stronger in the suꝑperficies of Glasse then of water; still stronger in yethe superficies of Chrystall, & strongest in yethe superficies of metalline Glasse. So in yethe confine of water & com̄mmon Glasse the reflexion is very weak, but yet stronger then in yethe confine of water & oyle or almost any other two liquors, & still stronger in the confine of water & Chrystall or Metalline Glasse: accordingly as those Mediums differ more or lesse in density. So in yethe confine of com̄mmon Glasse & Chrystall there is a weak reflexion & a stronger reflexion in the confine of com̄mmon & mealline Glasse. But in yethe confine of two Glasses of equall density, there is not any sensible reflexion as was shown in yethe 1st Observation. And yethe same may be understood of yethe superficies of two Chrystalls or liquors or any other substances in which no reflraction is caused. Whence it comes to passe that uniform Mediums have no sensible reflexion but in their externall superficies where they are adjacent to other Mediums of a different densiity.Prop: 2. The least parts of naturall bodies are in somemeasure transparent; And the opacity of those the multitude of reflexions caused caused in their That this is so will easily be granted been conversant wthwith Microscopes. And562558 And it may be also tried by applying any substance to a hole through which some light is immitted into a dark room For how opake sooner that substance may bseem in yethe open Air, it will by that means appear very manifestly transparent if it be of a sufficient thinnesse. Onely metalline bodies must be exempted wchwhich by reason of their excessive density seem to reflect almost all the light incident on their first superficies.Prop: 3: Between the parts of opake or coloured bodies are many interstices replenished wthwith mediums of other densities: as water between the tinging corpuscles wherwthwith any liquor is impregnated; Air between the aqueous globules that constitute Clouds or mists; & for the most part spaces void of both Air & water; but yet perhaps replenished wthwith some subtiler Medium between the parts of hard bodies.The truth of this is evinced by the two precedent Propositions, For by yethe second Proposition there are many reflexions from yethe internall parts of bodies, wchwhich by the 1st Proposition would not happen if the parts of those bodies were continued wthwithout any such interstices between them, because reflexions are caused only in superficies wchwhich intercede Mediums of a differing density.But further that this discontinuity of parts is the Principall cause of the opacity of bodies will appear by considering that opake substances become transparent by filling their pores wthwith any substance of equall or almost equall density wthwith their parts. Thus paper dipped in water or oyle, the oculus mundi stone steeped in water, linnen cloth oyl'd or varnished, & many other substances soaked in such liquors as will intimately pervade their little pores become by that means more transparent then otherwise. So on the contrary the most transparent substances may by seperating their parts become rendered sufficiently opake, as Glasse by being reduced to powder or otherwise flawed, water by being formed into many small bubbles either alone in the form of froth, or by shaking it together wthwith oyle of Turpentine or some other convenient liquor wthwithwchwhich it will not incorporate, & Horn by being scraped.ToTo the increase of of the opacity of these bodies it conduces something that by yethe 23d Observation the reflexions of very thin transparent substances are considerably stronger then those made by yethe same substances are considerablyof a greater thicknesse. And to the reflexion of solid bodies it may further add, that the interstices of their parts are void of Air. For that for yethe most part they are so is reasonable to beleive considering yetheineptitude wchwhich Air hath to pervade small cavities as appears by yethe ascention 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 yethe difficulty wherewthwith Air pervades yethe pores of a Bladder through wchwhich water finds ready passage. And according to yethe 11th Observation the cavities thisus void of Air will cause the same kind of effects as to reflexion wchwhich those doe ytthat are replenished wthwith 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 & precedent Proposition, & declined many other very excellent things concerning yethe 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 thickness to render ymthemopake & coloured. For yethe opakest bodies if their parts be subtily divided (as Metalls by being dissolved in acid Menstruums, &c) become perfectly transparent. And you may also remember that in yethe 8th Observation there was no reflexion at yethe superficies of yethe Object-glasses where they were very near one another though they did not absolutely touch. And in yethe 17th Observation the water-bubble where it became thin insensible so as to cause the apparitions spots.5635On these grounds I conceive it is that water, salt, glasse, stones & such like substances are transparent For upon divers considerations they seem to be as porous as other bodies, but yet their pores & parts too small to cause any opacity.Prop: 5. The transparent parts of bodies according to their severall sizes must reflect rayes of one colour, & transmitt those of another, on the same grounds that thinne plates or Bubbles doe reflect or transmitt those rayes. And this I take to be yethe ground of all their colours.For If if a thin'd or plated body wchwhich being of an eaven thicknesse appeares all over of one uniform colour should be broken into fragments of yethe same thicknesse wthwith the plate, I see noe reason why a heap of those fragments should not constitute a powder of yethe same colour wchwhichyethe Plates exhibited before it was broken. And the parts of all naturall bodies being like soe many fragments of a Plate must on yethe same grounds exhibit the same colours.Now that they doe soe will further appear by yethe affinity of their properties. As that yethe infusion of Nephritic wood & many other substances reflect one colour & transmitt another like thin bodies in yethe 9th & 20th Observations. That yethe colours of silks, cloths, & other substances wchwhich water or oyle can intimately penetrate become more faint & obscure by being im̄mmmerged in those liquors, & recover their vigor again by being dryed much after the manner declared of thin bodies in yethe 10th & 21 Observations. And that some of those coloured powders wchwhichPainters 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 yethe same manner that yethe colour of a Plate is changed by varying its thicknesse. For which reason also it is that many flowers by being bruised become more transparent then before; or at least in in some degree or another change their colours. Nor is it much lesse to my purpose that by mixing divers liquors very odd & remarkable productions & changes of colours may be effected, of wchwhich no cause can be more obvious & naturall than that the saline corpuscles of one liquor do variously act upon or unite wthwith the tinging corpuscles of another, so as to make them swell or shrink (whereby not only 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 wee 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 wee consider the various Phæ nomena of the Atmoshphere, wee may observe that when vapours are first raised, they hinder not yethe transparency of the Air, being divided into parts too small to cause any reflectxion 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 & transmitt others, may constitute clouds of various clolours acording to their sizes. And I see not what can be rationally conceived in so transparent a substance as water for the production of these colours besides the various sizes of these its parcells, wchwhich seem to affect a globular figure most; but yet perhaps not wthwithout some instability in yethe smallest of them by reason that those are most easily agitated by heat or any trembling motions in yethe Air.Prop: 6. Then parts of Bodies on which their colours depend are denser then yethe medium wchwhich pervades the interstices.This will appear by considering that yethe colour body depends not only on yethe rayes wchwhich are pendicularly on its parts, but on those alsodent at all other angles. And that 564559 7th Observation a very little variation of obliquity will change yethe reflected colour where the thinne body or small particle is rarer then yethe ambient Medium, in so much 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 grey then any other colour, or at best it must be 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 by the variation of obliquity, that the rayes which are reflected least oblily, may predominate over the rest so much as to cause a heap of such particles to appear very intensly of their colour.It conduces also something to this Proposition that according to 22th Observation, the colours exhibited by the denser thin body wthwithin yethe rarer are more bris then those exhibited by the rarer wthwithin yethe denser.Prop: 7th. The bignesse of yethe component parts of naturall bodies may be conjectured by their colours.For since yethe parts of those bodies by pProp: 5 doe most probably exhibit the same colours wthwith a plate of equall thicknesse, provided they have yethe same refractive density; & since their parts seem for yethe most part to have much yethe same density wthwith water or glasse, as by many circumstances is obvious to collect: to determine yethe sizes of those parts you need only have recourse to yethe precedent Tables in which yethe thicknesse of water or Glasse exhibiting any colour is expressed. Thus if it be desired to yethe diameter of a corpuscle which being of equall density wthwith glasse, shall reflect Green of yethe third order; the number showes it to be about parts of an inch. The greatest difficulty is here to know of what order the colour the colour of any body is. And for this end wee must have recourse to yethe 4th & 18th Observations from whence may be collected these perticulars.Scarlets & other Reds, Oreanges & yellowes if they be pure & intense are most probably of yethe second order. Those of yethe first & third order also may be pretty good, only yethe Orange & Red of the third order have too great a mixture of Violet & Blew.There may be good Greens of yethe fourth order but the purest are of the third. And of this order yetheGreen of all Vegetables seems to be, partly by reason of yethe intensnesse of their colours, & partly because when they wither some of them turn to a greenish yellow, & others to a more ꝑperfect yellow or Orange or perhaps to Red, passing first through all yethe aforesaid intermediate colours. which changes seem to be effected by the exhaling of the moisture wchwhich may leave the tinging corpuscles more dense, & something augmented by yethe accretion of yethe oyley & earthy part of that moisture. Now yethe Green wthwithout doubt is of yethe same order wthwith those colours into wchwhich it changeth because the changes are graduall & those colours though usually not very pure yet for yethe most part are too pure & lively to be of yethe fourth order.Blewes & Purples may be either of yethe second or third order. But yethe best are of the third. Thus yethe colour of Violets seems to be of that order, because their syrrup by acid liquors turnes red and by urinous & alcalizate turn green. For since it is of yethe nature of acids to dissolve or attenuate, & of Alcalies to precipitate or incrassate, if the purple colour of yethe syrrup was of yethe second order, an acid liquor by attenuating its tinging corpuscles would change it to Red of yethe first order, & an Alcaly by incrassatethem would change it to a green of yethe second red & Green, especially yethe Green, seem be yethe colours produced by these changes5656 But if the said purple be supposed of the third order its change to red of yethe second & green of the third may wthwithout any inconvenience be allowed.If there be found any body of a deeper & lesse reddish purple then that of Violets its colour most probably, is of yethe second order. But yet there being no body com̄mmonly known whose colour is constantly more deep then theirs, I have made use of their naname to denote the deepest & least reddish Purples, such as manifestly transcend their colour in purity.The Blew of yethe first Order though very faint & little, may possibly be yethe colour of some substances. And perticularly yethe Azure colour of yethe 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 theis being the first colour which vapors begin to reflect, it ought to be yethe colour of yethe 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 either that in yethe first order of colours, of wchwhich sort perhaps is yethe colour of white lead; or else it is a mixture of those succeeding the third or fourth order, such as is yethe colour of paper, linnen, & most white substances. If corpuscles of various sizes exhibiting the colours of yethe second & third order be mixed, they should rather constitute an imperfect whitenesse or Grey of wtchwhich I have already spoken. But yet it seems not impossible for them to exhibit an intense whitenesse ifthey be disposed to transmitt all yethe light wchwhich they reflect not, & do not retain & stifle much of it. For thus I told you that froth at a distance hath appeared very white & yet near at hand yethe severall Bubbles of wchwhich it was constituted were seen tinged all over wthwith Rings of colours of yethe four or five first orders.Lastly for yethe production of Black the corpuscles must be lesse then any of those which exhibit colours. For at all greater sizes there is too much light reflected to constitute this colour. But if they be supposed a little lesse then is requisite to reflect the blew of the first order they will according to the 4th , 8th, 17th, & 18th Observations reflect so very little light as to appear intensely black, & yet may perhaps variously reflect refract it to & fro wthwithin themselves so long untill it happen to be stifled & lost, Bby which meanes they will appear black in all positions of the eye wthwithout 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 wchwhichthey are applied;* why Glasse ground very elaborately wthwith sand together wthwith what is worn of from the glasse & Copper become very black; why black substances doe on a Copper plate till it be well polished makes yethe sand together wthwith what is worne off from the Glasse, & copper become very black; why black substances do soonest of all others become hot & burne, wchwhich effect may proceed partly from yethe multitude of refractions in a little room, & partly from yethevery easie com̄mmotion 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 reflextion from black substances, which will usually appear of a blewish white. And the reason is that black bodies on yethe obscure blew of yethe first order, as described in yethe18th Observation, whence the corpuscles of black substances are most apt to reflect that colour.In these descriptions I have been the more popular because I hopeits not impossible but that Microscopes willmaybe improved to the discovery of corpuscles wchwhich their colours depend. For if those be so far improved as wthwith sufficient represent objects five or six hundred 566 then at a foot distance they appear to orour naked eyes I should hope that wee might be able to discover some of the greatest of those corpuscles. And by one that would magnifie three or four thousand times perhaps they might all be discovered but those which produce blacknesse. In the mean while I see nothing material that rationally can be doubted of excepting this Position,. That transparent corpuscles of the same thicknesse & density with a Plate, do exhibit the same colour. And this I would have understood not wthwithout some latitude, as well because those corpuscles may be of irregular figures, & many rayes must be oblily incident, & so have a shorter way through them then yethelength of their diameter, as because the straitnesse of yethe Medium pent in on all sides may a little alter 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 Microspcope they have appeared of yethe same colour at their edges & corners where included the Medium was terminated, wchwhich they appeared of in other places. However it willwould add much to orour satisfaction if those corpuscles can be discovered wthwith Microscopes, wchwhich if wee shall at lengtheverattain to I fear it will be yethe utmost improvement of this sense. For it seems impossible to see yethe more secret & noble works of Nature wthwithin those corpuscles by reason of their transparency.This may suffice concerning the constitution of naturall bodies on which their colours depend. But for further understanding the nature of reflexions I shall add these two following Propositions.Prop: 8. The cause of rReflexion is not yethe impinging of light on yethe solid & impervious parts of bodies, as is com̄mmonly supposed.This will appear by the following consideration First that in yethe passage of light out of glasse into Air theirre is a reflexion as strong or stronger then in its passage out of Air into Glasse & by many degrees stronger then in its passage out of Glasse into water. And it seems not probable that Air should have more reflecting parts then water or Glasse. But if that should possibly be supposed it will availe nothing: for the reflexion is as strong if not .perhaps stronger when the Air is drawn away from the Glasse, (suppose in the Air-Pump invented by Mr Boyle) as when it is adjacent to it. Secondly, if light in its passage out of Glasse into Air be incident more oblily 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 meetwthwith pores enough in the Air to transmitt the greater ptpartof it, & at another degree of obliquity meet wthwith nothing but parts to reflect it wholly: especially considering that in its passage out of Air into Glasse, how oblique soever be its incidence it finds pores enough in the Glass to transmitt yethe greatest part of it; If any man suppose that it is not reflected by the Air but by yetheutmost superficiall parts of yethe 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 yethe rayes suppose of 45 or 46 degrees at wchwhich they are all reflected where the Air is adjacent to yethe Glasse, they shall be in great measure transmitted where the water is adjacent to it: wchwhich argues that their reflexion or transmission depends on yethe constitution of yethe Air & water. hind the glass, & not on the parts of yethe glass Thirdly if yethe colours made by a Prism placed at yetheof a beam of light into a darkned Room cast on a second Prism placed at a the former, in such manner that they incident upon it: the second Prism 597567 to the incident rayes that those wchwhich are of a blew colour shall be all reflected by it, & yet those of a red colour pretty copiously transmitted. Now if the reflexion be caused by the parts of Air or Glasse I would ask why at yethe same obliquity of incidence the blew should wholly impinge on those parts so as to be all reflected & yet the red find pores enough 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 rayes should not impinge on the parts of Glasse when contiguous to one another Glasse as much as when contiguous to Air. Firstly when the top of a water-Bubble (in yethe 17th Observation) by the continuall subsiding & exhaling of the water grew very thin there was such a little & almost insensible quantity of light reflected from it that it appeared intensly black, wheras round about that black spot where the water was thicker the reflexion was soe strong as to make yethe 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 yethe 15thObservation the rayes of yethe same colour were by turnes transmitted at one thicknesse & reflected at another thicknesse for an indeterminate number of successions. And yet in yethe suꝑperficies of the thin'd body where it is of any one thicknesse there are as many parts for the rayes to impinge on, as where it is of any other thicknesse. Lastly if reflexion were caused by the parts of reflecting bodies it would be impossible for thin Plates or Bubbles at yethe same place to reflect the rayes of one colour & transmitt those of another as they do according to the 13 & 15 Observations. For it is not to be īimmagined that at one place the rayes which for instance exhibit a blew colour should have the fortune to dash upon the parts & those wchwhich exhibit a Red to let upon the Pores Pores of the body; & then at another place soethe Body is either a little thicker, or a little thinner that on yethe contrary the Blew should hit upon its pores & the Red upon its parts.Prop: 9. 'Tis most probable ytthatyethe rayes wchwhich impinge on yethe solid parts of any body, are not reflected but shifted & lost in that body.This is consentaneous to yethe Precedent Proposition & will further appear by considering that if all the rayes should be reflected which impinge on yethe internall parts of clear water or Chrystall, those substances should rather have a cloudy then so very clear transparency. And further there would be no Principle of the obobscurity or blacknesse wchwhich some bodies have in all positions of yethe eye. For to produce this effect it is necessary that many rayes be retained & lost in yethebody & it seems not probable that any rayes can be stopt & retained in it wchwhich do not impinge on its parts.If you now ask how rayes are reflected wthwithout impinging on yethe parts of a body, & how those wchwhich impinge on its parts may be stopped & shifted, it requires an Hypothesis to explain it by, the description of wchwhich is besides my design. And so yethe manner how severall rayes are unequally refrangible & reflecxtible & originally indued wthwith severall colours remains to be explained Hypothetically. But I shall content my selfe in having shown that de facto the rayesrayes of light are indued wthwith those properties.SrSir I am IYorYour humble servant Isaac Newton.