3970 (B)
Trin: Coll Cambridge. Feb. 6. 167
SrSir
To perform my late promise to you, I shall wthwithout further ceremony acquaint you, that in yethe beginning of yethe year 1666 (at wchwhich time I applyed my self to yethe grinding of Optick glasses of other figures then Sphericall) I procured me a triangular glasse Prisme to try therewthwith yethe celebrated phænomena of colours. And in order thereto having darkned my chamber & made a small hole in my window-shuts to let in a convenient quantity of yethe sun's light, I placed my Prism at its entrance that it might be thereby refracted to yethe opposite wall. It was at first a very pleasing divertisement to view the vivid & intense colours produced thereby; but after a while applying my selfe to consider them more circumspectly, I became surprized to see them in an oblong form, wchwhich according to yethe received lawes of refraction I expected should have been circular.
They were terminated at yethe sides wthwith streight lines, but at the ends the decay of light was so graduall that it was difficult to determine justly what was their figure, yet they seemed semicircular.
Comparing yethe length of this Coloured Spectrum wthwith its bredth I found it about five times greater, a disproportion soe extravagant that it excited me to a more then ordinary curiosity of examining frōom whence it might proceed; I could scarce think that yethe various thicknesse of yethe glasse, or yethe termination wthwith shaddow or darknesse could have any influence on light to produce such an effect, yet I thought it not amis amisse to examine first those circumstances, & soe tryed wtwhat would happen by transmitting light through parts of yethe glasse of divers thicknesses, or through holes in yethe window of divers bignesses, or by setting the Prism wthwithout, so that the light might passe through it & bee refracted before it was terminated by yethe hole: but I found none of those circumstances materiall. The fashion of yethe colours was in all these cases yethe same.
Then I suspected whither by any unnevenesse in yethe glass or other contingent irregularity these colours might be thus dilated. And to try this I took another prism like yethe former & soe placed it that yethe light passing through them both might be refracted contrary wayes & so by the latter returned into that course frōom which yethe former had diverted it. ffor by this meanes I thought the regular effects of yethe first Prism would be destroyed by the sceond Prism, but yethe irregular ones more augmented by yethe multiplicity of refractions. The event was that yethe light wchwhich by the first Prism was diffused into an oblong form was by the second reduced into an orbicular one wthwith as much regularity as when it did not at all passe through them. So that what ever was yethe cause of that length 'twas not any contingent irregularity.
I then proceded to examine more critically what might be effected by the difference of yethe incidence of rayes com̄mming frōom divers parts of yethe Sun, & to that end measured the severall lines & angles belonging to yethe image. Its distance frōom yethe hole or Prism was 22 foot, its utmost length 13 inches, its breadth 2 inches, the diameter of yethe hole of an inch, the angle which yethe rayes tending towards the midle of yethe image made wthwith those lines in wchwhich they would have proceeded wthwithout refraction 44 degr. 56'. And yethe verticall angle of yethe Prism 63 degr 12'. Also the refractions on both sides the Prism, that is of yethe incident & emergent rayes were as near as I could make them, equall, & consequently about 54 degr 4'. And yethe rayes fell perpendicularly upon yethe wall. Now subducting the diameter of yethe hole frōom yethe length & breadth of yethe image there remains 13 inches yethe length & 2 the breadth comprehended by those rayes wchwhich passed through yethe center of yethe said hole & consequently the angle at yethe hole wchwhich that breadth subtended was about 31' answerable to yethe sun's diameter, but yethe angle wchwhich its length subtended was more then five such diameters, namely 2 degr 49'.
Having made these observations I first computed from them the refractive power of that glasse & found it measured by yethe ratio of yethe sines 20 to 31. And then by that ratio I computed the refractions of two rayes flowing frōom opposite parts of yethe sun's discus, so as to differ 31' in their obliquity of incidence, & found that the emergent rayes should have comprehended an angle of about 31' as they did before they were incident.
But because this computation was founded on yethe Hypothesis of the proportionality of yethe Sines of incidence & refraction, wchwhich, though by my own & others experience, I could not imagine to be so erroneous as to make that angle but 31' wchwhich in reality was 2 degr 49' yet my curiosity caused me again to take my Prism. And having placed it at my window as before I observed that by turning it a little about its axis to & fro, so as to vary its obliquity to yethe light more then by an angle of 4 or 5 degrees, the colours were not thereby sensibly translated from their place on yethe wall, & consequently by that variation of incidence, the quantity of refraction was not sensibly varied. By this experiment461
By this experiment therefore as well as by the former computation it was evident that the difference of yethe incidence of rayes flowing from divers parts of yethe Sun, could not make them after decussation diverge at a sensibly greater angle then that at wchwhich they before converged. which being at most but about 31. or 32 minutes, there still remained some other cause to be found out frōom whence it could be 2 degr 49'.
Then I began to suspect whither the rayes after their trajection through the Prism did not move in curve lines & acording to their more or lesse curvity tend to divers parts of yethe wall. And it increased my suspiciton when I remembred that I had often seen a Tennis-ball struck wthwith an oblique Racket describe such a curve line. for a circular as well as a progressive motion being communicated to it by that stroak, its parts on that side where the motions conspire must presse & beat the contiguous air more violently then on yethe other, & there excite a reluctancy & reaction of yethe air proportionally greater. And for yethe same reason, if yethe rayes of light should possibly bee globular bodyes & by their oblique passage out of one medium into another acquire a circulating motion, they ought to feel yethe greater resistance from yethe ambient Æther on that side where the motions conspire & thence be continually bowed to the other. But notwthwithstanding this plausible ground of suspicion when I came to examine it I could observe noe such curvity in them. And besides (wchwhich was enough for my purpose) I observed ytthat the difference betwixt yethe length of yethe image & diameter of yethe hole through wchwhich yethe light was transmitted, was proportionable to their distance.
The graduall removall of these suspicions at length led me to yethe experimentum crucis; which was this. I took two boards, & placed one of them close behind the Prism at yethe window, so that yethe light might passe through a small hole made in it for that purpose & fall on yethe other board wchwhich I placed at about twelve foot distance, having first made a small hole in it also for some of that incident light to passe through. Then I placed another Prism behind this second board so that the light trajected through both yethe boards might passe through that also & & be again refracted before it arrived at yethe wall. This done, I took the first Prism in my hand & turned it to & fro slowly about its axis so much as to make yethe severall parts of yethe image cast on yethe second board, successively passe through the hole in it, that I might observe to what places on yethe wall the second Prism would refract ymthem. And I saw by yethe variation of those places, that the light tending to that end of yethe image towards wchwhich the refraction of yethe first Prism was made, did in yethe second Prism suffer a refraction considerably greater then yethe light tending to yethe other end. And soe the true cause of yethe length of that image was detected to be noe other then that light consists of rayes differently refrangible wchwhich wthwithout any respect to a difference in their incidence were according to their degrees of refrangibility transmitted towards diverse parts of yethe wall.
When I understood this, I left of my aforesaid glassworkes, for I saw that yethe ꝑperfection of Telescopes was hitherto limited, not so much for want of glasses truly figured according to yethe prescriptions of Optick writers Authors, wchwhich all men have hitherto imagined, as because that light it selfe is a heterogeneous mixture of differently refrangible rayes. So that were a Glasse so exactly figured as to collect any one sort of rayes into one point, it could not collect those also into the same point which having yethe same incidence upon yethe same medium are apt to suffer a different refraction. Nay I wondered that seing the differentce of refrangibility was soe great as I found it, Telescopes should arrive to that ꝑperfection thay are now at. ffor measuring the refractions in one of my Prisms I found that supposing the cōommon sine of incidence upon onone of its planes was 44 parts, the sine of refraction of yethe utmost rayes on yethe red end of yethe colours made out of yethe glasse into yethe air would be 68 parts, & yethe sine of refraction of yethe utmost ray rayes on yethe other end 69 parts. So that the difference is about a 24th or 25t part of yethe whole refraction. And consequently the objectglasse of any Telescope cannot collect all yethe rayes wchwhich come frōom one point of an object, so as to make them convene at its focus in lesse room then in a circular space whose diameter is yethe 50th part of yethe diameter of its spectrum. which is an irregularity462 which is an irregularity some hundreds of times greater then a circularly figured lens, of so small a section as yethe objectglasses of long Telescopes are, would cause by yethe unfitnesse of its figure, were light uniform.
This made me take reflections into consideration; & finding them regular, so that yethe angle of reflection of all sorts of rayes was equall to their angle of incidence I understood that by their mediation Optick instruments might be brought to any degree of ꝑperfection imaginable if provided a reflecting substance could be found which would polish as finely as glasse, & reflect as much light as glasse transmitts, & yethe art of com̄mmunicating to it a Parabolick figure bee also attained. But those seemed very great difficulties, & I almost thought them insuperable when I further considered that every irregularity in a reflecting superficies makes yethe rayes stray 5 or 6 times more out of their due course then yethe like irregularities in a refracting one. So that a much greater curiosity would be here requisite then in figuring glasses for refraction.
Amidst these thoughts I was forced frōom Cambridge by the intervening Plague & it was more then two years before I proceeded further. But then having thought on a tender way of polishing propper for mettall, & wherby as I imagined the figure also would be corrected to yethe last, I began to try what might be effected in this kind, & by degrees so far perfected an instrument (in yethe essential parts of it like that I sent to London) by wchwhich I could discern Iupiters four concomitants, & showed them divers times to two others of my acquaintance. I could also discern the moon-like phase of Venus, but not very distinctly nor wthwithout some nicenesse in disposing the instrument.
ffrom that time I was interrupted till this last Autumn, when I made the other. And as that was sensibly better then yethe first, (especially for day Objects) so I doubt not but they will be still brought to a much greater ꝑperfection by their endeavours who as you inform me are taking care about it at London.
I have sometimes thought to make a microscope wchwhich in like manner should have instead of an objectglasse a relecting piece of metall. And this I hope they will also take into consideration for those For those instruments seem as capable of improvement as Telescopes, & perhaps more because but one reflective peice of metall is requisite in them, as you may perceive by yethe annexed diagram where A B representeth the object metall, C D the eyeglasse, F their cōommon focus, & O the other focus of yethe metall in wchwhich yethe object is placed. But to return frōom this digression, I told you that light is not similar or homogeneall but consists of difform rayes, some of wchwhich are more refrangible then others. So that of those wchwhich are alike incident on yethe same medium, some shall be more refracted then others, & that not by any virtue of yethe glasse or other externall cause, but from a predisposition wchwhich every particular ray hath to suffer a particular degree of refraction. I shall now proceed to acquaint you wthwith another more notable difformity in its rayes wherin the origen of colours is infolded. A naturalist would scarce beleive expect to see yethe science of those become mathematicall, & yet I dare affirm that there is as much certainty in it as in any other part of Opticks. ffor what I shall tell concerning them is not an Hypothesis but most rigid consequence, not conjectured by barely inferring 'tis thus because not otherwise or because it satisfies all phænomena (the Philosophers universall Topick,) but evinced by yethe mediation of experiments concluding directly & wthwithout any suspicion of doubt. To continue the historicall narration of these experiments would make a discourse too tedious & confused, & therefore I shall rather lay down the doctrine first, & then for its examination give you an instance or two of yethe experiments as a Specimen of yethe rest.
The Doctrine you will find comprehended & illustrated in yethe following propositions.
1. As yethe rayes of light differ in degrees of refrangibility, so they also differ in their disposition to exhibit this or that particular colour. Colours are not qualifications of light derived frōom refractions or reflections of naturall bodies as 'tis generally beleived, but originall & connate properties, wchwhich in diverse rayes are divers, some rayes are disposed to exhibit a red colour & noe other, some a463 some a yellow & noe other, some a green & noe other & so of yethe rest. Nor are there only rayes propper & peculiar to yethe more eminent colours, but even to all their intermediate gradations.
2. To the same degree of refrangibility ever belongs the same colour, & to yethe same colour ever belongs the same degree of refrangibility. The least-refrangible rayes are all disposed to exhibit a red colour & contrarily those rayes which are disposed to exhibit a red colour are all yethe least refrangible. So yethe most refrangible rayes are all disposed to exhibit a deep violett colour & contrarily those wchwhich are apt to exhibit such a violet colour are all the most refrangible. And so to all the intermediate colours in a continued series belong intermediate degrees of refrangibility. And this Analogy 'twixt colours & refrangibility is very precise & strict: the rayes always either exactly agreeing in both or proportionally disagreeing in both.
3. The Species of colour & degree of refrangibility propper to any particular sort of rayes, is not mutable by refraction, nor by reflection frōom naturall bodies, nor by any other cause that I could yet observe. When any one sort of rayes hath been well parted frōom those of other kinds it hath afterwards obstinately retained its colour notwthwithstanding my utmost endeavours to chang it. I have refracted it wthwith Prismes, & reflected it wthwith bodies wchwhich in day light were of other colours; I have intercepted it wthwith yethe coloured film of air interceding two compressed plates of Glasse, transmitted it through coloured mediums & through mediums irradiated wthwith other sort of rayes, & diversly terminated it, & yet could not produce any new colour out of it. It would by contracting or dilating become more brisk or faint, & by yethe losse of many rayes in some cases very obscure & dark, but I could never see it changed in Specie.
4 Yet seeming transmutations of colours may be made where there is any mixture of divers sorts of rayes: ffor in such mixtures the component colours appear not, but by their mutuall allaying each other constitute a midling colour. And therefore if by refraction or or any other of yethe aforesaid causes the difform rayes latent in such a mixture bee seperated, there shall emerge colours different frōom yethe colour of yethe composition. Which colours are not new generated but only made apparent by being parted. ffor if they be again intirely mixed & blended together, they will again compose that colour wchwhich they did before seperation. And for yethe same reason transmutations made by yethe convening of divers colours are not reall; for when yethe difform rayes are again severed they will exhibit the very same colours which they did before they entered the composition. As you see blew & yellow powders when finely mixed appear to yethe naked eye green & yet the colours of yethe component corpuscles are not thereby really transmuted, but only blended. ffor when veiwed wthwith a good Microscope they still appear blew & yellow interspersedly.
5 There are therefore two sorts of colours. The one originall & simple; the other compounded of these. The originall or primary colours are Red, yellow, Green, Blew, & a violet purple; together with Orang, Indico, & an indefinite varietie of intemediate gradations.
6 The same colours in Specie wthwith these primary ones may be also produced by composition: ffor a mixture of yellow & blew makes green, of red & yellow makes Orang of Orang & yellowish green makes yellow. And in generall if any two colours be mixed, which in yethe series of those generated by yethe Prism are not too far distant one from another, they by their mutuall alloy compound that colour wchwhich in yethe said series appeareth in yethe midway between them. But those which are situated at too great a distance doe not soe. Orang & Indico produce not the intermediate green, nor Scarlet & green yethe intermediate yellow.
7 But the most surprizing & wonderfull composition was that of whitenesse. There is noe one sort of rayes wchwhich alone can exhibit this. Tis ever compounded & to its composition are requisite all the aforesaid primary colours mixed in a due proportion. I have often wthwith admiration beheld that all yethe colours of yethe Prism being made 464 made to converge & thereby to be again mixed as they were in yethe light before it was incident upon yethe Prism, reproduced light intirely & perfectly white, & not at all sensibly differing from yethe direct light of yethe sun; unlesse when the glasses I used were not sufficiently clear, for then they would a little tinge incline it wthwith to their colour.
8. Hence therefore it comes to passe that whitenesse is yethe usuall colour of light: ffor light is a confused agregate of rayes indued wthwith all sorts of colours as they are promiscuously darted from yethe various parts of luminous bodies. And of such a confused agregate as I said is generated whitenesse if there be a due proportion of yethe ingredients; But if any one predominate, the light must incline to that colour as it happens in yethe blew flame of Brimstone, the yellow flame of a candle & the various colours of yethe fixt starrs.
9. These thinges considered, the manner how colours are produced by yethe Prism is evident. ffor of yethe rayes constituting yethe incident light since those wchwhich differ in colour proportionally differ in refrangibility, they by their unequall refractions must be severed & dispersed into an oblong form in an orderly succession frōom yethe least refracted scarlet to yethe most refracted violedt. And for yethe same reason it is that objects when looked upon through a Prism appear coloured. ffor yethe difform rayes by their unequall refractions are made to diverge toward severall parts of yethe Retina & there expresse the images of thinges coloured as in yethe former case they did the sun's image upon a wall. And by this inequality of refractions they become not only coloured but also very confused & indistinct.
10 Why the colours of yethe rainbow appear in falling drops of rain is also frōom hence evident. ffor those drops wchwhich refract the rayes disposed to appear purple in greatest quantity to yethe Spectators eye, refract yethe rayes of other sorts soe much lesse as to make ymthem passe beside it; & such are yethe dropes in yethe inside of yethe primary Bow & on yethe outside of yethe second or exterior one. So those drops which refract in greatest plenty the rayes apt to appear red toward yethe Spectators eye, refract those of other sorts soe much more as to make them passe beside it, & such are yethe drops on yethe exterior part of yethe Primary & interior part of yethe secondary Bow.
11. The odd Phænomenona of an infusion of Lignum Nephriticum, leaf-Gold, fragments of coloured glasse, & some other transparently coloured bodies appearing in one position of one colour, & of another in another are on these grounds noe longer riddles. ffor those are substances apt to reflect one sort of light & transmit another; as may be seen in a dark Room by illustratingminating them wthwith similar or uncompounded light. ffor then they appear of that colour only wthwith wchwhich they are illuminated, but yet in one position more vivid & luminous then in another, accordingly as they are disposed more or lesse to reflect or transmit the incident colour.
12 ffrom hence also is manifest the reason of an unexpected experiment wchwhich Mr Hook somewhere in his Micrographia relates to have made wthwith two wWedg-like transparent vessells filled the one wthwith a red, the other wthwith a blew liquor: Namely that though they were severally transparent enough yet both together became opake. ffor if one transmitted only red & yethe other only blew, no rayes could passe through both.
13 I might add more instances of this nature, but I shall conclude wthwith this generall one, that yethe colours of all naturall bodies have noe other origin then theis, that they are variously qualified to reflect one sort of light in greater plenty then another. And this I have experimented in a dark Room by illuminating those bodies wthwith uncompounded light of divers colours. ffor by that means any bodie may be made to appear of any colour. They have there noe appropriate colour but ever appear of yethe colour cast upon them, but yet wthwith this difference that they are most brisk & vivid in yethe light of their own day-light-colour. Minium appeareth there of any colour465 colour indifferently wthwith wchwhich 'tis illustrated, but yet most luminous in red; & so Bise appeareth indifferently of any colour wthwith wchwhich 'tis illustrated, but yet most luminous in blew. And therefore Minium reflecteth rayes of any colour, but most copiously those endowed wthwith red; & consequently when illustrated wthwith day-light, that is wthwith all sorts of rayes promiscuously blended, those qualified wthwith red shall abound most in the reflected light, & by their prevalence cause it to appear of that colour. And for yethe same reason Bise reflecting blew most copiously, shall appear blew by yethe excesse of those rayes in its reflected light; & yethe like of other bodies. And that this is yethe intire & adequate cause of their colours is manifest, because they have noe power to change or alter the colour of any sort of rays incident apart, but put on all colours indifferently wthwith wchwhich they are enlightened.
These thinges being so, it can be no longer disputed whither there be colours in yethe dark, nor whither thesey be the qualityes of yethe objects wee see, no nor perhaps whether light be a bodie. ffor since colours are qualityes of light, having its rayes for their entire & imediate subject, how can wee think those rayes qualityes also, unlesse one quality may be yethe subject of & sustain another, wchwhich in effect is to call it substance. Wee should not know bodies for substances were it not for their sensible qualityes, & the principall of themose being now found due to something else, wee have as good reason to beleive that to be a substance also. Besides who ever thought any quality to be a heterogeneous agregate such as light is discovered to be. But to say determine more absolutely what light is, after what manner refracted, & by what modes or actions it produceth in orour minds the Phantasms of colours, is not so easie. . And I shall not mingle conjectures wthwith certaintyes.
Reviewing what I have written, I see yethe discours it selfe will lead to divers experiments sufficient for for its examination: And therefore I shall not trouble you farther then to describe one of those which I have already insinuated.
In a darkned Room make a hole in yethe shutt of a window, whose diameter may conveniently be about a third part of an inch, to admitt a convenient quantity of yethe Sun's rayes light. And then place a clear & colourlesse Prism to refract the entering light towards the further part of yethe Room: wchwhich as I said will thereby be diffused into an oblong coloured image. Then place a lens of about three foot radius (suppose a broad object-glasse of a three foot Telescope) at yethe distance of about 4 or 5 foot frōom thence, through wchwhich all those colours may at once be transmitted, & made by its refraction to convene at a further distance of about 10 or 12 foot. If at that distance you intercept this light wthwith a sheet of white paper, you will see the colours converted into whitenesse again by being mingled. But it is requisite that the Prism & lens be placed steddy, & that the paꝑper on wchwhich yethe colours are cast bee moved to & fro; for by such motion you will not only find at what distance the whitenesse is most ꝑperfect, but also see how yethe colours gradually convene & vanish into whitenesse, & afterwards having crossed one another in that place where they compound whitenesse, are again dissipated & severed & in an inverted order retain yethe same colours wchwhich they had before they entered the composition. You may also see that if any of the colours at yethe lens be intercepted, the whitenesse will be changed into yethe other colours. And therefore that the composition of whitenesse be perfect, care must be taken that none of yethe colours fall beside the lens.466
In the annexed designe of this experiment A B C representeth yethe Prism set endwise to sight, close by yethe hole F of yethe window E G. Its vertical angle A C B may conveniently be about 60 degrees. M N designes yethe lens. Its breadth 2 or 3 inches. S F one of yethe streight lines in wchwhich difform rayes may be conceived to flow successively from yethe Sun. F P & F R two of those rayes unequally refracted, which yethe Lens makes to converg towards Q, & after decussation to diverge again. And H I yethe paper at divers distances on which yethe colours are projected: which in Q constitute whitenesse, but are red & yellow in R, r, & ρ; & blew & purple in P, p & π.
If you proceed further to try yethe impossibility of changing any uncompounded colour, wchwhich I have asserted in yethe 3d and 13th propositions; 'tis requisite ytthat yethe Room be made very dark, least any scattering light mixing wthwith yethe colour, disturb & allay it & render it compound, contrary to yethe design of yethe experimtment 'Tis also requisite that there be a ꝑperfecter seperation of yethe colours then after yethe manner above described can be made by yethe refraction of one single Prism; & how to make such further seperations will scarce be difficult to them that consider yethe discovered lawes of refractions. But if tryall shall be made wthwith colours not throughly seperated there must be allowed changes proportionable to the mixture. Thus if compound yellow light fall upon blew bBise, the Bise will not appear ꝑperfectly yellow, but rather green, because there are in yethe yellow mixture many rayes indued wthwith green, because there are in yethe yellow & green being lesse remote frōom yethe usuall blew colour of Bise then yellow, is more copiously reflected by it.
In like manner if any of yethe Prismatick colours, suppose Red, be intercepted on design to try yethe asserted impossibility of reproducing that colour out of yethe others which are pretermitted 'tis necessary either that yethe colours be very well parted before yethe red be intercepted, or that together wthwith yethe red yethe neighbouring colours into which any red is secretly dispersed (that is the yellow & perhaps green too) bee intercepted, or else that allowance be made for yethe emerging of so much red out of yethe yellow & green as may possibly have been diffused & scatteringly blended in theose colours. And if these thinges be observed the new production of Red or any intercepted colour will be found impossible.
This I conceive is enough for an introduction to experimtsments of this kind; which if any of the R Society shall be so curious as to prosecute, I should be very glad to be informed wthwith what successe. That if any thing seem to be defective or to thwart this relation, I may have an opportunity of giving further direction inabout it, or iof acknowledging my errors if I have cōommitted any.
YorYour humble servtvant
Isaac Newton