BOOK III.
AFTER this view which has been taken of Sir Isaac Newton’s mathematical principles of philosophy, and the use he has made of them, in explaining the system of the world, &c. the course of my design directs us to turn our eyes to that other philosophical work, his treatise of Optics, in which we shall find our great author’s inimitable genius discovering it self no less, than in the former; nay perhaps even more, since this work gives as many instances of his singular force of reasoning, and of his unbounded invention, though unassisted in great measure by those rules and general precepts, which facilitate the invention of mathematical theorems. Nor yet is this work inferior to the other in usefulness; for as that has made known to us one great principle in nature, by which the celestial motions are continued, and by which the frame of each globe is preserved; so does this point out to us another principle no less universal, upon which depends all those operations in the smaller parts of matter, for whose sake the greater frame of the universe is erected; all those immense globes, with which the whole heavens are filled, being without doubt only design’d as so many convenient apartments for carrying on the more noble operations of nature in vegetation and animal life. Which single consideration gives abundant proof of the excellency of our author’s choice, in applying himself carefully to examine the action between light and bodies, so necessary in all the varieties of these productions, that none of them can be successfully promoted without the concurrence of heat in a greater or less degree.
2. ’Tis true, our author has not made so full a discovery of the principle, by which this mutual action between light and bodies is caused; as he has in relation to the power, by which the planets are kept in their courses: yet he has led us to the very entrance upon it, and pointed out the path so plainly which must be followed to reach it; that one may be bold to say, whenever mankind shall be blessed with this improvement of their knowledge, it will be derived so directly from the principles laid down by our author in this book, that the greatest share of the praise due to the discovery will belong to him.
3. In speaking of the progress our author has made, I shall distinctly pursue three things, the two first relating to the colours of natural bodies: for in the first head shall be shewn, how those colours are derived from the properties of the light itself; and in the second upon what properties of the bodies they depend: but the third head of my discourse shall treat of the action of bodies upon light in refracting, reflecting, and inflecting it.
4. The first of these, which shall be the business of the present chapter, is contained in this one proposition: that the sun’s direct light is not uniform in respect of colour, not being disposed in every part of it to excite the idea of whiteness, which the whole raises; but on the contrary is a composition of different kinds of rays, one sort of which if alone would give the sense of red, another of orange, a third of yellow, a fourth of green, a fifth of light blue, a sixth of indigo, and a seventh of a violet purple; that all these rays together by the mixture of their sensations impress upon the organ of sight the sense of whiteness, though each ray always imprints there its own colour; and all the difference between the colours of bodies when viewed in open day light arises from this, that coloured bodies do not reflect all the sorts of rays falling upon them in equal plenty, but some sorts much more copiously than others; the body appearing of that colour, of which the light coming from it is most composed.
5. That the light of the sun is compounded, as has been said, is proved by refracting it with a prism. By a prism I here mean a glass or other body of a triangular form, such as is represented in fig. 121. But before we proceed to the illustration of the proposition we have just now laid down, it will be necessary to spend a few words in explaining what is meant by the refraction of light; as the design of our present labour is to give some notion of the subject, we are engaged in, to such as are not versed in the mathematics.
6. It is well known, that when a ray of light passing through the air falls obliquely upon the surface of any transparent body, suppose water or glass, and enters it, the ray will not pass on in that body in the same line it described through the air, but be turned off from the surface, so as to be less inclined to it after passing it, than before. Let A B C D (in fig. 122.) represent a portion of water, or glass, A B the surface of it, upon which the ray of light E F falls obliquely; this ray shall not go right on in the course delineated by the line F G, but be turned off from the surface A B into the line F H, less inclined to the surface A B than the line E F is, in which the ray is incident upon that surface.
7. On the other hand, when the light passes out of any such body into the air, it is inflected the contrary way, being after its emergence rendred more oblique to the surface it passes through, than before. Thus the ray F H, when it goes out of the surface C D, will be turned up towards that surface, going out into the air in the line H I.
8. This turning of the light out of its way, as it passes from one transparent body into another is called its refraction. Both these cases may be tried by an easy experiment with a bason and water. For the first case set an empty bason in the sunshine or near a candle, making a mark upon the bottom at the extremity of the shadow cast by the brim of the bason, then by pouring water into the bason you will observe the shadow to shrink, and leave the bottom of the bason enlightned to a good distance from the mark. Let A B C (in fig. 123.) denote the empty bason, E A D the light shining over the brim of it, so that all the part A B D be shaded. Then a mark being made at D, if water be poured into the bason (as in fig. 124.) to F G, you shall observe the light, which before went on to D, now to come much short of the mark D, falling on the bottom in the point H, and leaving the mark D a good way within the enlightened part; which shews that the ray E A, when it enters the water at I, goes no longer straight forwards, but is at that place incurvated, and made to go nearer the perpendicular. The other case may be tryed by putting any small body into an empty bason, placed lower than your eye, and then receding from the bason, till you can but just see the body over the brim. After which, if the bason be filled with water, you shall presently observe the body to be visible, though you go farther off from the bason. Let A B C (in fig. 125.) denote the bason as before, D the body in it, E the place of your eye, when the body is seen just over the edge A, while the bason is empty. If it be then filled with water, you will observe the body still to be visible, though you take your eye farther off. Suppose you see the body in this case just over the brim A, when your eye is at F, it is plain that the rays of light, which come from the body to your eye have not come straight on, but are bent at A, being turned downwards, and more inclined to the surface of the water, between A and your eye at F, than they are between A and the body D.
9. This we hope is sufficient to make all our readers apprehend, what the writers of optics mean, when they mention the refraction of the light, or speak of the rays of light being refracted. We shall therefore now go on to prove the assertion advanced in the forementioned proposition, in relation to the different kinds of colours, that the direct light of the sun exhibits to our sense: which may be done in the following manner.
10. If a room be darkened, and the sun permitted to shine into it through a small hole in the window shutter, and be made immediately to fall upon a glass prism, the beam of light shall in passing through such a prism be parted into rays, which exhibit all the forementioned colours. In this manner if A B (in fig. 126) represent the window shutter; C the hole in it; D E F the prism; Z Y a beam of light coming from the sun, which passes through the hole, and falls upon the prism at Y, and if the prism were removed would go on to X, but in entring the surface B F of the glass it shall be turned off, as has been explained, into the course Y W falling upon the second surface of the prism D F in W, going out of which into the air it shall be again farther inflected. Let the light now, after it has passed the prism, be received upon a sheet of paper held at a proper distance, and it shall paint upon the paper the picture, image, or spectrum L M of an oblong figure, whose length shall much exceed its breadth; though the figure shall not be oval, the ends L and M being semicircular and the sides straight. But now this figure will be variegated with colours in this manner. From the extremity M to some length, suppose to the line n o, it shall be of an intense red; from n o to p q it shall be an orange; from p q to r s it shall be yellow; from thence to t u it shall be green; from thence to w x blue; from thence to y z indigo; and from thence to the end violet.
11. Thus it appears that the sun’s white light by its passage through the prism, is so changed as now to be divided into rays, which exhibit all these several colours. The question is, whether the rays while in the sun’s beam before this refraction possessed these properties distinctly; so that some part of that beam would without the rest have given a red colour, and another part alone have given an..orange, &c. That this is possible to be the case, appears from hence; that if a convex glass be placed between the paper and the prism, which may collect all the rays proceeding out of the prism into its focus, as a burning glass does the sun’s direct rays; and if that focus fall upon the paper, the spot formed by such a glass upon the paper shall appear white, just like the sun’s direct light.
The rest remaining as before, let P Q. (in fig. 127.) be the convex glass, causing the rays to meet upon the paper H G I K in the point N, I say that point or rather spot of light shall appear white, without the least tincture of any colour. But it is evident that into this spot are now gathered all those rays, which before when separate gave all those different colours; which shews that whiteness may be made by mixing those colours: especially if we consider, it can be proved that the glass P Q does not alter the colour of the rays which pass through it. Which is done thus: if the paper be made to approach the glass P Q, the colours will manifest themselves as far as the magnitude of the spectrum, which the paper receives, will permit. Suppose it in the situation h g i k, and that it then receive the spectrum l m, this spectrum shall be much smaller, than if the glass P Q were removed, and therefore the colours cannot be so much separated; but yet the extremity m shall manifestly appear red, and the other extremity l shall be blue; and these colours as well as the intermediate ones shall discover themselves more perfectly, the farther the paper is removed from N, that is, the larger the spectrum is: the same thing happens, if the paper be removed farther off from P Q than N. Suppose into the position θ γ η ϰ, the spectrum λ μ painted upon it shall again discover its colours, and that more distinctly, the farther the paper is removed, but only in an inverted order: for as before, when the paper was nearer the convex glass, than at N, the upper part of the image was blue, and the under red; now the upper part shall be red, and the under blue: because the rays cross at N.
12. Nay farther that the whiteness at the focus N, is made by the union of the colours may be proved without removing the paper out of the focus, by intercepting with any opake body part of the light near the glass; for if the under part, that is the red, or more properly the red-making rays, as they are styled by our author, are intercepted, the spot shall take a bluish hue; and if more of the inferior rays are cut off, so that neither the red-making nor orange-making rays, and if you please the yellow-making rays likewise, shall fall upon the spot; then shall the spot incline more and more to the remaining colours. In like manner if you cut off the upper part of the rays, that is the violet coloured or indigo-making rays, the spot shall turn reddish, and become, more so, the more of those opposite colours are intercepted.
13. This I think abundantly proves that whiteness may be produced by a mixture of all the colours of the spectrum. At least there is but one way of evading the present arguments, which is, by asserting that the rays of light after passing the prism have no different properties to exhibit this or the other colour, but are in that respect perfectly homogeneal, so that the rays which pass to the under and red part of the image do not differ in any properties whatever from those, which go to the upper and violet part of it; but that the colours of the spectrum are produced only by some new modifications of the rays, made at their incidence upon the paper by the different terminations of light and shadow: if indeed this assertion can be allowed any place, after what has been said; for it seems to be sufficiently obviated by the latter part of the preceding experiment, that by intercepting the inferior part of the light, which comes from the prism, the white spot shall receive a bluish cast, and by stopping the upper part the spot shall turn red, and in both cases recover its colour, when the intercepted light is permitted to pass again; though in all these trials there is the like termination of light and shadow. However our author has contrived some experiments expresly to shew the absurdity of this supposition; all which he has explained and enlarged upon in so distinct and expressive a manner, that it would be wholly unnecessary to repeat them in this place[280]. I shall only mention that of them, which may be tried in the experiment before us. If you draw upon the paper H G I K, and through the spot N, the straight line w x parallel to the horizon, and then if the paper be much inclined into the situation r s v t the line w x still remaining parallel to the horizon, the spot N shall lose its whiteness and receive a blue tincture; but if it be inclined as much the contrary way, the same spot shall exchange its white colour for a reddish dye. All which can never be accounted for by any difference in the termination of the light and shadow, which here is none at all; but are easily explained by supposing the upper part of the rays, whenever they enter the eye, disposed to give the sensation of the dark colours blue, indigo and violet; and that the under part is fitted to produce the bright colours yellow, orange and red: for when the paper is in the situation r s t u, it is plain that the upper part of the light falls more directly upon it, than the under part, and therefore those rays will be most plentifully reflected from it; and by their abounding in the reflected light will cause it to incline to their colour. Just so when the paper is inclined the contrary way, it will receive the inferior rays most directly, and therefore ting the light it reflects with their colour.
14. It is now to be proved that these dispositions of the rays of light to produce some one colour and some another, which manifest themselves after their being refracted, are not wrought by any action of the prism upon them, but are originally inherent in those rays; and that the prism only affords each species an occasion of shewing its distinct quality by separating them one from another, which before, while they were blended together in the direct beam of the sun’s light, lay conceal’d. But that this is so, will be proved, if it can be shewn that no prism has any power upon the rays, which after their passage through one prism are rendered uncompounded and contain in them but one colour, either to divide that colour into several, as the sun’s light is divided, or so much as to change it into any other colour. This will be proved by the following experiment[281]. The same thing remaining, as in the first experiment, let another prism N O (in fig. 128.) be placed either immediately, or at some distance after the first, in a perpendicular posture, so that it shall refract the rays issuing from the first sideways. Now if this prism could divide the light falling upon it into coloured rays, as the first has done, it would divide the spectrum breadthwise into colours, as before it was divided lengthwise; but no such thing is observed. If L M were the spectrum, which the first prism D E F would paint upon the paper H G I K; P Q lying in an oblique posture shall be the spectrum projected by the second, and shall be divided lengthwise into colours corresponding to the colours of the spectrum L M, and occasioned like them by the refraction of the first prism, but its breadth shall receive no such division; on the contrary each colour shall be uniform from side to side, as much as in the spectrum L M, which proves the whole assertion.
15. The same is yet much farther confirmed by another experiment. Our author teaches that the colours of the spectrum L M in the first experiment are yet compounded, though not so much as in the sun’s direct light. He shews therefore how, by placing the prism at a distance from the hole, and by the use of a convex glass, to separate the colours of the spectrum, and make them uncompounded to any degree of exactness[282]. And he shews when this is done sufficiently, if you make a small hole in the paper whereon the spectrum is received, through which any one sort of rays may pass, and then let that coloured ray fall so upon a prism, as to be refracted by it, it shall in no case whatever change its colour; but shall always retain it perfectly as at first, however it be refracted[283].
16. Nor yet will these colours after this full separation of them suffer any change by reflection from bodies of different colours; on the other hand they make all bodies placed in these colours appear of the colour which falls upon them[284]: for minium in red light will appear as in open day light; but in yellow light will appear yellow; and which is more extraordinary, in green light will appear green, in blue, blue; and in the violet-purple coloured light will appear of a purple colour; in like manner verdigrease, or blue bise, will put on the appearance of that colour, in which it is placed; so that neither bise placed in the red light shall be able to give that light the least blue tincture, or any other different from red; nor shall minium in the indigo or violet light exhibit the least appearance of red, or any other colour distinct from that it is placed in. The only difference is, that each of these bodies appears most luminous and bright in the colour, which corresponds with that it exhibits in the day light, and dimmed in the colours most remote from that; that is, though minium and bise placed in blue light shall both appear blue, yet the bise shall appear of a bright blue, and the minium of a dusky and obscure blue: but if minium and bise be compared together in red light, the minium shall afford a brisk red, the bise a duller colour, though of the same species.
17. And this not only proves the immutability of all these simple and uncompounded colours; but likewise unfolds the whole mystery, why bodies appear in open day-light of such different colours, it consisting in nothing more than this, that whereas the white light of the day is composed of all sorts of colours, some bodies reflect the rays of one sort in greater abundance than the rays of any other[285]. Though it appears by the fore-cited experiment, that almost all these bodies reflect some portion of the rays of every colour, and give the sense of particular colours only by the predominancy of some sorts of rays above the rest. And what has before been explained of composing white by mingling all the colours of the spectrum together shews clearly, that nothing more is required to make bodies look white, than a power to reflect indifferently rays of every colour. But this will more fully appear by the following method: if near the coloured spectrum in our first experiment a piece of white paper be so held, as to be illuminated equally by all the parts of that spectrum, it shall appear white; whereas if it be held nearer to the red end of the image, than to the other, it shall turn reddish; if nearer the blue end, it shall seem bluish[286].
18. Our indefatigable and circumspect author farther examined his theory by mixing the powders which painters use of several colours, in order if possible to produce a white powder by such a composition[287]. But in this he found some difficulties for the following reasons. Each of these coloured powders reflects but part of the light, which is cast upon them; the red powders reflecting little green or blue, and the blue powders reflecting very little red or yellow, nor the green powders reflecting near so much of the red or indigo and purple, as of the other colours: and besides, when any of these are examined in homogeneal light, as our author calls the colours of the prism, when well separated, though each appears more bright and luminous in its own day-light colour, than in any other; yet white bodies, suppose white paper for instance, in those very colours exceed these coloured bodies themselves in brightness; so that white bodies reflect not only more of the whole light than coloured bodies do in the day-light, but even more of that very colour which they reflect most copiously. All which considerations make it manifest that a mixture of these will not reflect so great a quantity of light, as a white body of the same size; and therefore will compose such a colour as would result from a mixture of white and black, such as are all grey and dun colours, rather than a strong white. Now such a colour he compounded of certain ingredients, which he particularly sets down, in so much that when the composition was strongly illuminated by the sun’s direct beams, it would appear much whiter than even white paper, if considerably shaded. Nay he found by trials how to proportion the degree of illumination of the mixture and paper, so that to a spectator at a proper distance it could not well be determined which was the more perfect colour; as he experienced not only by himself, but by the concurrent opinion of a friend, who chanced to visit him while he was trying this experiment. I must not here omit another method of trying the whiteness of such a mixture, proposed in one of our author’s letters on this subject[288]: which is to enlighten the composition by a beam of the sun let into a darkened room, and then to receive the light reflected from it upon a piece of white paper, observing whether the paper appears white by that reflection; for if it does, it gives proof of the composition’s being white; because when the paper receives the reflection from any coloured body, it looks of that colour. Agreeable to this is the trial he made upon water impregnated with soap, and agitated into a froth[289]: for when this froth after some short time exhibited upon the little bubbles, which composed it, a great variety of colours, though these colours to a spectator at a small distance discover’d themselves distinctly; yet when the eye was so far removed, that each little bubble could no longer be distinguished, the whole froth by the mixture of all these colours appeared intensly white.
19. Our author having fully satisfied himself by these and many other experiments, what the result is of mixing together all the prismatic colours; he proceeds in the next place to examine, whether this appearance of whiteness be raised by the rays of these different kinds acting so, when they meet, upon one another, as to cause each of them to impress the sense of whiteness upon the optic nerve; or whether each ray does not make upon the organ of sight the same impression, as when separate and alone; so that the idea of whiteness is not excited by the impression from any one part of the rays, but results from the mixture of all those different sensations. And that the latter sentiment is the true one, he evinces by undeniable experiments.
20. In particular the foregoing experiment[290], wherein the convex glass was used, furnishes proofs of this: in that when the paper is brought into the situation θ γ η ϰ, beyond, beyond N the colours, that at N disappeared, begin to emerge again; which shews that by mingling at N they did not lose their colorific qualities, though for some reason they lay concealed. This farther appears by that part of the experiment, when the paper, while in the focus, was directed to be enclined different ways; for when the paper was in such a situation, that it must of necessity reflect the rays, which before their arrival at the point N would have given a blue colour, those rays in this very point itself by abounding in the reflected light tinged it with the same colour; so when the paper reflects most copiously the rays, which before they come to the point N exhibit redness, those same rays tincture the light reflected by the paper from that very point with their own proper colour.
21. There is a certain condition relating to sight, which affords an opportunity of examining this still more fully: it is this, that the impressions of light remain some short space upon the eye; as when a burning coal is whirl’d about in a circle, if the motion be very quick, the eye shall not be able to distinguish the coal, but shall see an entire circle of fire. The reason of which appearance is, that the impression made by the coal upon the eye in any one situation is not worn out, before the coal returns again to the same place, and renews the sensation. This gives our author the hint to try, whether these colours might not be transmitted successively to the eye so quick, that no one of the colours should be distinctly perceived, but the mixture of the sensations should produce a uniform whiteness; when the rays could not act upon each other, because they never should meet, but come to the eye one after another. And this thought he executed by the following expedient[291]. He made an instrument in shape like a comb, which he applied near the convex glass, so that by moving it up and down slowly the teeth of it might intercept sometimes one and sometimes another colour; and accordingly the light reflected from the paper, placed at N, should change colour continually. But now when the comb-like instrument was moved very quick, the eye lost all preception of the distinct colours, which came to it from time to time, a perfect whiteness resulting from the mixture of all those distinct impressions in the sensorium. Now in this case there can be no suspicion of the several coloured rays acting upon one another, and making any change in each other’s manner of affecting the eye, seeing they do not so much as meet together there.
22. Our author farther teaches us how to view the spectrum of colours produced in the first experiment with another prism, so that it shall appear to the eye under the shape of a round spot and perfectly white[292]. And in this case if the comb be used to intercept alternately some of the colours, which compose the spectrum, the round spot shall change its colour according to the colours intercepted; but if the comb be moved too swiftly for those changes to be distinctly perceived, the spot shall seem always white, as before[293].
23. Besides this whiteness, which results from an universal composition of all sorts of colours, our author particularly explains the effects of other less compounded mixtures; some of which compound other colours like some of the simple ones, but others produce colours different from any of them. For instance, a mixture of red and yellow compound a colour like in appearance to the orange, which in the spectrum lies between them; as a composition of yellow and blue is made use of in all dyes to make a green. But red and violet purple compounded make purples unlike to any of the prismatic colours, and these joined with yellow or blue make yet new colours. Besides one rule is here to be observed, that when many different colours are mixed, the colour which arises from the mixture grows languid and degenerates into whiteness. So when yellow green and blue are mixed together, the compound will be green; but if to this you add red and purple, the colour shall first grow dull and less vivid, and at length by adding more of these colours it shall turn to whiteness, or some other colour[294].
24. Only here is one thing remarkable of those compounded colours, which are like in appearance to the simple ones; that the simple ones when viewed through a prism shall still retain their colour, but the compounded colours seen through such a glass shall be parted into the simple ones of which they are the aggregate. And for this reason any body illuminated by the simple light shall appear through a prism distinctly, and have its minutest parts observable, as may easily be tried with flies, or other such little bodies, which have very small parts; but the same viewed in this manner when enlighten’d with compounded colours shall appear confused, their smallest parts not being distinguishable. How the prism separates these compounded colours, as likewise how it divides the light of the sun into its colours, has not yet been explained; but is reserved for our third chapter.
25. In the mean time what has been said, I hope, will suffice to give a taste of our author’s way of arguing, and in some measure to illustrate the proposition laid down in this chapter.
26. There are methods of separating the heterogeneous rays of the sun’s light by reflection, which perfectly conspire with and confirm this reasoning. One of which ways may be this. Let A B (in fig. 129) represent the window shutter of a darkened room; C a hole to let in the sun’s rays; D E F, G H I two prisms so applied together, that the sides E F and G I be contiguous, and the sides D F, G H parallel; by this means the light will pass through them without any separation into colours: but if it be afterwards received by a third prism I K L, it shall be divided so as to form upon any white body P Q the usual colours, violet at m, blue at n, green at o, yellow at r, and red at s. But because it never happens that the two adjacent surfaces E F and G I perfectly touch, part only of the light incident upon the surface E F shall be transmitted, and part shall be reflected. Let now the reflected part be received by a fourth prism Δ Θ Λ, and passing through it paint upon a white body Ζ Γ the colours of the prism, red at t, yellow at u, green at w, blue at x, violet at y. If the prisms D E F, G H I be slowly turned about while they remain contiguous, the colours upon the body P Q shall not sensibly change their situation, till such time as the rays become pretty oblique to the surface E F; but then the light incident upon the surface E F shall begin to be wholly reflected. And first of all the violet light shall be wholly reflected, and thereupon will disappear at m, appearing instead thereof at y, and increasing the violet light falling there, the other colours remaining as before. If the prisms D E F, G H I be turned a little farther about, that the incident rays become yet more inclined to the surface E F, the blue shall be totally reflected, and shall disappear in n, but appear at x by making the colour there more intense. And the same may be continued, till all the colours are successively removed from the surface P Q to Ζ Γ. But in any case, suppose when the violet and the blue have forsaken the surface P Q, and appear upon the surface Ζ Γ, Ζ Γ, the green, yellow, and red only remaining upon the surface P Q; if the light be received upon a paper held any where in its whole passage between the light’s coming out of the prisms D E F, G I H and its incidence upon the prism I K L, it shall appear of the colour compounded of all the colours seen upon P Q; and the reflected ray, received upon a piece of white paper held any where between the prisms D E F and Δ Θ Σ shall exhibit the colour compounded of those the surface P Q is deprived of mixed with the sun’s light: whereas before any of the light was reflected from the surface E F, the rays between the prisms G H I and I K L would appear white; as will likewise the reflected ray both before and after the total reflection, provided the difference of refraction by the surfaces D F and D E be inconsiderable. I call here the sun’s light white, as I have all along done; but it is more exact to ascribe to it something of a yellowish tincture, occasioned by the brighter colours abounding in it; which caution is necessary in examining the colours of the reflected beam, when all the violet and blue are in it: for this yellowish turn of the sun’s light causes the blue not to be quite so visible in it, as it should be, were the light perfectly white; but makes the beam of light incline rather towards a pale white.