ISAAC NEWTON

 

1642-1727

 

NEWTON'S FIRST public paper tells of a series of experiments he started at the age of twenty-four, the results of which would have made him famous had he accomplished nothing more. A New Theory of Light and Colors shows his power to make a few simple experiments reveal long-hidden secrets of nature. He reports how he set up a prism to see the "celebrated phenomena of colors," perhaps for amusement; how he noticed that the patch of colors thrown on the wall was not shaped like the round spot of light in the prism, but very much longer; how lie compared this appearance with the known laws of light, and found, finally, that the red light was always re­fracted less than the yellow, the yellow than the green, the green than the blue or violet, and that the original white light was actually a combination of all the colored lights in the spectrum.

The paper was read in 1672 before the Royal Society of London and published in English in that year in the Philosophical Transactions. During the next few years that journal devoted much space to articles in support and rebuttal of the new theory. But it stands today still correct, save that we now know that light exhibits properties of waves as well as those of corpuscles.

 

 

NEW THEORY OF LIGHT AND COLORS

 

Sir:

To perform my late promise to you, I shall without further ceremony acquaint you that in the beginning of the year 1666 (at which time I applied myself to the grinding of optick glasses of other fig­ures than spherical), I procured me a triangular glass prism, to try therewith the celebrated phenomena of colors: And in order thereto having darkened my chamber, and made a small hole in my window. shuts, to let in a convenient quantity of the sun's light, I placed my prism at its entrance, that it might thereby be refracted to the oppo­site wall. It was at first a very pleasing divertisement to view the vivid and intense colors produced thereby; but after a while applying myself to consider them more circumspectly, I became surprised to see them in an oblong form; which, according to the received laws of Refraction, I expected should have been circular.

They were terminated at the sides with straight lines, but at the ends the decay of light was so gradual that it was difficult to de­termine justly what was their figure; yet they seemed semicircular.

Comparing the length of this colored spectrum with its breadth, I found it about five times greater; a disproportion so extravagant that it excited me to a more than ordinary curiosity of examining from whence it might proceed. I could scarce think that the various thicknesses of the glass, or the termination with shadow or dark­ness could have any influence on light to produce such an effect; yet I thought it not amiss, first to examine those circumstances, and so tried what would happen by transmitting light through parts of the glass of divers thicknesses, or through hole. In the window of divers bignesses, or by setting the prism without so that the tight might pass through it, and be refracted before it was terminated by the hole. But I found none of those circumstances material. The fashion of the colors was in all these cases the same.

Then I suspected, whether by any unevenness in the glass or other contingent irregularity, these colors might be thus dilated. And to try this, I took another prism like the former, and so placed it that the light, passing through them both, might be refracted contrary ways, and so by the latter returned into that course from which the former had diverted it. For by this means I thought the regular effects of the first prism would be destroyed by the second prism, but the irregular ones more augmented by the multiplicity of the refractions. The event was that the light, which by the first prism was diffused into an oblong form, was by the second reduced into an orbicular one with as much regularity as when it did not at all pass through them. So that, whatever was the cause of that length, 'twas not any contingent irregularity . . . .

(Newton then attempted to explain the oblong shape of the colored light spectrum by the fact that light is bent when it enters or leaves a dense medium. But he found the length of the oblong to be far greater than could be explained in this way.)

Then I began to suspect whether the rays, after their trajection through the prism did not move in curve lines, and according to their more or less curvity tend to divers parts of the wall. And it increased my suspicion when I remembered that I had often seen a tennis ball, struck with an oblique racket, describe such a curve line. For, a circular as well as a progressive motion being communicated to it by that stroke, its parts on that side where the mo­tions conspire must press and beat the contiguous air more violently than on the other, and there excite a reluctancy and reaction of the air proportionately greater. And for the same reason, if the rays of light should possibly be globular bodies, and by their oblique pass­age out of one medium into another acquire a circulating motion, they ought to feel the greater resistance from the ambient ether, on that side, where the motions conspire, and thence be continually bowed to the other. But notwithstanding this plausible ground of suspicion, when I came to examine it, I could observe no such curvity in them. And besides (which was enough for my purpose) I observed that the difference 'twixt the length of the image and diameter of the hole, through which the light was transmitted, was proportionable to their distance.

Newton's experiment on bending colored light. When Newton turned the first prism, light of each color came to the slit in the board, The second prism caused no further color change, but red light was bent least far up the wall. Green light was bent more than yellow, and extreme blue was bent most of all.

 

The gradual removal of these suspicions at length led me to the Experimentum Crucis, which was this: I took two boards and placed one of them close behind the prism at the window, so that the light might pass through a small hole made in it for the purpose and fall on the other board, which I placed at about 12 feet distance, having first made a small hole in it also, for some of that incident light to pass through. Then I placed another prism behind this second board, so that the light, trajected through both the boards might lass through that also, and be again refracted before it arrived at the wall. This done, I took the first prism in my hand and turned it to and fro slowly about its axis, so much as to make the several parts of the image cast on the second board successively pass through the hole in it, that I might observe what places on the wall the second prism would refract them. And I saw by the variation of those places that the light, tending to that end of the image, towards which the refraction of the first prism was made, did in the second prism suffer a refraction considerably greater than the light tending to the other end. And so the true cause of the length of that image was detected to be no other than that light consists of rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

When I understood this, I left off my aforesaid glass works, for I saw that the perfection of telescopes was hitherto limited, not so much for want of glasses truly figured according to the prescriptions of optick authors (which all men have hitherto imagined) as because that light itself is a heterogeneous mixture of differently refrangible rays. So that, were a glass so exactly figured as to collect any one sort of rays into one point, it could not collect those also into the same point, which having the same incidence upon the same medium are apt to suffer a different refraction. Nay, I wondered that, seeing the difference of refrangibility was so great as I have it, telescopes should arrive to that perfection they are now at . . . .

But to return from this digression, I told you that light is not similar or homogeneal, but consists of difform rays, some of which are more refrangible than others: so that of those which are alike incident on the same medium, some shall be more refracted than others, and that not by any virtue of the glass, or other external cause, but from a predisposition, which every particular ray hath to suffer a particular degree of refraction.

I shall now proceed to acquaint you with another more notable difformity in its rays, wherein the origin of colors is unfolded; concerning which I shall lay down the doctrine first, and then, for its examination, give you an instance or two of the experiments as s specimen of the rest.

The doctrine you will find comprehended arid illustrated in the following propositions.

1. As the rays of light differ in degrees of refrangibility so they also differ in their disposition to exhibit this or that particular color. Colors are not qualifications of light, derived from refractions, or reflections of natural bodies (as 'tis generally believed), but original and connate properties, which in divers rays are diverse. Some rays are disposed to exhibit a red color and no other; some a yellow and no other, some a green and no other, and so of the rest. Nor are there only rays proper and particular to the more eminent colors, but even to all their intermediate gradations.

2. To the same degree of refrangibility ever belongs the same color, and to the same color ever belongs the,same degree of re­frangibility. The least refrangible rays are all disposed to exhibit a red color, and contrarily those rays which are;disposed to exhibit a red color, are all the least refrangible. So the most refrangible rays are disposed to exhibit a deep violet color, Ad contrarily those which are apt to exhibit such a color are the most refrangible. And so to all the intermediate colors in a continued series belong inter­mediate degrees of refrangibility. And this analogy 'twixt colors and refrangibility is very precise and strict; the rays Always either exactly agreeing in both, or proportionally disagreeing in both.

3. The species of color and degree of refrangibility proper to any particular sort of rays is not mutable by refraction, nor by reflection from natural bodies, nor by any other cause that I could yet observe. When any one sort of rays bath been well parted from those of other kinds, it bath afterwards obstinately retained its color, not­withstanding my utmost endeavors to change it. I have refracted it with prisms, and reflected it with bodies, which in daylight were of other colors; I have intercepted it with the colored film of air interceding two compressed plates of glass; transmitted it through colored mediums, and through mediums irradiated with other sorts of rays, and diversely terminated it; and yet could never product any new color out of it. It would by contracting or dilating become more brisk, or faint, ,and by the loss of many rays, in some cases very obscure and dark; but I could never see it changed in specie (in kind.

4. Yet seeming transmutations of colors may be made, where there is any mixture of divers sorts of rays. For in such mixtures, the component colors appear not, but, by their mutual allaying each other, constitute a middling color. And therefore, if by refraction, or any other of the aforesaid causes, the difform rays, latent in such a mixture, be separated, there shall emerge colors different from the color of the composition. Which colors are not new generated, but only made apparent by being parted; for if they be again entirely mixt and blended together, they will again compose that color which they, did before separation. And for the same reason, transmutations: made by the convening of divers colors are not real; for when the difform rays are again severed, they will exhibit the very same colors which they did before they entered the composition; as you see, blue and yellow powders, when finely mixed, appear to the naked eye green, and yet the colors of the component corpuscles are not thereby really transmuted, but only blended. For, when viewed with a good microscope, they still appear blue and yellow interspersedly.

5. There are therefore two sorts of colors. The one original and simple, the other compounded of these. The original or primary colors are red, yellow, green, blue, and a violet-purple, together with orange, indigo, and an indefinite variety of intermediate gradations.

6. The same colors in specie with these primary ones may be also produced by composition: for a mixture of yellow and blue makes green; of red and yellow makes orange; of orange and yellowish green makes yellow. And in general, if any two colors be mixed which in the series of those generated by the prism are not too far distant one from another, they by their mutual alloy compound that color which in the said series appeareth in the midway between them. But those which are situated at too great a distance do not so. Orange and indigo produce not the intermediate green, not scarlet and green the intermediate yellow.

7. But the most surprising and wonderful composition was that of whiteness. There is no one sort of rays which alone can exhibit this. 'Tis ever compounded; and to its composition are requisite all the aforesaid primary colors, mixed in a due proportion. I have often with admiration beheld that all the colors of the prism being made to converge, and thereby to be again mixed as they were in the light before it was incident upon the prism, reproduced light, entirely and perfectly white, and not at all sensibly differing from a direct light of the sun, unless when the glasses I used were not sufficiently clear; for then they would a little incline it to their color.

8. Hence, therefore, it comes to pass that whiteness is the usual color of light; for light is a confused aggregate of rays indued with all sorts of colors, as they are promiscuously darted from the various parts of luminous bodies. And of such a confused aggregate, as I said, is generated whiteness, if there be a due proportion of the ingredients; but if any one predominate the light must incline to that color; as it happens in the blue flame of brimstone, the yellow flame of candle, and the various colors of the fixed stars.

9. These things considered, the manner how colors are produced by the prism is evident. For of the rays constituting the incident light, since those which differ in color proportionally differ in re­frangibility, they by their unequal retractions mast be severed and dispersed into an oblong form in an orderly succession from the least refracted scarlet to the most refracted violet. And for the same reason it is that objects, when looked upon through a prism, appear colored. For the difform rays, by their unequal refractions, are made to diverge toward several parts of the retina, and these express the images of things colored, as in the former case they did the sun's irnagc upon a wall. And by this inequality of retractions they be­come not only colored, but also very confused and indistinct.

10. Why the colors of the rainbow appear in falling drops of rain, is also from hence evident. For those drops which refract the rays disposed to appear purple, in greatest quantity to the spectator's eye, refract the rays of other sorts so much less, as to make them pass beside it; and such are the drops on the inside of the primary bow, and on the outside of the secondary or exterior one. So those drops which refract in greatest plenty the rays apt to appear red, to the spectator's eye, refract those of other sorts so much more as to make them pass beside it; and such are the drops on the exterior part of the primary, and interior part of the secondary bow.

11. The odd phenomena of an infusion of Lignum Nephriticum, leaf gold, fragments of colored glass, and some other transparently colored bodies, appearing in one position of one  color, and of an, other in another, are on these grounds no longer riddles. For those are substances apt to reflect one sort of light and transmit another; as may be seen in a dark room, by illuminating them with similar or uncompounded light. For then they appear of that color only, accordingly as they are disposed more or less to reflect or transmit the incident color.

12. From hence also is manifest the reason of an unexpected experiment, which Mr. Hook somewhere in his Micrography re­lates to have made with two wedge-like transparent vessels, tilled the one with a red, the other with a blue liquor: namely, that though they were severally transparent enough, yet both together became opaque. For, if one transmitted only red, and the other only blue, no rays could pass through both.

13. I might add more instances of this nature, but I shall con elude with this general one, that the colors of all natural bodies have no other origin than this, that they are variously qualified to reflect one sort of light in greater plenty than another. And this I have experimented in a dark room by illuminating those bodies with un­compounded light of divers colors. For by that means any body may be made to appear of any color . . . .

This, I conceive, is enough for an introduction to experiments of this kind which if any of the Royal Society shall be so curious as to prosecute, I should be very glad to be informed with what success; that if anything seem to be defective or to thwart this relation, I may have an opportunity of giving further direction about it, or of acknowledging my errors, if I have committed any.