590 L I (I H T The law of single refraction was put in a form equi valent to this (all but one word) for the first time by Snell in Leyden, some time before 1626. It was first published in 1637 by Descartes, who undoubtedly obtained it from Snell ; but he gave it without any mention of its author. The one word referred to is homogeneous as applied to the incident light. For the fact that white light consists of innumerable different homogeneous constituents, which are separated from one another by refraction, was first established by Newton. We quote his own account of this important discovery from his letter to Oldenburg, dated Feb. , 167^: Newton "To perform my late promise to you, I shall without further on the ceremony acquaint you, that in the year 166(5 (at which time I composi- applied myself to the grinding of optick-glasses of other figures tion of than spherical) I procured me a triangular glass-prism, to try there- white with the celebrated phtenomena of colours. And in order thereto, light. 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 be thereby retracted to the opposite wall. It was at first a very pleasing divertisement, to view the vivid and intense colours 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 refractions, I expected should have been cir cular. 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 semi circular. " Comparing the length of this coloured 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 thickness of the glass, or the termination with shadow or darkness, could have any influence on light to produce such an efFegt : yet I thought it not amiss, first to examine those circum stances, and so tried what would happen by transmitting light through parts of the glass of divers thicknesses, or through holes in the window of divers bignesses, or by setting the prism without, so that the light 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 colours was in all these cases the same. "Then I suspected, whether by any unevenness in the glass, or other contingent irregularity, these colours 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 con trary 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 multi plicity of refractions. The event was, that the light, which by the first prism was diffused into an oblong form, was by the second re duced 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, it was not any contingent irregularity. "I then proceeded to examine more critically, what might be effected by the ditference of the incidence of rays coming from divers parts of the sun ; and to that end, measured the several lines and angles belonging to the image. Its distance from the hole or prism was 22 foot ; its utmost length 13J inches ; its breadth 2| ; the diameter of the hole ^ of an inch. The angle which the rays, tending towards the middle of the image, made with those lines, in which they would have proceeded without refraction, was 44 deg. 56 min. and the vertical angle of the prism 63 deg. 12 min. Also the refractions on both sides the prism, that is, of the incident and emergent rays, were, as near as I could make them, equal ; and consequently about 54 deg. 4 min. And the rays fell perpendicu larly upon the wall. Now subducting the diameter of the hole from the length and breadth of the image, there remains 13 inches in the length, and 2| the breadth, comprehended by those rays which passed through the center of the said hole ; and consequently the angle of the hole, which that breadth subtended, was about 31 min. answerable to the sun s diameter ; but the angle which its length subtend 1, was more than 5 such diameters, namely, 2 deg. 49 min. "Having made these observations, I first computed from them the refractive power of that glass, and found it measured by the ratio of the sines 20 to 31 ; and then by that ratio I computed the refractions of two rays flowing from opposite parts of the sun s discus, so as to differ 31 min. "in their obliquity of incidence, and found that the emergent rays should have comprehended an angle of about 31 min. as they did before they were incident. " But because this computation was founded on the hypothesis of the proportionality of the sines of incidence and refraction, which though by my own experience I could not imagine to be so errone ous, as to make that angle but 31 min. which in reality was 2 deg. 49 min. yet my curiosity caused me again to take my prism : and having placed it at my window, as before, I observed, that by turn ing it a little about its axis to and fro, so as to vary its obliquity to the light, more than an angle of 4 or 5 degrees, the colours were not thereby sensibly translated from their place on the wall ; and consequently by that variation of incidence, the quantity of refrac tion was not sensibly varied. By this experiment, therefore, as well as by the former computation, it was evident, that the differ ence of the incidence of rays, flowing from divers parts of the sun, could not make them after decussation diverge at a sensibly greater angle, than that at which they before converged ; which being at most but about 31 or 32 min. there still remained some other cause to be found out, from whence it could be 2 deg. 49 min. "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 com municated to it by that stroke, its parts, on that side where the motions conspire, must press and beat the contiguous air more vio lently than on the other, and there excite a reluctancy and re-action of the air proportionably 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 aether, 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 betwixt the length of the image and the diameter of the hole, through which the light was transmitted, was proportionable to their distance. " The gradual removal of these suspicions at length led me to the cxperimcntum 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 pur pose, 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 pass 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, suc cessively pass through the hole in it, that I might observe to 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 is not similar orhomogeneal, but consists of difform rays, some of uiliich arc, more refrangible than others ; so that without any difference in their incidence on the same medium, some .shall be more refracted than others ; and therefore that, according to their particular degrees of rcfrangibility , they were transmitted through the prism to divers parts of the opposite wall." The constant ratio mentioned in the above statement of llefrac- the law of refraction is called the refractive index. Its tive numerical value depends upon the nature of the two media, imlex - and also upon the quality of the homogeneous light. It is usually greater for orange light than for red, for yellow than for orange, and so on, so that the violet rays are often called the "more refrangible" rays. 1 The following experimental facts are additions to the law. When refraction takes place from a rarer into a denser medium, the angle of refraction is usually less than that of incidence. If the refractive index for a particular kind of light from a medium A into another B be u., that from B to A is M In other words, a refracted ray may be sent back by the path by which it came. If //.j be the refractive index for a particular ray from A 1 This statement is, however, liable to some very singular excep tions, which will be mentioned later, when we are dealing with anomalous dispersion.
