LIGHT 609 it, and the undulation would be reversed, a rarefaction returning in place of a condensation ; and this will perhaps be found most con sistent with the phenomena." This idea, of a rarefaction returning by reflexion when a condensation is incident, is equivalent to a loss or gain of half a wave-length when light in a denser body is reflected at the surface of a rarer body. Whether, then, the plate be denser or rarer than the medium surrounding it, one or other of the two interfering rays loses half an undulation more than the other in the mere act of reflexion. This completely removes the difficulty. But Young went farther, and pointed out that if a thin plate be interposed between two media, one rarer, the other denser than the plate, this half wave-length effect should disappear. He verified this conjecture by direct experiment, founded on a modification of a process due to Newton. ewton s Newton had, long before, devised and carefully employed n s s - an excessively ingenious (because extremely simple and effective) method of studying the colours of thin plates. It consisted merely in laying a lens of long focus on a flat plate of glass. The film of air or other fluid between the spherical surface and its tangent plane has a thickness which is directly proportional to the square of the distance from the point of contact. When such an arrangement is looked at in homogeneous light, the lens having been pressed into contact with the flat plate, there is seen a central black spot, surrounded by successive bright and dark rings, whose number appears to be practically unlimited. The radii of the successive bright rings were found by Newton to be as the square roots of the odd numbers 1, 3, 5, &c. Hence the thicknesses of the film of air are directly as these numbers. When rays of higher refrangibility are used the rings diminish in diameter. Hence when white light is employed we have a superposition of coloured rings of all sizes, but it is no longer possible to trace more than four or five alternations of bright and dark rings the colours being then more and more compound. This series of coloured rings is named after Newton, and the successive colours, gradually more and more composite, form Newton s scale of colours. Thus we read, in books more than thirty years old, of a red or blue of the third order, meaning those colours as seen in the third bright ring round the central dark spot. oloursof Many of the most vivid colours of natural and artificial rooved bodies are due to one or other of the forms of interference maces. we nave r0 ughly explained. Thus Barton s buttons (once employed for ornament as they produce an effect very similar to that of diamonds) were simply polished metal plates stamped by a die of hardened steel, on whose surface a pattern had been engraved consisting of small areas ruled in different directions with close equidistant parallel grooves. That the colours of a pearl and of mother-of- pearl are due to a similar surface corrugation was proved by Bre vster, who took impressions from such substances in black wax, and found that it was thus rendered capable of giving the same play of colours. The scales from the wings of butterflies owe their bright colours to a delicate Thin ribbed structure. On the other hand, the thin transparent plates, wings of the house-fly, earwig, &c., owe their colours to their thinness. The same is true of the temper colour of steel, Xobili s rings, &c. Very beautiful examples of thin plates scaled off from decayed glass (found in Roman exca vations) have been figured, with their play of colours, by Brevvster. 1 Refrac- Here we can only say a word or two about the probable tive in- relation between the wave-length of homogeneous light . e r x "* , and its refractive index for any isotropic medium. The wave- existence of dispersion was attributed by Cauchy to the length, fact that even the most homogeneous media, such as water, Trans. Roy. Soc. Edin., 1861. have grained or heterogeneous structure of dimensions not incomparably smaller than the average length of a wave of light. This grained structure has been recently proved to exist, by several perfectly independent processes arising from totally unconnected branches of physics ; and its dimensions have been assigned, at least in a roughly approximate manner. See ATOM, and CONSTITUTION OF BODIES. It appears from the theory of disturbances in such a medium that the velocity of a ray depends upon its wave length in a manner which is expressed by a series of even inverse powers of that wave-length. Hence we have a relation such as in which, from our present ignorance of the precise con nexion between matter and ether, we must be content to find the multipliers of the various terms by direct measure ment. If we neglect all but the first two terms, we may determine a and /? from the known wave-lengths of two of Fraunhofer s lines, and their refractive indices for a particular medium. We can then test the accuracy of the formula by its agreement with the corresponding numbers in the same medium for others of the fixed lines. Thus, taking the data for water given above, we have, from the numbers for the two hydrogen- lines C and F, the values a = l-3243, = 0-00000000319. Calculating from these, and the wave-length of H, we have for its refractive index 1 3447, instead of 1-3442 as deter mined by Fraunhofer. So far as we may trust this theory, which certainly accords fairly with the experimental data for substances of moderate dispersive powers, though by no means well with those for substances of high dis persive power such as oil of cassia, the value of the quantity a is the refractive index for the longest jwssz 6/e waves ; i.e., it is that of the inferior limit of the spectrum. DOUBLE REFRACTION. We now come to phenomena Double which cannot be even roughly explained by processes refrac- based on the vague analogies of sound and water waves tion - which have hitherto sufficed for our elementary treatment of the subject. These phenomena were first observed in Iceland spar. Iceland They were described in a general way by Bartholinus, who s P ar - showed that one of the two rays into which a single incident ray is divided by this substance follows the ordinary law of refraction. Huygens, who studied the subject only eight years later, verified the greater part of the results of Bartholinus, and added many new ones. From his point of view it was of course obvious that the ordinary ray is propagated by spherical waves, i.e., its velocity is the same in all directions inside the crystal. To explain the extraordinary ray, he assumed that it was Wave- propagated in waves of the form of an ellipsoid of revolu- surface tion, the simplest assumption he could make. To test its of extn accuracy he first noticed that a rhombohedral crystal of Iceland spar behaves in precisely the same way whichever pair of parallel faces light passes through. Hence he acutely concluded that the axes of the ellipsoids of revolu tion (if such were the form of the waves for the extra ordinary ray) must be symmetrically situated with regard to each of these planes. The only such lines in a rhombo- hedron are parallel to that which joins those corners which are formed by the meeting of three equal plane angles. In the case of Iceland spar these equal angles are obtuse. Huygens then verified, by experiments well contrived, though carried out by a very rough mode of measurement, the general agreement of his hypothesis with the fact ; and he further tested it by comparing its indications as to the position of the two images for any position of the crystal XIV. - 77
