REFRACTION 477 REFRACTION ray, and is therefore in the "plane of incidence," and the sine of the angle of incidence bears to the sine of the angle of refraction a ratio which remains con- stant, for any two media, whatever be the angle of incidence. The observed fact that light is dif- ferently bent in its course by different refracting media shows that there is a difference between bodies in their power of receiving light through their bound- ing surface. Newton, in accordance with his corpuscular theory, interpreted this as showing that when the luminous corpuscles come very near the surface of a denser substance they are as it were jerked or made to swerve out of an oblique path and hurried in by the at- traction of the denser substance so as to enter that substance more directly; and that when the light quits the denser substance it is retarded by a similar at- traction. The consequence of this would be that light would travel in the denser medium perhaps not appreciably faster than in air, but with a mean velocity certainly not less. On the undulatory theory, however, refraction is a neces- sary consequence of a slower travel of ether-disturbances in the denser medium. When a spherical wave impinges on a plane surface it is modified into a hyper- boloid, the center of curvature of the cen- tral portion of which is farther away than or nearer than the center of the sphere in the ratio of the refractive in- dex of the second medium to that of the first. An eye within a rarer medium will thus see the image of a point situ- ated within the denser medium as if it were nearer than it really is;^ hence a stick appears bent when partly immersed obliquely in water; and, owing to differ- ences in the amount of refraction at dif- ferent angles, the bottom of a tank looked down upon appears sunk in the middle. Why ether disturbances of differing wave lengths are differently refracted in such a medium as glass is not yet per- fectly clear. The fact that ether dis- turbances of grreater frequencies are propagated more slowly through opti- cally denser matter may be fairly in- ferred to arise from a mutual interac- tion of the ether, periodically stressed and released, and the matter amid whose molecules the disturbance is propagated. The question is complicated by the down- right absorption or non-transmission of many particular wave lengths, and by the peculiar behavior of some particular transparent substances which produce "anomalous dispersion"; for example, iodine vapor refracts red light more than blue, and blue more than violet; and
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fuchsine refracts blue and violet light less than it does red, orange, and yellow, while it absorbs the rest. Further, it is found that in these cases of anomalous dispersion the substance generally has in the solid form a surface color different from that seen through its solution; and there are always absorption bands, on the red side of which the refrangibility is increased, while on the other side it is diminished, as if the molecules them- selves took up oscillations of particular periods and hurried on the propagation of slightly slower or retarded that of slightly more rapid oscillations of the ether. It appears as if this kind of ac- tion were never wholly absent; the spec- trum produced by a prism never wholly coincides with the diffraction spectrum in which the deviation for each wave length depends directly on the wave length it- self; and the spectrum produced by a prism say of crown glass does not ex- actly coincide in its visible distribution of colors with a spectrum of equal length made by a flint-glass prism. This is called the "irrationality of dispersion.'* If we take two prisms, one of crown, the other of flint-glass and pass a beam of light through; then, if the angles of these prisms be suitable, the rays dis- persed by the one will be collected by the other, and there will on the whole be deviation wdthout dispersion; but not absolutely so, on account of the irration- ality of dispersion of both prisms, the effect of which is that a calculated ratio of angles and refractive indices which will cause deviation without dispersion for any given pair of wave lengths will, to a very slight extent in most cases, fail to do so for the other wave lengths present in the mixed light transmitted through the system. By the use of three prisms three wave lengths may similarly be achromatized. Double Refraction. — The wave surface developed when a disturbance originates at a point in a homogeneous mediurn, like glass, is spherical in form. In uni- axial crystals the disturbance _ travels with two wave fronts, one spherical, the other ellipsoidal; and the two wave fronts are coincident along the directioa of the optic axis. Of such crystals some are "positive," such as quartz and ice, and in these the sphere incloses the el- lipsoid; in "negative" crystals, such as Iceland spar and tourmaline, the ellip- soid incloses the sphere. In biaxial crystals the three optical axes are dis- similar, and the wave surfaces become complex; there are two refracted rays. If a doubly refracting substance be put between two crossed Nicol's prisms light passes; and by this means it is found 31
