Extreme Red, the wavelength in air is 0.0000266 " Violet, 0.0000167 These are, roughly, the 40100 and the of an inch. Seeing, then, that light describes 200,000 m. per second, the number of waves which enter the eye per second are: Extreme Red 460 millions of millions.
" Violet 730 " These numbers, compared with those of sonorous waves (see Souxn), show the extra ordinary difference in delicacy between the optic and auditory nerves. But whereas the range of the car is somewnere about 12 octaves, that of the eye is less than one.
has been already illustrated in a previous volume.
Dispersion. —We have just seen that, by Fresnel's interference experiment, waves of different length are separated (for in the last figure the position of the bright line, V, dependS on the length of the waves which produce it). But the different colors are also separated by common refraction, as iu Newton's celebrated experiment. Site SPECTRUM. This shows, of course, that in refracting media, waves of different colors move with different velocities; and, as the violet arc more retracted than the red, it appears that the shorter waves move more slowly in glass or water than the longer ones. in free space, waves of all lengths travel with equal speed, else (see ABERRATION) all stars ought to appear drawn out into spectra, in consequence of the earth's animal motion. Also. a star suddenly breaking out, or suddenly vanishing (a phenomenon several times observed), should flash out first red, and gradually become white, or should gradually decay from white to violet., which is not observed to be the case. These facts are the most difficult to explain of any to which the undulatory theory has .•et been applied. Fresnel, in deed, appears to have been in possession of a solution of the difficulty, but the appendix to one of his paperA, to which lie more than once refers as containing this explanation, was not found among his MSS. Cauchy and others have, however, by delicate iuvesti gatious. shown that, if the forces exerted by the molecules of a reputing body on the ether are exerted through distances comparable with the length of a WaVe, the velocity of light will then depend on the The velocity is, in fact, shown to be represented by a formula such as this: • A— where A and B arc constant quantities for a given medium, and A. is the length of a wave, The larger A is, the less is the second term of the formula, and therefore the velocity is the greater. A very singular result follows from this formula—viz., that the velocity becomes more and more nearly equal to A. as the wave length is greater. Hence, waves of low radiant heat, which (see HEAT) arc merely waves of light which are incapable of produc ing vision, must be crowded together toward a limit, not very far beyond the red end of the spectrum.
Poittrization.—We now come to a set of phenomena which give us some further infor mation as to the nature of huniniferous waves. When two beams of light, such as those in Fresael's experiment, are polarized in planes perpendicular to each other (see POLAR IZATION) before they meet, they do not interfere. This is in accordance with the assump
tion required for the explanation of the existence of polarization itself—viz., that the vibrations of the ether take place t•ansrersely to the direction of the ray.
Double assumptions, forced upon us by experimental results, are now far complete that we may proceed. after Fresno], to apply them to the explanation of double refraction. See POLARIZATION; REFRACTION, DOUBLE. This explanation is extremely beautiful, cud when published, was justly hailed as the greatest step in physical science which had been made' since Newton deduced the facts of physical astronomy front the law of gravitation. • As we have seen above, in treating of simple reflection and refraction, that the form and velocity in and with which a disturbance spreads from any point of a wave, is all that is required for the determination of the course of a ray, we must endeavor to find the form iu which a disturbance spreads in a double-refracting crystal; and this should lead us to a construction for each of the two rays.
Huygens had already pointed out that one of the two rays produced by Iceland spar follows the ordinary law of refraction. Hence the disturbances which give rise to this ray are propagated in spherical waves in the crystal. He showed also that the other ray could be accounted for, if the disturbances to which it is due were propagated in the form of an oblate spheroid touching the sphere with the extremities of its axis, that axis being parallel to the crystallographic axis of the mineral. The following dia gram (fig. 6) will make this clear: P is the point where the ether is disturbed. Two waves spread from P in the form shown in the cut, the line ABP being the axis of rota tion of the spheroid, and parallel to the axis of the crystal. Thus, let rays aA, etc. (11g. 7), of which AB is the wave-front, fall upon the surface Ab of such a crystal; and let AC be the direction of its axis. Draw, about A as a center, the sphere and spheroid into which the disturbance at A spreads in the crystal while light in air passes from B to b. Then if planes be drawn through the line b (perpendicular to the paper) so as to touch the sphere in and the spheroid in these planes will touch respectively all the intermediate spheres and spheroids produced by disturbances at points between A and b. [This is evident from simple geometry.] Thus, and are the new wave fronts; and the ray ai, falling on the crystal, is divided. into the two and Of these, Afi, is the ordinary ray, and being produced by spherical waves, has all the properties of a ray ordinarily refracted. It obviously moves perpendicularly to its front, as is perpendicular to But it is otherwise with which is, in general, not perpendicular to itafront, fi,b. Again, if AC, the axis of the crystal, be not in the plane of incidence, the ray is not in that plane; so that here we have refraction out of the plane of incidence.