VELoctrY OF 1.mirr. The fact that time is re quired for the propagation' of whatever it is that causes light has been known since the observation of lotoemer on the eclipses of Jupiter's satellites in 1675. It follows then—granting that light is due to wave-motion—that there is a medium filling all interstellar space, which serves to convey these waves. This medium is called `the ether' (q.v.), and it is evident from the prop erties of transparent bodies that it permeates them. and may in fact be regarded as a universal medium. There is every reason, both theoretical and experimental, for believing that the velocity of waves of all lengths is the same in the pure ether, e.g. in interstellar space: but inside of ordinary matter the velocity of ether-waves varies with the wavelength. and of course their velocity is different in different media. It will be shown below under Refraction that the index of refrac tion of one medium reference to another for a definite color is the ratio of the velocity in the second medium of these ether-waves which correspond to that color to that of the same waves in the first medium. There are two experimental methods for the determination of the velocity of ether-waves in air—or, as often expressed, the `velocity of light' in air. One depends upon the use of a toothed wheel. which is made to revolve rapidly in front of a source of light, thus al lowing intermittent flashes of light to be seen through the teeth. These waves coining through the gaps between the teeth traverse a considerable distance—several miles—fall upon a mirror which reflects them back in their original direc tion. if on their return to the wheel the latter has turned so far that they strike a tooth, they are stopped; if, however, the wheel has turned farther so that they find an opening between the teeth, they will pass through and their return may be observed by suitable means. if the speed of the wheel is thus exactly r'ght, the WIIVOS pass ing out through one opening will return through the next ; if the speed is now increased, the re turning waves will be stopped by a tooth: if the speed is still further increased, the waves passing out through any one opening will return through the next but one, etc. Therefore, if the speed of
the wheel, the size of the teeth, and the distance between the wheel and the mirror are known, the velocity of the waves may be calculated. This method is due to Fizeau and has been used by him, by Cornu, by Forbes and Young, and more recently by Perrotin. The other method consists in allowing light from a narrow slit to fall upon a mirror, he reflected to another distant mirror which reflects it back to the first mirror and thus back to the source. This first mirror is not sta tionary, however, but is made to revolve rapidity; so that, when the waves return from the distant mirror, the angles of incidence and reflection are not quite what they were before, and so the waves are not reflected directly hack to the slit-source, but are deflected slightly. If the amount of this deflection, the distance apart of the two mirrors, and the rate of revolution of the rotary mirror are known, the velocity of the waves may he cal culated. This method is due in part to Fizeau also, but in the main to Fonean1t. It has been used by the latter, by :Michelson. and by New comb. For several reasons this method is not as good as that of the toothed wheel. (See Corm], Reports of International Congress of Physics, Paris, 1900, p. 225.) The accepted value for the velocity of light in air is 300.000 kilometers or 3 X em. per second. with a possible error of less than one part in 1000. This is ahont ISO, GOO miles per second. Foucault showed by direct experiment that the velocity of light in air is greater than that in water, an observation which definitely overthrows Newton's corpuscular theory of light.