S\2 FOURTH UKPORT — 1834. 



this fact, in order to be able to decide between them. This 

 seemed to be accomplished by the reasonings of Young. From 

 the laws of interference it appears that homogeneous light, 

 in its progress in space, passes through certain periodically re- 

 turning states, the intervals of which are constant in the same 

 medium ; while in different media they are proportional to the 

 velocities of propagation, since the number of such intervals in 

 a given quantity of light cannot be supposed to vary. Now it 

 followed from the experiments of Newton that the intervals, 

 by which he explained the phenomena of thin plates, were di- 

 miiiished in the denser medium ; and as these intervals have 

 been shown by Young to be identical Mith those deduced from 

 the law of interference, it followed that the velocity of light was 

 slower in the denser medium *. Newtpn had even found the 

 ratio of the magnitudes of the intervals to be the same with that 

 of the sines of incidence and refraction ; and this is precisely as 

 it should be on the principles of the wave-theory. 



But the retardation of light in the denser medium has been 

 directly established by M. Arago. If two pencils be made to 

 interfere and produce fringes, as in the experiment of Fresnel, 

 and if a thin plate of a denser medium be intei-posed in the 

 path of one of them, the whole system of fringes will be shifted 

 to one side or the other, according as the light has been accele- 

 rated or retarded within the plate. The result of this import- 

 ant and decisive experiment was in favour of the theory of 

 waves f. 



The refractive index being equal to the ratio of the velocities 

 of light in the two media, direct or inverse, it follows, which- 

 ever theory we adopt, that any change in the velocity of the in- 

 cident ray must cause a variation in the amount of refraction, 

 unless the velocity of the refracted ray be altered propoi-tionally. 

 Now the relative velocity of the light of a star is altered by the 

 earth's motion ; and the amount of the change is obviously the 

 resolved part of the earth's velocity in the direction of the star. 

 It w^as therefore a matter of much interest to determine how, 

 and in what degree, this change affected the refraction. By the 

 observation of this effect, it was hoped, we should have an easy 

 and accurate method of determining the constant of aberration ; 

 Ave should be enabled to compare the light of different stars, 

 and detect any difference which might exist in their velocities ; 

 and lastly, we might compare these velocities with that of light 



* " Experiments and Calculations relative to Physical Optics," Phil. Trans. 

 1803. 



t Amuiles de Cliimie, lorn. i. See also the account of Mr. Potter's rcfcti- 

 tion of this experiment, Phil. Mag., vol. iii. p. 333. 



