38 REPORT ON PHYSICAL OPTICS. 



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

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

 if a thin plate of a denser medium be interposed 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 accelerated or retarded 

 within the plate. The result of this important and decisive expe- 

 riment was in favour of the theory of waves.* 



The refractive index being equal to the ratio of the velocities of 

 light in the two media, direct or inverse, it follows, whichever 

 theory we adopt, that any change in the velocity of the incident 

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

 velocity of the refracted ray be altered proportionally. 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 was 

 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 ; we 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 emanating from 

 other sources. The experiment was undertaken by M. Arago, at 

 the request of Laplace, f An achromatic prism was attached in 

 front of the object-glass of the telescope of a repeating circle, so as 

 to cover only a portion of the lens. The star being then observed 

 directly through the uncovered part of the lens, and afterwards in 

 the direction in which its light was deviated by the prism, the 

 difference of the angles read off gave the deviation. The stars 

 selected for observation were those in the ecliptic, which passed the 

 meridian nearly at 6 A.M. and 6 P.M., the velocity of the earth 

 being added to that of the star in the former case, and subtracted 

 from it in the latter. No difference whatever was observed in the 

 deviations ; and the result was the same whatever was the origin 



* Amulet de Chimie, torn. i. See also the account of Mr. Potter's repetition of 

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



t The idea of detecting a difference in the velocity of the light of the fixed stars, 

 by its effect upon the amount of refraction, seems to have first occurred to Mr. Michell. 

 Such a difference of velocity, he conceived, must, necessarily arise from the different 

 attractions of the stars upon the emitted molecules, and he has computed the dimi- 

 nution of the original velocity of emission arising from this cause. Phil.Traiu., 1784.. 



