REFLEXION AND REFRACTION OF LIGHT. 39 



of the light.* Fraunhofer has likewise compared the light of 

 several of the fixed stars with respect to its refrangibility. No 

 difference whatever was observed, although the method employed 

 was adequate to the detection of a difference so small as the 

 10,000th part of the whole refraction nearly.f 



This remarkable and unexpected result can be reconciled to 

 the theory of emission,:}: as M. Arago has observed, only by the 

 hypothesis already adverted to namely, that the molecules are 

 emitted from the luminous body with various velocities ; but that 

 among these velocities there is but one which is adapted to our 

 organs of vision, and which produces the sensation of light. The 

 wave-theory has been more successful in its explanation. If the 

 ether which encompasses our globe were like its atmosphere, and 

 partook of its motion, the refraction would be precisely the same as 

 if the whole were at rest. This, however, we have seen, cannot be 

 the case ; and the phenomena of aberration compel us to admit 

 that the ethereal medium which encompasses the earth is not dis- 

 placed by its motion. This being assumed, it follows that the ether 

 which is carried along by the refracting medium, is that which con- 

 stitutes the excess of its density above that of the surrounding ether. 

 On this supposition Fresnel has calculated the length of a wave in 

 the moving medium, and thence also the actual change in the 

 direction of the refracted ray produced by the earth's motion. 

 This change is found to be opposite, and exactly equal to that pro- 

 duced by the same cause in the apparent direction of the ray ; so 

 that the ray is actually seen in the same direction as if the earth 

 were at rest, and the apparent refraction is unaltered by the earth's 

 motion. These results, it may be observed, are precisely the same 

 for terrestrial objects, the velocity of wave-propagation being inde- 

 pendent of the motion of the luminous body. 



* Biot, Astronomic Physique, vol. iii. 



f Edinb. Journ. of Science, viii. p. 7. 



J M. Prevost has endeavoured to reconcile the experimental result of M. Arago 

 with the ordinary suppositions of the theory of emission, and to show that a change in 

 the relative velocity in the light of the stars, caused by the motion of the refracting 

 plane, does not affect the refraction in the same manner as an equal change in the 

 absolute velocity. " De 1'Effet duMouvement d'un plan refringcnt sur laEefraction. "- 

 Geneva Memoirs, vol. i. His reasonings do not appear to be conclusive. 



The sine of the change is to the sine of the total deviation of the ray in the ratio 

 of the velocity of the earth to that of light. Fresnel's result is much more com- 

 plicated, but it will be easily seen to reduce itself to this. " Sur I'Infltience du Mouve- 

 ment terrestre dans quelques Phenomenes d'Optique." Annales de Chimie, torn. ix. 



