NATURAL PHILOSOPHY. 123 



hypothesis was proposed by Fresnel, and constructed for the purpose of 

 equally satisfying the phenomena of aberration, and an experiment of 

 Arago, by which it has been proved that the motion of the earth has 

 no influence upon the refraction which the light of the stars suffers in 

 a prism. 



We may determine the value which in each of these hypotheses it is 

 necessary to attribute to the velocity of light in bodies, when the bodies 

 are supposed to be in motion. If the jether is supposed to be wholly 

 carried with the body in motion, the velocity of light ought to be in- 

 creased by the whole velocity of the body, the ray being supposed to have 

 the same direction as the motion. If the aether is not supposed to be 

 free and independent, the velocity of light oiight not to be changed at 

 all. Lastly, if only one part of the asther is carried along, the velocity 

 of light would be increased, but only by a fraction of the velocity of the 

 body, and not, as in the first hypothesis, by the whole velocity. Although 

 the Velocity of light is enormous comparatively to such as we are able 

 to impart to bodies, we are at the present time in possession of means 

 of observation, of such extreme delicacy, that it seems possible to de- 

 termine, by direct experiment, what is the real influence of the motion 

 of bodies upon the velocity of light. We are indebted to M. Arago 

 for a method, based upon the phenomena of interference, which is capa- 

 ble of indicating the most minute variations in the index of refraction 

 of bodies. It is by adopting the same principle, and joining the double 

 tube of M. Arago to the conjugate telescopes, which were employed 

 for determining the absolute velocity of light, that I have been able to 

 study directly, in two mediums, the effects of the motion of a body upon 

 the light which traverses it.* 



I will now endeavor to describe what was the course of the light in 



the experiment. From the focus of a cylindrical lens, the solar rays 



penetrated almost immediately into the first telescope by a lateral open- 



ino- very near to its focus. A transparent mirror, the plane of which 



made an angle of 45 with the axis of the telescope, reflected the rays 



in the direction of the object-glass. On leaving the object-glass, the 



rays, having become parallel among themselves, encountered a double 



chink, each opening of which corresponded to the mouth of one of the 



tubes. A very narrow bundle of rays thus penetrated into each tube, 



and traversed its entire length. The two bundles, always parallel to 



each other, reached the object-glass of the second telescope, were then 



refracted, and by the effect of the refraction reunited again at its focus. 



They there encountered the reflecting plane of a mirror, perpendicular 



to the axis of the telescope, and underwent a reflection back again 



towards the object-glass ; but, by the effects of this reflection, the rays 



had changed their route in such a way, that that which was to the 



right before was to the left after reflection, and vice versa. After having 



again passed the object-glass, and been thus rendered parallel to each 



other, they penetrated a second time into the tubes ; but as they were 



inverted, those which had passed though one tube in going, passed 



through the other on returning. After their second transit through 



* See Annual of Scientific Discovery, 1850, pp. 145-6 ; 1851, 137-8. 



