Intelligence and Miscellaneous Articles. 569 



Or, lastly, according to a third hypothesis, which includes both 

 the former ones, only a portion of the aether is free, the other por- 

 tion being attached to the molecules of bodies and participating in 

 their motion. 



This latter hypothesis was proposed by Fresnel, and con- 

 structed for the purpose of equally satisfying the phsenomena of 

 aberration, and a celebrated experiment of M. 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 

 IV is necessary to attribute to the velocity of light in bodies )yljen 

 the bodies are supposed to be in motion. u ^*i 



If the aether is supposed to be wholly carried along with the't)0^ 

 in motion, the velocity of light ought to be increased by the whole 

 velocity of the body, the ray being supposed to have the same direc^ 

 tion as the motion. ' j 



If the aether is supposed to be free and independent, the veloci^ 

 of light ought not to be changed at all. v- 



Lastly, if only one part of the aether is carrie^ 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. 

 This consequence is not so obvious as the former, but Fresnel has 

 shown that it may be supported by mechanical arguments of great 

 probability. 



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 to me to be possible to determine by a 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 phae- 

 nomena of interference, which is capable of indicating the most 

 minute variations in the indexes of refraction of bodies. The expe- 

 riments of MM. Arago and Fresnel upon the difference between the 

 refractions of dry and moist air, have proved the extraordinary 

 sensibility of that means of observation. 



It is by adopting the same principle, and joining the double tube 

 of M. Arago to the conjugate telescopes which I employed for deter- 

 mining 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 attempt to describe, without the aid of a diagram, 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 opening 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 cor- 

 responded to the mouth of one of the tubes. A very narrow bundle 



