572 Intelligence and Miscellaneous Articles. 



This result was afterwards compared with those which have been 

 deduced by calculation from the different hypotheses relative to the 

 ather. 



According to the supposition that the sether is entirely free and 

 independent of the motion of bodies, the displacement ought to be 

 null. 



According to the hypothesis which considers the aether united to 

 the molecules of matter in such a way as to participate in its mo- 

 tions, calculation gives for the double displacement the value 0*92. 

 Experiment gave a number only half as great, or 0*46. 



According to the hypothesis by which the sether is partially carried 

 along, the hypothesis of Fresnel, calculation gives 0*40, that is to 

 say, a number very near to that which was found by experiment ; and 

 the difference between the two values would very probably be still 

 less if it had been possible to introduce into the calculation of the 

 velocity of the water a correction which had to be neglected from 

 the want of sufficiently precise data, and which refers to the un- 

 equal velocity of the different threads of fluid ; by estimating the 

 value of that correction in the most probable manner, it is seen that 

 it tends to augment a little the theoretical value and to approach 

 the value of the observed result. 



An experiment similar to that which I have just described had 

 been made previously with air in motion, and I have demonstrated 

 that the motion of the air does not produce any sensible displace- 

 ment in the fringes. In the circumstances in which that experiment 

 was made, and with a velocity of 25 metres a second, which was 

 that of the motion of the air, it is found that according to the hy- 

 pothesis by which the aether is considered to be carried along with 

 the bodies, the double displacement ought to be 0*82. 



According to the hypothesis of Fresnel, the same displacement 

 ought to be only 0*000465, that is to say, entirely imperceptible. 

 Thus the apparent immobility of the fringe in the experiment made 

 with air in motion is completely in accordance with the theory of 

 Fresnel. It was after having demonstrated this negative fact, and 

 while seeking for an explanation by the different hypotheses relating 

 to the aether in such a way as to satisfy at the same time the phae- 

 nomena of aberration and the experiment of M. Arago, that it aji- 

 peared to me to be necessary to admit with Fresnel that the motion 

 of a body occasions an alteration in the velocity of light, and that 

 this alteration of velocity is greater or less for different mediums, 

 according to the energy with which those mediums refract light, so 

 that it is considerable in bodies which are strongly refractive and 

 very feeble in those which refract but little, as the air. It follows 

 from this, that if the fringes are not displaced when light traverses 

 air in motion, there should, on the contrary, be a sensible displace- 

 ment when the experiment is made with water, the index of refrac- 

 tion of which is very much greater than that of air. 



An experiment of M. Babinet, mentioned in the ninth volume of 

 the Comptes Rendus, seems to be opposed to the hypothesis of an 

 alteration of velocity in conformity with the law of Fresnel, But 



