36 REFLEXION AND REFRACTION. 



tion in which its light was deviated by the prism, the dif- 

 ference 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. 



The experiment of Arago is, however, illusory. The eye 

 itself is in movement, and the aberration thus produced has 

 been shown to be equal and opposite to the change produced 

 by refraction. 



(44) M. Fizeau has attempted by other means to deter- 

 mine the effect of the motion of the refracting medium upon 

 the velocity of propagation. Two long tubes of equal length, 

 filled with water, are interposed in the paths of two interfer- 

 ing pencils, and the place of the fringes observed. A rapid 

 movement is then communicated to the water in one of the 

 tubes ; and it is observed that the fringes are displaced. 

 The amount of the displacement is, in truth, very small 

 about half the breadth of a fringe in the experiments of 

 M. Fizeau. But its magnitude and direction accord suffi- 

 ciently with the results of theory, and with the hypothesis of 

 Fresnel on the relation between the velocity of the body, and 

 that of the ether connected with it. 



