REPORT ON PHYSICAL OPTICS. 313 



emanating from other sources. The experiment was undertaken 

 by M. Arago, at the request of Laplace*. An achromatic 

 prism was attached in front of the object-glass of the telescope 

 of a repeating circle, so as to cover only a portion of the lens. 

 The star being then observed directly through the uncovered 

 part of the lens, and afterwards in the direction in which its 

 light was deviated by the prism, the difference 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 devia- 

 tions ; and the result was the same whatever was the origin of 

 light f . Fraunhofer has likewise compared the light of several 

 of the fixed stars with respect to its refrangibility. No differ- 

 ence 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 J. 



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 displaced by its motion. This being assumed, it 

 follows that the ether which is carried along by the refracting 

 medium, is that which constitutes the excess of its density above 



* The idea of detecting a difference in the velocity of the light of the fixed 

 stars, by its effect upon the amount of refraction, seems to have first occurred 

 to Mr. Michell. Such a difference of velocity, he conceived, must necessarily 

 arise from the different attractions of the stars upon the emitted molecules; and 

 he has computed the diminution of the original velocity of emission arising 

 from this cause. Phil. Trans. 1784. 



t Biot, Astronomie Physique, vol. iii. 



X Edinh. Journ. of Science, viii. p. 7. 



§ 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 of 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 of the absolute velocity, — " Dc I'Effet du Mouvcment d'un plan 

 refiingent sur la Refraction, "Ge/iei»a Memoirs, vol. i. His reasonings do not 

 appear to be conclusive. 



