IQO M. F. Eisenlohr on the Relation between the direction of 



case also, the effect of the longitudmal vibrations will be even 

 still more evident. The formula becomes 



tany^ ^-^ ^ (2) 



2Vsin^- 

 1+- 



X2 



From what precedes, it will be seen how peculiar an interest 

 the experiments referred to possess, even though they do not 

 afford so direct a solution as had been anticipated of the problem 

 immediately proposed. They are, in fact, the best means of 

 testing the existence of longitudinal vibrations in light ; and it is 

 impossible to exaggerate the importance to science of a thorough 

 and varied experimental investigation of these ph?enomena, since, 

 besides the means they afford of coming to a conclusion on the 

 soundness of the hypotheses here advanced, they throw light on 

 this question of the existence of longitudinal vibrations. To 

 return, however, to the point which gave rise to these experi- 

 ments. I may observe that the view which the above formula 

 involves, viz. that the aether vibrations in polarized light are per- 

 pendicular to the plane of polarization, receives so much support, 

 not only from the agreement above pointed out between the result 

 calculated from that formula and the experiments, but also from 

 other considerations, that we may, I think, consider it as proved. 



I have already mentioned, with reference to this subject, that 

 the remarkable confirmation which the calculations of Cauchy 

 received from the experiments of Jamin, is a striking evidence in 

 favour of the view concerning the direction of the tether vibi'a- 

 tions which the former propounded as the foundation of his cal- 

 culations, and which is the very one above stated. Another 

 evidence is to be found in an old theoretical work of Fresnel, 

 which unfortunately is too little known. 



From the phrenomenon of aberration, we have reason to believe 

 that the aether which surrounds the earth does not partake of its 

 motion. As, therefore, the light of a star which lies in the 

 direction of this motion undergoes some alteration on this 

 account in the rapidity of its propagation relatively to the earth, 

 as well in empty space as in the substance of bodies themselves, 

 and this, too, generally in different degrees, we might expect an 

 alteration in the refractive powers of transparent bodies for such 

 light, so that the image of a star refracted through a prism would 

 be more or less displaced. This displacement must, it would 



seem, depend on the first power of — , where c represents the ra- 

 pidity of the earth's motion, and w the rate of propagation of 

 light ; it must therefore, like aberration, be measurable, and be of 



