ETHER. 



coincided with the object, it could not be observed directly, but by 

 the penal by partial reflection at a plane surface of glass, it was 

 nd that Ue image of the finest spider line could be distinctly seen, though 

 light which Conned the image had passed twice through three prisms of 

 Tne appaimtua was fiwt turned so that the direction of the light in first 

 ^ rf through the second prism was that of the earth's motion in its orbit. 

 He apparatus WM afterwards placed so that the direction of the light was 

 opposite to that of the earth's motion. If the deviation of the ray by the 

 WM increased or diminished for this reason in the first journey, it would 



be diminished or increased in the return journey, and the image would appear 

 i one side of the object When the apparatus was turned round it would 

 appear on the other side. The experiment was tried at different times of the 

 rear, hut only negative results were obtained. We cannot, however, conclude 

 absolutely from this experiment that the sether near the surface of the earth 

 is earned along with the earth in its orbit, for it has been shown by Professor 

 Stokes* that according to Fresnel's hypothesis the relative velocity of the aether 

 within tlif prism would be to that of the aether outside inversely as the 

 aquare of the index of refraction, and that hi this case the deviation would 

 not be sensibly altered on account of the motion of the prism through the aether. 



Fizeaut, however, by observing the change of the plane of polarization of 

 light transmitted obliquely through a series of glass plates, obtained what he 

 supposed to be evidence of a difference in the result when the direction of 

 the ray in space was different, and Angstrom obtained analogous results by 

 diffraction. 'The writer is not aware that either of these very difficult experi- 

 ments has been verified by repetition. 



In another experiment of M. Fizeau, which seems entitled to greater con- 

 fidence, he has observed that the propagation of light in a stream of water takes 

 place with greater velocity in the direction in which the water moves than 

 in the opposite direction, but that the change of velocity is less than that 

 which would be due to the actual velocity of the water, and that the 

 phenomenon does not occur when air is substituted for water. This experiment 

 seems rather to verify Fresnel's theory of the aether; but the whole question 

 f the state of the luminiferous medium near the earth, and of its connexion 

 with gross matter, is very far as yet from being settled by experiment. 



I'kiL Hag. 1846, p. 53. t Ann. de Chimie et de Physique, Feb. 1860. 



