1892.] 



on the Motion of the Ether near the Earth. 



575 



*' doubly negative," and we require some guarantee that the condition 

 was right for seeing what might really have been in some sort there. 

 Hence Mascart and Jamin's modification of the experiment is prefer- 



FiG. 8, 



Hoek's arrangement. 



able (Fig. 9). The thing now looked for is a shift of already existing 

 interference bands, when the above apparatus is turned so as to have 

 different aspects with respect to the earth's motion ; but no shift was 

 seen. 



Fig. 9.: 



Arrangement of Mascart and Jamin. 



Interference methods all fail to display any trace of relative motion 

 between earth and ether. 



Try other phenomena then. Try refraction. The index of re- 

 fraction of glass is known to depend on the ratio of the speed of light 

 outside, to the speed inside, the glass. If then the ether be streaming 

 through glass, the velocity of light will be different inside it accord- 

 ing as it travels with the stream or against it, and so the index of 

 refraction will be different. Arago was the first to try this experiment 

 by placing an achromatic prism in front of a telescope on a mural 

 circle, and observing the deviation it produced on stars. 



Observe that it was an achromatic prism, treating all wave-lengths 



