OF ARTS AND SCIENCES. 391 



wires from which it started after passing through six prisms. The 

 experiment was tried when the light started in the direction of 

 the earth's motion, and when in tlie opposite ; also, at different sea- 

 sous of the year. In all cases the image of the wi;es coalesced with 

 the wires. 



Lodge states the case clearly thus : " If all the ether were free, 

 there would have been a displacement of the image of the wires. If 

 all the ether were bound to the glass, there would have been a differ- 

 ence on the other side. But, according to Fresnel's hypothesis, there 

 should be no ditfereuce either way. According to his hypothesis, the 

 free ether, which is the portion in relative motion, has nothing to do 

 with the refraction. It is the addition of the bound ether which 

 causes the refraction, and this part is stationary relatively to the 

 glass, and is not streaming through it at all. Hence the refraction is 

 the same whether the prism be at rest or in motion through space." 

 Maxwell is more guarded in his own statement of the case. He says: 

 '• We cannot conclude certainly that the ether moves with the earth. 

 For Stokes has shown from Fresnel's hypothesis that the relative 

 velocities of the ether in the prism and that outside are inversely as 

 the square of the index of refraction, and the deviation in this case 

 would not be sensibly altered ; the velocity of the earth being only 

 one ten-thousandth of the velocity of light." 



In 1879, Maxwell wrote to Professor D. P. Todd, then at the Nau- 

 tical Almanac Office in Washington, asking him if he had observed an 

 apparent retardation of the eclipses of Jupiter's satellites depending on 

 the geocentric position of the planet. Such observations, he thought, 

 would furnish the only method he knew of finding the direction and 

 velocity of the sun's motion through the surrounding medium. In 

 terrestrial methods of measuring the velocity of light, it returns on its 

 path, and the velocity of the earth in relation to the ether would alter 

 the whole time of passage by a quantity depending on the square of 

 the ratio of the velocities of the earth and light, and this is quite too 

 small to be observed. 



In 1839, Babinet made a very delicate experiment on the relation 

 of the luminiferous ether to the motion of the earth. He found that 

 when two pieces of glass of equal thickness were placed across two 

 beams of light which interfered so as to produce fringes, one of them 

 moving in the direction of the earth's motion and the other contrary 

 to it, the fringes were not displaced. The experiment was made 

 three times by Babinet, with new apparatus each time. He con- 

 cludes that here is a new condition to be fulfilled by all theories in 



