OPTICS. 605 



be done. His object was to determine the velocity of the electric 

 shock. His apparatus consisted in a small mirror, turning with great 

 velocity about an axis -which is in its own plane, like a coin spinning 

 on its edge. The velocity of spinning may be made so great, that an 

 object reflected shall change its place perceptibly after an almost incon- 

 ceivably small fraction of a second. The application of this contri- 

 vance to measure the velocity of light, was, at the suggestion of Arao-o, 

 who had seen the times of the rival theories of light, undertaken by 



O ' / 



younger men at Paris, his eyesight not allowing him to prosecute such 

 a task himself. It was necessary that the mirrors should turn more than 

 1000 times in a second, in order that the two images, produced, one 

 by light corning through air, and the other by light corning through 

 an equal length of water, should have places perceptibly different. 

 The mechanical difficulties of the experiment consisted in keeping up 

 this great velocity by the machinery without destroying the machinery, 

 and in transmitting the light without too much enfeebling it. These 

 difficulties were overcome in 1850, by M. Fizeau and M. Leon Foucault 

 separately : and the result was, that the velocity of light was found to 

 be less in water than in air. And thus the Newtonian explanation of 

 refraction, the last remnant of the Emission Theory, was proved to be 

 false. 



