CHAP. viii. 



PHYSICAL PRINCIPLES. 



the exact number of revolutions a second of the mirror, 

 which was shown by a simple clockwork connected with 

 it, the velocity of light was deduced as being 185,157 

 miles per second. 



It is evident that there are here several sources of 

 error. The short distance traversed by the light renders 

 it necessary for the revolving mirror to turn with ex- 

 treme rapidity, while the observed displacement of the 

 ray is very small. Minute errors in the various meas- 

 urements will therefore be enormously multiplied in the 

 result. To obviate these difficulties the concave mirror 

 has been placed much further away; and in the most 

 recent and most accurate experiments by Professor New- 

 combe at Washington, the distance between the revolv- 

 ing and the concave mirrors was about two and a half 

 miles, and the mirror revolved two hundred and thirty 

 times a second. This gave such a large displacement of 

 the returning ray that it could be measured with extreme 

 accuracy, and the average of numerous trials gave the 

 velocity of light as 186,327 miles per second. It thus 

 appears that Foucault's measurement in a small room 

 was only in error about T |-g-, or a little more than a half 

 of one per cent., a wonderful testimony to his skill as an 

 experimenter under such unfavorable conditions. Pro- 

 fessor Newcombe believes that his determination is cor- 

 rect within -nnhrg-, but he thinks that by placing the 

 mirrors twenty or thirty miles apart in the clear atmos- 

 phere of the Rocky Mountains a still greater approach 

 to perfect accuracy could be obtained. 1 



The same M. Leon Foucault who made these beautiful 



J For a more detailed account of Professor Newcombe's experi- 

 ments, see Nature, vol. xxxiv. p. 170. . 



