330 L. Page — A Century's Progress in Physics. 



calculated. This method was one of the first of obtaining 

 the value of this important quantity. 



More recently, terrestrial methods of great precision 

 have been devised for measuring the velocity of light. 

 The most accurate of these is that employed by the 

 French physicist Foucault in 1862. A ray of light is 

 reflected by a rotating mirror to a fixed mirror placed at 

 some distance, which in turn reflects the ray back to 

 the moving mirror. The latter, however, has turned 

 through a small angle during the time elapsed since the 

 first reflection, and consequently the direction of the ray 

 on returning to the source is not quite opposite to that in 

 which it had started out. This deviation in direction is 

 determined from the displacement of the image formed 

 by the returning light, and from it the velocity of light 

 is calculated. In order to make the deflection appreci- 

 able the distance between the two mirrors should be very 

 great. As originally arranged by Foucault, it was 

 found impractical to make this distance greater than 

 twenty meters, and consequently the displacement of the 

 image was less than a millimeter. Such a small deflection 

 limited the accuracy of the experiment to one percent. 

 In 1879, however, Michelson (18, 390, 1879), then a mas- 

 ter in the United States Navy, improved Foucault 's opti- 

 cal arrangements to such an extent that he was able to 

 use a distance of nearly seven hundred meters between 

 the two mirrors. With a rate of two hundred and fifty- 

 seven revolutions a second for the rotating mirror, the 

 displacement obtained was over thirteen centimeters. 

 This experiment gave 299,910 kilometers a second for 

 the velocity of light, with a probable error of one part in 

 ten thousand. Later investigations by Newcomb and 

 Michelson (31, 62, 1886) gave substantially the same 

 result. So great has been the accuracy of these terres- 

 trial determinations that recent practice has been to cal- 

 culate from them and the angle of aberration the earth's 

 orbital velocity, and hence the distance of the earth from 

 the sun. This indirect method of measuring the astro- 

 nomical unit has a probable error no greater than the best 

 parallax methods of the astronomer. (J. Lovering, 36, 

 161, 1863.). 



Aberration is a first order effect, i. e., it depends upon 

 the first power of the ratio of the velocity of the earth to 

 the velocity of light, and at first sight it seemed to prove 



