A CENTURY'S PROGRESS IN PHYSICS 363 



it would be necessary to keep the axis of the telescope 

 exactly at right angles to the plane of the earth's orbit. 

 However, as the earth is in motion, the telescope must be 

 pointed a little forward, just as in walking rapidly 

 through the rain an umbrella must be inclined forward so 

 as to intercept the raindrops which would otherwise fall 

 on the spot to be occupied at the end of the next step. 

 The angle through which the telescope has to be tilted is 

 known as the angle of aberration, and the tangent of this 

 angle may easily be shown to be equal to the ratio of the 

 velocity of the earth to the velocity of light. Knowing 

 the velocity of the earth, the velocity of light can then be 

 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 



