594 



Prof. Brace on the Change produced by the 



to the effect of the earth's motion upon the velocity of 

 propagation in the glass. The total increment for a single 

 plate would be 2h(fi — a) approximately. If now the ray be 

 reflected back through the second plate, in the same azimuth 



Fig.. 1. 



with respect to the incident ray, it would experience similarly 

 a rotation of 2(/3' — a') ; and, since its direction is reversed 

 with respect to that of the motion of the earth, it would 

 experience a decrement of 2B(/3' — a'). Hence the azimuth 

 of the ray will appear to an observer to have changed by an 

 amount equal to 4S(/3 — a), approximately, for the pair of 

 plates, if a! — a and /3' = /3. fc'or a second pair of plates, in 

 series with the first, the change would be 8S(/3 — a), and so 

 on. The full lines in the figure show the azimuths of the 

 ray, without the effect of the earth's motion, and the numbered 

 dotted lines, those with the supposed changes in the planes of 

 polarization. Thus, line 1 is the new azimuth after passing 

 the first plate, and line 2 the new azimuth after returning 

 through the second plate of the pair. Similarly, if the 



