ACOUSTICS AND GRAVITATION. 149 



tor, T the telescope. The rays from L follow the paths a'bab' and b'aba' and 

 again emerge toward L, but are caught in part and reflected by the plate of 

 glass m and sent to T. The same is true of the rays from L'. Hence four rays 

 pass the side of the ray rectangle and interfere in pairs at T. By moving the 

 set of mirrors NN f bodily right or left, these rays may be separated into two 

 pairs of two rays each, passing the corresponding plates of the compensator C. 

 The observer at the telescope T sees two vertical slit-images, which may be 

 adjusted to lie exactly side by side with their near edges just touching. Each 

 carries its horizontal achromatic interferences, nearly 



v 



but not quite of the same size, since the glass-paths of /\L> 



the two rays will not, in general, be rigorously the same. /y / .-^ 

 Hence, in moving the screw of the compensator C, intro- ^/ " a/ ' / 

 ducing a difference of glass-paths, the two sets of fringes <** ""/JT 184 

 move in the same direction, but are only periodically 



horizontal prolongations of each other. With the achromatics i to 3 complete 

 phase reversals were thus producible within the vertical limits of the slit- 

 image. The experiment succeeded best with sunlight and short collimators 

 close at hand, giving high slit-images. 



To make the experiment more useful, the four rays should be separated, so 

 so that if path-difference is introduced into one pair and not into the other, 

 the result may be accentuated. To do this requires a broader Billet compen- 

 sator than I possessed or a special device, and I did not therefore carry it out. 



The horizontal spectrum fringes, even if from a wide slit, require much light, 

 particularly in view of the plate-glass m. Sunlight with a condenser lens of 

 long focus is necessary for clearness. The two sets of fringes are then merged in 

 the same banded spectrum and alternately strengthen or destroy each other. 



