THEORY OF ETHER 



157 



angle, nearly 180, called a bi-prism, is placed so that light 

 diverging from the point d falls on the face a c. Part of the 

 light is refracted by the portion a b and part by the portion 

 c b of the bi-prism. A given ray d e is refracted and reaches a 

 point p, as if it na cl come from g, while another ray df is 



Fig. 53. 



refracted by the other portion of the bi-prism and reaches the 

 point p as if it had come from a point h. Hence the effect of 

 these two rays at the point p is the same as if they had come 

 respectively from the two points, g and h, which are close 



together, and hence a screen placed at p will exhibit inter- 

 ference effects. 



A carefully constructed optical bench allows this effect to 

 be observed, and the distance apart of the lines to be accu- 

 rately measured. It consists of a heavy iron bar, planed, 

 grooved, and graduated so that three upright stands may be 



