Short Spark- Discharges, 



695 



at C, the relative movement of the discharge-surfaces P and 

 N could at all times be followed. 



Fig. 2. 



I I 



Y 



The relation between the actual movement of the carriage 

 in which B is mounted and the increase in the length of the 

 path of the ray is given by the following equation. 

 Let a — angle of incidence. 



x = angle between mirrors A and B. 



D„= actual distance passed over by the ray between the 



mirror- due to a movement of the mirror B. 

 // = number of the order, i. e. n gives the number of 

 times the ray is reflected by the mirror B. 

 = actual movement of the mirror B along the original 



path of the ray. 

 = number of fringes that passed the cross-hair of the 

 microscope when the rath order of reflexions was u^ul. 

 = wave-length of the light used. 

 = d cos a. 



d 



