64 "N RAYS 



OB ; and this time the screen CD, carried 

 along with the frame in this movement, is 

 no longer interposed in the path of the rays. 

 The half-plate AO therefore receives the 

 action of the spark while subjected to the 

 rays. 



This being understood, the experiment is 

 as follows : first the plate is kept in the 

 first of the above-indicated positions during 

 five seconds, then in the second position also 

 for five seconds ; it is then brought back to the 

 first position, and the double operation just 

 described is repeated several times. 



After an interval of time equal to an even 

 multiple of five seconds for instance, one 

 hundred seconds each of the half-plates has 

 been exposed to the spark for an equal period, 

 only, while AO was exposed, " N " rays were 

 in action, and while OB was exposed there 

 were none. 



Thanks to an arrangement of guides and 

 buffer-stops, the to-and-fro motion of the frame 

 can be executed with perfect certainty and 

 regularity, in spite of the darkness. A metro- 

 nome is used to regulate the action. 



