263 



Photograiili V is taken with the aid oi' convex lenses, focussing the 

 light from the snlpliide screen on the ph:ite; by means of a lead plate. 



one-half of the luminous screen was screened from the source in such 

 a manner as is shown in Fig. IV. A lilack strip of paper is pasted across 

 the center of the screen to marlv the halves of the luminous sulphide- 



