362 O. U. VONWILLER. 



a glass rod, and the whole cooled rapidly and then heated 

 in an oven to temperatures varying in different cases from 

 170° O. to 213° 0., after which the cell was cooled, some- 

 times gradually and at others rapidly. The mica at the 

 back of the selenium was then carefully cut away so that 

 light could strike it directly on either surface. 



In fig. 1 is shown a horizontal section of a box prepared 

 for the test of such selenium cells. A B is a vertical par- 

 tition running nearly the whole length of the box; a c and 

 b c are two vertical plane mirrors inclined at 45° to Ac. 



Fig. 1. 



Di and D 2 are two doors, either or both of which may be 

 opened so that light from a source some distance in front 

 of the box may fall on either or both of the mirrors. The 

 selenium cell S is allowed to hang vertically in the plane 

 A B, the platinum wires being attached to two terminals 

 let through an ebonite piece in the cover of the box, not 

 shown in the diagram. All the surfaces inside the box 

 except the mirrors and the cell were black. 



In testing a cell, the source of light was placed at any 

 distance desired in front of the box, and first one door was 



