the Wave-Length of the Soft y Rays from Radium B. 857 



mounted on a small turn-table so that the axis o£ revolution 

 of the table passed accurately through the reflecting face ; 

 the angle of the crystal was measured on a metal scale by 

 means of a glass pointer attached to the table. The emana- 

 tion tube, lead block, and crystal were all placed between 



Fur. 1. 



the rectangular pole-pieces of a powerful electromagnet MM, 

 as indicated in the diagram ; the magnetic field usually 

 employed was 2500 gauss. 



The photographic plate was held with its film towards the 

 source of the rays in a special carrier mounted on a rotating 

 arm, the axis of rotation of which coincided with the axis of 

 the crystal turn-table ; in front of the film there was a single 

 thickness of black paper to protect the plate from stray light. 

 As it was possible that the position of the plate in the holder 

 might vary in successive experiments, it was necessary for 



