1)40 



Dr. Mamie Sieo-bal 



ill on 



The crystal (Kr), slit (Sp 1) and plate-holder (PI) are 

 mounted on one and the same base; slit and plate- 

 holder forming opposite sides in a pyramidal-shaped box. 

 The slit is made of gold, with a very small opening-angle. 



Fie-. 1. 



Fig-. 2. 



The box can be rotated about an axis through the slit. 

 A nonius at the back of the box opposite a fixed circle- 

 scale on the bedplate bearing the box serves to measure 

 the angles of rotation. The slit (Bl) for the incoming- 

 X-ray pencil is fixed on the bedplate. Before using the 

 spectrograph the second slit (Sp 2) is so adjusted that 

 the normal from the first slit (Sp 1) to the plate passes 

 through it. Then the crystal is fixed directly on the first 

 slit with the cleavage-faces (or reflexion-faces) parallel to 



