X-RAYS AND CRYSTALS 



375 



This beam was by no means completely absorbed by the crystal ; 

 it traversed it to fall upon the plate and when the plate was 

 developed after several hours' exposure, the effect produced was 

 visible as a circular dark spot of much greater dimensions than 

 the cross section of the incident beam on account of slight 

 scattering of the rays. But besides this intense spot, there 

 appeared around it, on the plate, a series of much weaker spots 

 apparently arranged in a geometrical pattern. Fig. 2 shows two 

 typical crystallographs obtained with a crystal of zinc blende. By 

 altering the distance of the photographic plate from the crystal, 



Fig. i. 



A, Anticathode. Al, Aluminium window. Sj, S 2 , S 3 , S 4 , Stops. Cr, Crystal. P, P, Photographic plate. 

 B, Light tight box. I, The incident beam. D, A diffracted beam. 



the small spots could be made to close into or move out from the 

 big central one and it was clear that they were formed by narrow 

 rectilinear pencils spreading from the piece of crystal. Some of 

 these pencils make quite large angles, as much as 40 , with the 

 direction of the undeviated ray. The spots hardly altered in 

 size as the distance of plate from crystal was increased, remain- 

 ing always of the same size as the smallest stop. Copper 

 sulphate forms triclinic crystals, belonging to one of the more 

 complicated systems ; in order to obtain results which could be 

 analysed more readily, a crystal was chosen which belongs to the 



