572 Trmisactions of the Boyal Society of South Africa. 
In order to obtain such a surface a biprism was cut out of a crystal 
of NaCl. 
The top-, bottom-, and back-surfaces are planes of natural cleavage, and 
the angle 179 degrees (Fig. 2a). 
i 
I 
^ //SO 
1 
C/ea^age Plane. 
Fig. 2a. 
A second prism, equilateral triangular, was also obtained from Gottingen. 
It seem si probable that these prisms will represent the required — even 
if irregular — ultramicroscopic echelon gratings (Fig. 26), 
Cleavage P/ane 
Fig. 21. 
Experiments with NaCl Prisms. 
The prism {ci) was set optically so that the Eontgen rays passed normally 
through the back-surface. 
A pair of films were placed 15 cm. behind the prism, so that the spots" 
may be fairly large. 
The photogram (Fig. 3a) shows that there are positions of zero intensity 
within the spots as well as of definite points of maximum intensity. 
The central spot, which is circular in the case of crystalline plates, is now 
elliptic. 
This indicates that the small increased thickness of the central part plays 
a prominent part in the shape of the spots. 
