On the " Lines " vnthin R'untgen Interference PJiotograms. 578 
On rotating the prism through a small angle and then oV^taining a 
photogram the effect is enlarged, because those points which move away 
from the central spot become drawn-out elliptical spots. 
These spots show the (probable) effect of the irregular echelon grating 
clearly (Fig. Sb). 
Similar results were obtained with the second prism (Fig. 4). 
Quartz Plates. 
An experiment on a plate of quartz cut perpendicular to the optic axis 
showed a series of parallel lines in all the spots (Fig. 5). 
Mg(0H)2. 
A plate of Mg(OH)o was examined microscopically, and it showed small 
leaflets of partially removed planes adhering to the surface. 
This substance, which does not cleave so cleanly as mica, showed irregular 
interference spots. 
The border of the spots are chiefly influenced. 
The "spots" are as irregular as those of silicon (Fig. 6). 
Silicon. 
The photogram of silicon seems to indicate that this substance is built 
up of microcrystalline units, the whole forming the crystal of silicon. 
The radial line indicates the presence of a two-dimensional grating as 
well as the usual three-dimensional grating. 
The spots are not as large as expected, nor are they circular. 
The spots B are almost lines ; D and F spots seem to come from the same 
planes, whereas E seems to be D and a reflected image of D ; C seems to be 
B projected at a different angle. 
Unlike the diamond, D' is almost invisible, whereas D is a maximum 
point. 
This "regular" microcrystalline structure may account for its inability 
to produce spots at an obtuse angle.* 
* M. V. Lane and the Author, " Die Naturwissenschaften," 13, 328, 1914. 
