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EADIAL LINES IN KONTdEN INTEEFEEENCE PATTEENS. 
By J. Steph. van dee Lingen. 
At the ' Naturforschersaiiimlung,' 1913, W. Friedrich described the 
appearance of radial lines in the interference patterns of common salt and 
sylvine.*) 
These radial lines he attributes to the two dimensional grating effect of 
the three dimensional grating of the crystal, and the points to the three 
dimension grating effect of the same space-lattice. 
M. von Laue and the author pointed out that the " point pattern " is 
determined by the three constants of the grating, and that if one of these 
constants vanish or become small in comparison to the remaining two then 
the grating is principally a two dimensional one which will produce radial 
interference lines. f) 
This view is supported by experiments on magnesium-hydroxide. 
This substance shows the usual point pattern, but after the water 
molecules are expelled by long-continued heating, then it becomes brittle 
and only produces radial lines in the pattern. 
Biotite produces the usual pattern, but after treatment with acids — 
" Baurit aus Biotit " — shows no pattern whatsoever in spite of the fact that 
it still appears — macroscopically — to be crystalline. | ) 
Microcrystalline substances generally show radial lines and some small 
points. The latter are probably due to " regular units " in the crystal. 
If a crystal is built up of elementary units which are arranged in all 
possible directions, then even if each unit be a miniature crystal, these will 
not produce an " interference pattern " but a high dispersion. 
Crystals from sublimated iodine show both radial lines and " shaded " 
points on the pattern. This seems to indicate that the third elastic constant 
is almost a negligible quantity. 
A crystal of iodine — cut at G-ottingen — only showed a large number of 
small points but no radial lines after the same exposure as the former 
crystal. 
* W. Friedrich, ' Phys. Z. S./ 14, 1079, 1913. 
t M. V. Laue and J. Steph. v. d. Lingen, ' Naturwissenschaf ten/ 13, 328, 1914, 
" Baurit aus Biotifc," prepared by Prof. Rinne, Leipzig. 
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