720 9 
Crystallography. — “On the orientation of microscopic crystal- 
sections.” By J. Scumurzer. (Communicated by Prof. C. E. A. 
WICHMANN). 
(Communicated in the meeting of December 24, 1910). 
If in a slide ‘a crystal shows the traces of three non-parallel planes 
it is possible to determine the orientation of the section without 
making use of the extinction angle. If the optic constants of an 
anisotropic mineral are known, it is sufficient to know the extinction 
angle and the apparent angle between two planes (crystal, cleavage- 
or twinning-planes) as will be more distinctly demonstrated in a 
subsequent communication. 
In fig. 1 the crystal planes £, V,, and V,, the former of which 
is supposed to be the projection plane, are cut by the secant plane 
S, producing with it the secants OB, OA,, and OA,. In stead of 
the two angles «‚ and «, one measures in the secant plane, conse- 
quently in the slide, between the planes V,: 4 and V,:Z the 
apparent angles A,OB — A, and A,OB=A,. 
Be the secant-plane S given by its pole P,, of which the height 
PQ =o is measured > 0 above and < 0 below the equator plane, 
whilst the azimuth C,bQ—=o is >O if measured in opposite 
direction to the hands of a clock, then is in ABA,C,: 
