( 2085") 
sections 6, c, and d have relation to the secant-planes S,,.S,, and S,. 
And in this way a choice between the four poles found has become 
possible with certainty. 
If the orientation of the crystal discussed here, must be determined 
with the help of 2 erystal-planes and one cleavage-plane, then two 
solutions remain satisfactory, as is easily seen, whilst with 1 erystal- 
plane and two cleavage-planes, or likewise with 8 cleavage planes all 
4 solutions may be taken mto consideration. 
Cristallography. — “On the determination of the optic axial-angle 
from the extinction-angle with regard to the trace of a dis- 
cretional plane in a discretional erystal-section”. By J. SCHMUTZER 
7 
(Communicated by Prof. C. B. A. Wichmann). 
(Communicated in the meeting of February 28, 1911). 
3e in fig. 1 the projection-plane perpendicular to the bisectrix of 
the optie-axes, A and B and be further S(s) the secant-plane, then 
the directions of the vibration in the slide are given by the planes 
that halve the angle As/ and its supplement. 
Be angle w the inclination of the biseetrix (VY) on the secant- 
plane S; ~/KMH=—= « the angle, enclosed by the secant-lines of 
the planes AOB and S with the projection-plane, then, as was 
A 
Fie. 1. 
dedueted before, in the plane S the extinction with regard to the 
secant-line O/7 can be found from the equation : 
