( 1045 ) 
uncertain results, if differs little from O° or from + a and con- 
sequently the pole of the secant plane lies with a value of @ that 
u 4 3 : 
differs little from v = 0, og or 5%, or with a little value of o. 
With regard to the last condition, one is in a more favourable cir- 
cumstance with a large axial-angle than with a little one; whilst 
with J’ = 90° the curve 7 = 89° remains in the immediate vicinity 
3 
Jt ; 
of the vertical axes — and > a (comp. fig. 4) and the horizontal 6=0, 
with J” == 10° the corresponding curve does not approach the hori- 
zontal 6 =O within a latitude of + 40°. 
* 
T 5 ; ai Rives 
If y differs little from O° or + a then 27 lies in the vicinity of 
O° or +2. In (10) cot 2y furnishes then, with differences of a few 
minutes, which cannot be measured with certainty in ordinary cir- 
cumstances, already values differing so much from one another, that 
the final result is greatly influenced by it. If we take e.g. the feld- 
spar of Bamle discussed above. For y, was found the value 89°51’ 
whilst from the calculation according to RoseNBesen would follow 
Yi, == Oe 40°. 
The difference between ly cot 2yq and ly cot 2y'q is 
2,28100 —- 2,00478 = 0,27622, 
so that here an observation-mistake of 8’ counterbalances 8°35/ 
with y= + 45°. Consequently one calculates from the value, given 
by Rosrnsuscn for the extinction, with regard to the trace of P (O01) 
in the secant-plane M (010), «= y', —h—=—12°16’ (instead of 
— 12°8’), a value for V = 55°50’ (instead of 46°35’). The pole of 
the seeant-plane J/, fastened to the axes-plane AB (comp. fig. 3) 
and the plane La, ties with v = 172°58’, o=1°6’. So the result 
was to be expected. 
Crystallography. — “On the determination of an unknown plane 
from its traces in tivo orientated crystal-sections.” By J. SoumuTzeER. 
EN 
(Communicated by Prof. C. E. A. Wi1cHMANN). 
(Communicated in the meeting of February 25, 1911). 
[f one calls the coordinates of the pole of an unknown plane / 
(erystal-, cleavage- or twinning-plane) u and py, if two known secant- 
‘ 
planes S, and 5, are given by the poles s, (9,,6,) and s, (9,, 0) 
V1? 
and if further the angles, which the trace of P in these planes 
