(1041 ) 
~The angle between the long ellipse-axis and the trace of P is conse- 
quently given by (ef. 6) 
Va OS OPEL Se 78°17" — 167° 218" a ZPB 
XOSENBUSCH gives (vide above) for the same mineral as calculated 
value: =) 140465 
The method of calculation followed above, is the base for the 
graphical method, according to which, in a discretional crystal-section 
the extinction-angle with regard to the trace of a discretional-plane 
can be determined from the size of the axis-angle. One proceeds, 
for doing so, from the coordinaies uw,» of (V) and v,o of (S). 
If we take the oligoclase discussed above, we find, in the way 
as was explained last time, the value A in the diagram for 
ud mt 
tr MAD 11618 "op AG i Ns 
ZET 
EN A A ae 
Allee EN SLE ADI i ES 
RN EE 25 A PET 
‘ 4 rd 4 ge / ZG G Uy / /| ey / 7 
NONLIN OVE 
fi / ve Wf ae i Gs Y) NS ae EN 4 
appears from the scheme of the /-diagram’) a value ) = — « corre- 
sponds to a secant-plane (vu == 22, on —= 0); one finds consequently 
likewise in our case for h a value, deviating but little from — 78°. 
To determine the optic extinction with regard to the fictive trace of 
the plane 17 (1 a), one uses a second diagram, which has been cal- 
culated with the help of the ratio (2), and gives for a definite axis- 
angle the value y as f(w,v). In fig. 4 the y-diagram for V—= 30°. 
is represented; in the hatched octants (II, IV, V, VII) 7 has a positive 
value, in the others a negative one. For a further explanation of 
this diagram we may refer to what was said above about the 
signification of the ratio (2) and to a former communication in which 
a similar diagram was discussed *). The y-diagram for VW = 46°35’ 
1) Compare These Proc XIII, p 731. fig. 4—5, 
2) These Proc. X, p. 375 et seq. 
