( 130 ) 



the azimuth (it) along the great circle c/?, beginning from the point c, 

 and calling positive a direction contrary to the hands of a clock 



Fig. 2. 



whilst the height (r) is measured from the circle cJi, the direction 

 towards the pole n being positive, then the planes J/(010) and 7^(001) 

 are given by the coordinates of their poles 



M{v,):[i, = 172°58', r, = 1°6' 

 Fiv,) : lu = 86^45', r, = - 11°53'. 



Be further in a slide the angle between the traces of P and M 

 equal to n^=1t^ — Ji^ = — 101°45' whilst the optic extinction, with 

 regard to the trace of M, amounts to an angle ,? ^ y — A^ r= lo°50'. 

 In order to determine now the direction of the slide-plane S{q,g) 

 one combines in the first place the A-diagrams for P and M, the 

 diagram for M being removed over an angle [i^ — (i. = 86°13' 

 with regard to that for P (ci. \^\. I, U). Then in the upper octant to the 

 left (PI. I) the curve TgAg represents the geometrical place of all poles of 

 secant-planes, in which the traces of P and J/ include an angle of 

 — 101°4:b' = h^ — Aj. This curve is found by interpolation between 

 the curves ^^A^ and S^^^ which, as appears from the shape of the 

 h^- and A^-curves indicated in the figure, represent the G. P. of the poles 

 of the secant-planes, in which r< = A, — h, = — 100° resp. — 110°. 

 Now the curve ( TA) occurs only in 4 octants i.e. — if we call the 

 octant just spoken of the P' — in the octants I, III, VI, and VIII. If one 

 regards the left-octant below on PI. I (oct. V), then here a = h^ — h^ 

 should have the value rr — 101 °45' = 78°15'; from the figure it 

 appears, however, that the 0, 10, 20 . . . etc. cur\es of P{v^) do not 

 intersect with the — 70, — 60, — 50 . . . etc. -curves of M{v^), so 

 that here the curve (TA) does not appear. The same holds good for 

 the octants Tl, IV, and VII. 



