200 F. E. Wright — Transmission of Light through 



figure. As noted above, the rotatory effects of the surfaces, 

 both of the crystal plate itself and of the glass mounts and 

 lenses, are disregarded in this construction. These effects are 

 small but still noticeable, and the method in consequence is 

 only an approximate method. 



Professor Becke 1 has described another method for finding 

 the direction of vibration for a dark point P of the interference 

 figure. His method consists in drawing, in stereographic pro- 

 jection, the great circle which is tangent to a line through H 

 parallel with the plane of vibration 1 T/ 7J (fig. 11). The inter- 

 section F of this great circle with the polar circle of H is then 

 the desired direction. The point can also be found, as Profes- 



Fig. 12. 



Y' 





5^""^~~ 



\ 



\ 



,H \ 



/"""-""--^Wr"""" 





- — 4 





AC 



iZ' ,■:/ 



^E 



L-Y' N 



sor Becke has shown recently, 2 by noting that it is at the inter- 

 section of the straight line HFY' (fig. 12) and the polar circle 

 to H. This direction of vibration F is not, however, contained 

 in the plane Y'Z' (fig. 12), the extinguishing plane of the upper 

 nicol, in which case the point H cannot be perfectly dark, if the 

 above reasoning be correct. If the extinguishing plane of the 

 nicol were Z' X' instead of Y' Z', the point C would be the 

 direction of vibration for a dark point H, while Gr would be 



1 Tschermak's Min. Petr. Mitteil. , xxiv, 39-40, 1905. 



2 Tschermak's Min. Petr. Mitteil., xxviii, 293, 1907. 



3 It may be of interest to note that in this figure the line HY' cuts the 

 great circle HL at F, as Professor Becke has shown ; also that the line HD 

 intersects the horizontal circle at L; that the angles LM, X'Y', KN. are 

 right angles ; and that the angle CD is equal to KL, the angle between the 

 lines of projection of the lines OF and OG on the horizontal plane. In fig. 

 13. the angle X'M is equal to the angle DI, and also to the angle ZHX' or 

 180 —6 of the spherical triangle ZHZ'. 



