54 



TRANSACTIONS OF THE 



[Nov. 16, 



C parallel to C of plate. Then there will be a phase difference and 

 resultant color due to the sum of the phase differences of crys- 



tal and gypsum. If now there is a rotation around b, the phase 

 difference will change, because C will vary from C to a, and at 

 some point, which will be the point of emergence of an optic 

 axis, the phase difference of the crystal will be zero, that is, C = lb ^ 

 and the color will be the red of the g3^psum. With further rota- 

 tion C will be greater than h, and the red color will be lowered. 



With a rotation around ( there is no such change, for through- 

 out the elasticity in the direction if is the greater. 



In sections approximately normal to C,in corresponding posi- 



Fio. 6. 



