22 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
struction the radii of the outer and inner circles are Ri = R and R Z = .R. 
n n 
Then any ray DB incident on the sphere at B intersects the outer circle at 
FIG. 10. 
A . The line BA 'D' joining B and the point of intersection of the radius CA 
with the inner circle is then the refracted ray from the incident ray BD. 
FIG. 1 1 . 
This is evident from tin- similarity of the two triangles CBA and CBA' 
whereby the angle CBA = BA 'C or a = u' and 
sin u' BC n' 
sin a' A'C n 
sin a 
sin a' 
