306 
The American Geologist. November, 1905 
to meet the plane between the two media at the point n, at 
a distance above the point x of 1.87 times the length x-y. 
Ray 2 meets it at m, a distance equal to 1.56 times x-y. 
Rays 3 and 4, since B has higher index, are refracted to meet 
the plane at points m' and n', higher than the similar rays 
in A, or at distances above y of 1 .76 and 2.18 times x-y, re- 
spectively. 
At the surface of contact between A and B the critical 
angle is 62 10' and so all rays incident on y-z from B at an 
angle greater than 62 10' are totally reflected back into B. 
On the other hand, a portion of the light from A incident 
upon y-z is refracted into B. The ratio between the amount 
reflected and the amount refracted depends upon several 
factors. In proportion as the contact surfaces of A and B 
are highly polished, more light is reflected and less refract- 
ed; as the angle of incidence increases more light is re- 
flected and less refracted; and as the difference in the in- 
