REFRACTIVE INDICES. 
8 9 
It is evident, therefore, that at the junction between the two minerals, 
refraction and total reflection take place and if the objective be first focussed 
on a plane /i A/2, Fig. 56, a bright point D will be observed at the junction 
line, all the rays from the condenser being brought approximately to focus at 
this point. On raising the objective until it is in focus with the top of the 
section, MN, differences in intensity of illumination will be observed, the 
distance ML, Fig. 57, being lighted by the cone of rays included between 
rays 6 and 7, while LN receives the rays between A and/ and also the rays 
between A and 7. The intensity of illumination of the three areas FL, LH 
and HG is obviously different. By narrowing down the cone of rays from 
the condenser it is possible to cut out the light to LF entirely, in which case 
the bright area or strip LH will appear relatively at its brightest. If the 
cone of rays be not stopped down two white lines will appear, one, LF, 
moving toward M ( = i) and the second, LG, moving toward N ( = th) on 
raising the microscope tube. 
4 
4 \ 
j > 
6 \ \ 
FIG. 56. 
FIG. 57- 
The limiting angle for which all the rays from a 2 are reflected varies with 
the difference in refractive indices w 2 and n\, as illustrated by the following 
numerical examples : 
(i) Let ifi =1.550 
Then for a 2 = 30 
i-30 
= 1-555 
n*i = 0.0155 
But for a 2 = 7 8' t?i = o (limiting angle) 
ai = 78' t? 2 =10 13' 
Let n\ =1.550 w 2 =i.55i 2 2 n 2 i = 0.0031 
Then for a 2 = 30 t?i = 29 47' 
a! = 30 
But for a 2 = 3 12' 
01-3 12' 
= 3 o 12 
= o (limiting angle) 
