6 ELEMENTARY PKINCIPLES OF MICROSCOPICAL OPTICS 



Let us now suppose that in fig. 2 the ray is travelling in the op- 

 posite direction, G F in the denser medium will now be the incident 

 ray, and F E in the rarer medium will be the refracted ray. Now, 

 if the angle B F G be increased, the angle A F E will also be in- 



FIG. 3. The phenomenon of total reflexion. (From the ' Forces of Nature,' 

 published by Macmillan.) 



creased in a greater proportion, and the ray F E will approach the 

 surface F D. 



When F E coincides with F D, G F is said to be incident at the 

 critical angle of the medium. When this critical angle is reached, 

 none of the incident light will pass out of the denser medium, but it 



