and
the artisan knowing these, so cuts the diamond as to leave no avenue of
escape for the entrapped light but the front of the gem where it
entered. Jumping from wall to wall of the transparent enclosure, the
rays try them all with points of light in vain, until they reach again
the gate of entry, and even this must be properly approached if they
would pass through.
The
reason of it is this: that ray of light traversing a denser medium, in
its efforts to escape back into the rarer medium, air, meets with an
obstacle called " total reflection."! Though light may in some degree
enter from the rarer to the denser medium at any angle, it can return
only within certain bounds.
Inside
the limits of freedom, light passing from the diamond in a slanting
direction into the air is also refracted as it passes the surface of
the stone, but in a contrary direction. It is then bent or refracted
from instead of toward the perpendicular, and the sine of the angle of
incidence is less than that of the angle of refraction. The direction
of the ray is a simple reversal of that taken on entering the stone.
The angle of total reflection is variously given as 240 13'
to 24 ° 24'. A ray of light impinging on the inner surface of a
diamond slightly within or less than this angle, will on passing
through to the air be refracted so that it will pass along the stone
near the outer surface, as a brilliant shot of light. But if the ray
falls upon the inner surface at a greater angle or more obliquely, it
will be totally reflected; no part of it can escape into the air. It is
for this reason a diamond is shaped and proportioned as it is now.
Light entering the face of a properly-cut diamond reaches the back
facets at angles of total
