than in any other. For instance, the indices of refraction for spinel
and garnet, which are also, like the diamond, singly refractive stones,
are 1.71 and 1.77; that of the diamond is 2.43.
obtain the index of this refraction, draw a line perpendicular to the
surface of the body through the point of entry of the ray, and a circle
around with this point as a center. A straight horizontal line from the
point where the circle intersects the ray, to the perpendicular line,
is the sine of the angle of incidence, and the sides of the angle
follow the ray and the perpendicular line to the impinging point as a
vertex, thereby forming the angle of incidence. The ray on entering the
body is bent or refracted toward the perpendicular line. The point in
the circle, therefore, within the body, where it is intersected by the
refracted ray, would be nearer the perpendicular line, on the other
side of it. A line drawn between these two points would be the sine of
the angle of refraction and with the sides forming a vertex at the
central point with the vertex of the angle of incidence, would be the
angle of refraction. It is the comparative lengths of these sines
which give the index of refraction. In water it is as 1.33 to 1; in
spinel 1.71, in garnet 1.77, and in diamond it is as 2.43 to 1.
light, which falling upon the surface of the diamond is sent flashing
on, constitutes the surface brilliancy, and that which finds entry, by
the gem's power to hold and return it, forms the internal brilliancy.
light which has entered the stone is now in the grip of more exacting
laws. It has lost the full freedom of the air. The denser medium sets