urface. For each material there is a
definite angle outside of which light that is passing as above
described, is _totally reflected_ within the material.

[Illustration: FIG. 9.

_AB_ represents the back surface of a piece of diamond.

_CD_ is a line perpendicular to _AB_.

Angle _CDE_ is about 24 degrees.

Dotted line, _FDH_ represents the course taken by a ray of light which
is totally reflected at _D_ in such fashion that angle _FDA_ equals
angle _HDB_.

Any light proceeding towards _AB_ but between _E_ and _C_, would fail to
be totally reflected. Most of it would penetrate _AB_.]

TOTAL REFLECTION. For diamond this _critical angle_, as it is called, is
very nearly 24 deg. from a perpendicular to the surface. If now, we shape a
diamond so that most of the light that enters it from the front falls
upon the first back surface that it meets, at an angle greater than 24 deg.
to a perpendicular to that surface, the light will be totally reflected
within the stone. The angle at which it is reflected will be the same as
that at which it meets the surface. In other words the angles of
incidence and of reflection are equal. See Fig. 9 for an illustration of
this point.

THEORY OF THE "BRILLIANT." In the usual "brilliant" much of the light
that enters through the front surface is thus totally reflected from the
first rear facet that it meets and then proceeds across the stone to be
again totally reflected from the opposite side of the brilliant. This
time the light proceeds toward the top of the stone. See Fig. 10--(From
G. F. Herbert-Smith's _Gem-Stones_).

The angles of the top of a brilliant are purposely made so flat that the
up coming light fails to be totally reflected again and is allowed to
emerge to dazzle the beholder. In the better made brilliants the angle
that the back slope makes with the plane of the girdle is very nearly
41 deg. and the top angle, or angle of the front slope to the plane of the
girdle is about 35 deg.. Such well made brilliants when held up to a bright
light appear almost black--that is, they fail to pass any of the light
through them (except through the tiny culet, which, being parallel to
the table above, passes light that comes straight down to it).

[Illustration: FIG. 10.--COURSE OF THE RAYS OF LIGHT PASSING THROUGH A
BRILLIANT.]

In other words, instead of allowing the light to penetrate them,
well-made brilliants almost totally reflect it back toward its source,
that is, t