a ray of light impinges upon the surface of a gem, part of it passes
through it, in accordance with well-known optical laws, and part of it
is thrown back or reflected, in obedience to the following laws :—
(a.) The angle of reflection is equal to'the angle of incidence.
(b.) Both the incident and the reflected ray are in the same plane, and this is perpendicular to the reflecting surface.
amount of light thus reflected is different in different gems, and it
varies also in proportion to the obliquity with which the incident ray
falls upon the stone. The amount of light reflected increases up to a
certain angle—differing in different stones ; and under certain
conditions total reflection takes place. To this property is
due the superior brilliancy of the Diamond, as every incident ray which
strikes a face, inside the stone, at a greater angle than 24° 13 ' is
totally reflected. We thus see the supreme importance of cutting a
Diamond, not only of a graceful and handsome outline, but having each
facet cut with such mathematical precision as to secure the greatest
amount of reflection from its internal surfaces.
the reflected part of the ray, and passing on to notice that part which
is transmitted through the gem, we find that this transmission is
regulated by laws which may be thus enunciated :—
(a). A ray of light passing from a rarer into a denser