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Appendix A - Discrimination of Precious Stones

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Appendix.
intensity and delicacy of hue and beauty ; in a word, the optical characters constitute a great guff that divides the real stone from the imitation.
Reflection and Refraction.
When 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.
The 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.
Leaving 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
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