addition to the physical properties already dealt with there are a few
other optical properties, which are of great value in the determination
of gem-stones. These properties are determined by optical instruments
such as the dichroscope, refractometer, and the polarizing microscope.
A brief explanation of the essential properties of light is given
before the optical instruments and their uses are described.
Reflection and Refraction of Light
When a ray of light falls on a mirror or a polished surface, it is reflected so that the angle of incidence XON is equal to the angle of reflection YON (see fig. 26) and that the reflected and incident rays lie in the same plane with the normal to the reflecting surface.