Jewelry Stones

The stone under test is in contact with a hemisphere, or prism, of optically very dense glass. The contact liquid between the stone and the glass insures perfect contact. Lens A admits light to the instrument, which passes into the dense glass. A portion of the light will be totally reflected at the surface dividing the gem stone under test and the glass. The totally reflected light passes through lens £ on to a scale, part of which is brightly illuminated. The dark portion of the scale represents light not totally reflected that has escaped out through the stone. The bounding edge between the light and dark portion of the scale is distinct, and will give a direct reading of the refractive index of the stone under test. Since the glass of the instrument is always the same, the position of the shadow edge will entirely depend on the stone under test. Finally, the light rays pass into a glass prism which has the function of reflecting them, so that the scale can be conveniently viewed from above