popular.
It would be preferable to use monochromatic light. The usual source of
such a light is a non-luminous gas flame coloured by salt of lithium,
sodium and thallium. In the case of gem-stones by using ordinary light
sufficiently accurate results are obtained.
Tully's Refractometer
The
essential part of the instrument is a polished hemisphere of glass of a
specially high refractive index, about 1*80 or 1*90 for yellow sodium
light. The exposed surface of the sphere is a flat polished surface.
The cut stone to be tested is placed on to the glass hemisphere B.
