part
of the scale and the Test remains dark. A sharp shadow line is seen
across the field of view. The portion of the boundary between the
lighter and darker parts on the scale is a function of the critical
angle of the stone with reference to the sphere. This angle depends on
the relative refractive index of the gem and glass. The refractive
index of glass is known, the scale is so graduated that the position of
the boundary of the illuminated area will indicate the refractive index
of the stone. The divisions of the scale correspond to 0.01 but the
index could be estimated to 0.001. The range of the instrument is
limited by the refractive index of the glass used and may be as high as
1.88. The liquid used between the gem and the hemisphere must have its
refractive index higher than that of the stone, used. Methylene iodide
could be used for stones with refractive index up to 1.74, whereas a
solution of sulphur in methylene iodide permits indices up to 1.79 to
be determined.
As
light is reflected both from the liquid film and the stone two readings
could be made on the scale—one due to the liquid and the other the
stone. When examined under white light, the shadow line is not sharp,
but is a coloured band or a narrow spectrum. The width of the band
gives a measure of the refraction. If accurate results are required,
monochromatic light should be used. But determinations are usually made
with ordinary light. A coloured band is seen and readings should be
made in the middle of the coloured band. If the stone has two indices
of refraction, two boundaries are observed each denoting a different
index.
The
limitations are: (i) a flat polished surface is essential which faceted
gems could give; (ii) natural crystal faces could be used, but they may
not always be smooth; (iii) the gem must be gently pressed on the glass
hemisphere, as it is very soft and liable to get scratched easily.
After use it should be cleaned with a piece of blotting paper.
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