On
pages 6 to 20 there have been described easy methods of ascertaining
the density, pleochroism and hardness of precious stones. The index of
refraction may also now be determined, thanks to Mr. G. F. Herbert
Smith's improved Kefractometer, which is available for all gems of
which the index of refraction lies between 1.45 and 1.74. The
instrument, as made by Mr. J. H. Steward, is accompanied by
instructions how to use it and how to interpret the observations made ;
also by a table of indices corresponding to each division of the
graduated scale. The Refractometer is standardized for use with the
yellow light of a sodium flame, but white light may be employed if
approximations suffice.
Place
a small drop of a highly refractive liquid—a bromonaphthaline, for
example—upon the plane surface of the hemisji'ierical lens. Then press
a flat face of the gem into the liquid, so that the latter has a
uniform thickness. Light being admitted through the ground glass "
window " of the instrument, and the scale focussed, it will be seen
that different parts of the scale are differently illuminated; the
edges or boundaries of these regions where they coincide with the
scale-divisions are to be noted. With glass and other singly-refracting
substances, the edges due to these solids are single, with
doubly-refracting minerals double. ft is essential that the edge
arising from the refractive liquid itself should be beyond the edge due
to the stone. So when spodumene, peridot and spinel, with indices
ranging between l-63 and 1.73, have to be tested, methylene iodide, with a refractive index of 1.74, must be substituted for u bromonaphthaline, the refractive index of which is only 1.658.
