the instrument the
_refractive index_, as it is called, of any precious stone that is not
too highly refractive. (Its upper limit is 1.80. This would exclude very
few stones of importance, _i. e._, zircon, diamond, sphene, and
demantoid garnet.)

Those readers who wish to make a more intensive study of the
construction and use of the refractometer will find a very full and
complete account of the subject in _Gem-Stones and their Distinctive
Characters_, by G. F. Herbert-Smith, New York; James Pott & Co., 1912.
Chapter IV., pp. 21-36. The Herbert-Smith refractometer is there
described fully, its principle is explained and directions for using it
are given. The price of the refractometer is necessarily so high (duty
included) that its purchase might not be justified in the case of the
smaller retailer. Every large dealer in colored stones, whether
importer, wholesaler, or retailer, should have one, as by its use very
rapid and very accurate determinations of stones may be made, and its
use is not confined to unmounted stones, for any stone whose table facet
can be applied to the surface of the lens in the instrument can be
determined.

LESSON III

DOUBLE REFRACTION

EXPLANATION OF DOUBLE REFRACTION. In Lesson II. we learned what is meant
by _refraction_ of light. While glass and a small number of precious
stones (diamond, garnet, and spinel) bend light as was illustrated in
Fig. 1, practically all the other stones cause a beam of light on
entering them to separate, and the path of the light in the stone
becomes double, as shown in Fig. 2.

This behavior is called _double refraction_. It may be used to
distinguish those stones which are doubly refracting from those which
are not. For example, in the case of a stone which is doubly refracting
to a strong degree, such as a peridot (the lighter yellowish-green
chrysolite is the same material and behaves similarly toward light),
the separation of the light is so marked that the edges of the rear
facets, as seen through the table, appear _double_ when viewed through a
lens. A zircon will also similarly separate light and its rear facets
also appear double-lined as seen with a lens from the table of the
stone. The rarer stones, sphene and epidote, likewise exhibit this
property markedly. Some colorless zircons, when well cut, so closely
resemble diamonds that even an expert might be deceived, if caught off
his guard, but this simple test of looking for the doubled