such
an angle as will give the greatest amount of light back again on the
exposed aspect of the gem, so that its brilliance will be as great as
possible. Another effect of this repeated reflection is to cause the
ray to pass over a much longer path in the stone, and thus the
dispersion produced is greater, and therefore the prismatic colour
effect is more marked.
For
the accurate measurement of refraction an instrument called a
refractometer is used. Its action depends on the fact that the
refractive index can be calculated when the angle of total reflection
is known ; in the improved form devised by G. F. H. Smith (and
described by him in the " Mineralogical Magazine," vol. xiv. p. 83),
the instrument consists of a hemisphere of optically dense glass, so
mounted that its plane surface makes an angle of 26 degrees with the
axis of the instrument; on the opposite aspect of the mount carrying
the hemisphere is a piece of ground glass ; arrangement is made for the
adjustment of this portion of the instrument in relation to the axis of
the tube ; a correcting convex lens is placed close to the hemisphere
and in the tube; at the other end of the tube is a positive eye-piece
capable of sliding adjustment, and provided with a totally reflecting
glass prism to allow of the instrument being held in a convenient
position. A scale is provided by means of which the index can be read
to two places of decimals at least. In using the instrument the gem to
be observed is placed on the plane surface of the hemisphere and
moistened with a drop of a liquid of higher refractive index than
itself; on looking through the eyepiece a shadow is seen crossing the
scale in the form of a circular arc. By using a hemieylindrical glass
and a
