26	PRECIOUS STONES.
	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