When
examining a stone for dichroism it is necessary that the specimen
should be looked through in some direction other than that (along a
certain optic axis) in which the crystal is only singly refracting.
Trials in different positions, for the optimum effect, having been
made, the stone should be fixed in such a way that it can be placed
close to the square opening of the dichroiscope. A disc of millboard
having a hole in the middle may be used as a holder, the specimen being
fixed in position by means of a little blackened beeswax. Then the two
images of the square opening of the dichroiscope should be focussed
sharply by means of the sliding cap. It will be observed that one image
of the opening is nearly central ; this represents the ordinary ray,
for which, in the present volunv, the symbol w is employed. The
other image, formed by the extraordinary ray and expressed by the
symbol e, is more displaced, and is distinguished by a narrow blue
border on its outer edge, and by a narrow red border on its inner edge.
On turning the instrument round the greatest differences of hue between
the two images furnished by a dichroic stone will be seen four times
during a single rotation; four times the two squares will be identical
in colour. These phenomena correspond to eight positions, all 45
degrees apart in the circle of 360 degrees. With coloured glasses and
all other singly-refracting substances the images are alike in all
positions.
Further discussion of the optical properties of precious stones, including the colour-effects produced by diffraction, absorption, and interference, would
be out of place in a handbook of elementary character. For detailed
description of the phenomena in question reference may be made to any
treatise on experimental optics; for a brief account the author's
little book on " Colour " (Cassell & Co.) may be consulted.
Hardness.—One
of the characters by which gem-stones.may be distinguished from each
other and from their imitations is that of the degree in which they
possess the power of resisting abrasion.
