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 volume, the symbol ω is employed. The other image, formed by the extraordinary ray and expressed by the symbol ε, 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,
