Page:Handbook of Precious Stones.djvu/26

 A sliding cap at one end has a square perforation of about .12 inch; at the other end is a lens, or combination of lenses, of such focal length as to show a distinct image of the square opening when the cap is pulled out a quarter of an inch or so. With an instrument so constructed the pleochroism of the vast majority of gemstones may be determined at a glance. Of course this quality is so conspicuous in some species (tourmaline and iolite) that no instrument is usually needed to discern it. For it is easy to notice that the colours of some crystals, seen by transmitted light, vary with the direction in which they are viewed. If the transmitted ray be analysed by a Nicol's prism, its colour will be found to vary as the prism is turned round its axis; in fact, the two differently coloured beams are polarized in opposite planes. It is of course only in doubly refracting crystals that this phenomenon of dichroism occurs. In the descriptions given further on, of the several species of stone, these twin colours, as seen by the dichroiscope, are duly recorded. Here, however, it may be useful to group a few conspicuous instances of dichroism together; several are illustrated by Figs. 1 to 9 of the Frontispiece.