assume that a beam of ordinary or natural light, freely traversing any medium, has what we may call identical properties on all its sides, then, should that beam encounter any obstacle, as by reflection or refraction, it exhibits to a greater or less extent different properties on different sides—is, in fact, polarized. One quality of this polarized light is that it cannot be again reflected at a certain angle, nor can it again traverse in a certain direction the crystal in which it has suffered this change. But the amateur of precious stones is mainly concerned with these two facts, that in some doubly refracting crystalline minerals the two oppositely polarized beams are of different colours; and, secondly, that some transparent gem-stones are more or less opaque, in one direction at least, to one of the two oppositely polarized beams. Thus it will be clear that upon double refraction and its concomitant polarization depends that property of many gems which is known as pleochroism, and which may be mostly easily recognised by that useful little instrument, the dichroiscope.
Pleochroism.—When a distinctly coloured precious stone is examined by means of a dichroiscope it will invariably show two images of the same hue or of different hues. Should the two images of the square opening of the instrument be identical in colour," then the specimen may be a garnet, a spinel, or a diamond; it cannot be a ruby, a topaz, or a beryl, all of which show twin colours differing in a perfectly recognizable degree from each other. However, before proceeding with the description of the special applications of the dichroiscope, a word on the construction of the instrument may be introduced. It consists of a cleavage rhombohedron of Iceland spar, having its longer edges nearly an inch long and its shorter edges about three tenths of an inch each. In the original form of the instrument a small glass prism of 18° was cemented on each of the small end faces of the prism; but this may be done away with if these end faces be ground and polished so as to be perpendicular to the length of the prism.
