ing through the dichroscope the square hole appears double, since both the ordinary and extraordinary ray give an image. If a piece of mineral or cut stone is held in front of it, two images of this are likewise seen. These images are of different colors if the mineral is a doubly refracting one, since the two rays are differently absorbed in passing through such a mineral. The two images being side by side even slight differences of color can be perceived. The following are some of the twin colors exhibited by the more important gems when viewed in this manner, as stated by Church:
	The dichroscope thus affords a convenient and accurate means of distinguishing gems. Any one of those in the above list, for example, could be distinguished from a glass imitation by the fact that any glass substitute would give two images of exactly the same color, instead of different colors, as would the genuine stone. Such gems as diamond, garnet, and spinel cannot, however, be distinguished from glass or each other in this manner, since they give similarly colored images.
	33