Another curious phenomenon, called single refraction, enables the observer to distinguish the mineral from all other gems except the garnet and spinel, the others having double refraction, or, in other words, giving a double image of a candle-light when it is viewed through their facets. For the purpose of observing this phenomenon, Sir David Brewster invented an instrument which he called a lithoscope. It consisted of a small glass prism which moved around a fixed joint so that the lower surface of it could be laid upon the surface, or a facet of the stone to be examined. In this position, the two surfaces being parallel, the image reflected from the lower surface of the prism would coincide with that reflected from the surface of the stone. A drop of the oil of cassia or of sulphuret of carbon is placed between the prism and the facet, and then the observer turns a screw to raise the prism a little round its joint. The effect of this is to separate the image of a light or a small luminous aperture as given by the prism from that given by the facet; and the difference in the intensity and the color of these two images is an infallible indication of the nature of the stone. The image from the diamond will be many times brighter than that reflected from the face of the prism when testing any of the other precious stones. A simpler mode is sometimes adopted by experts, but it requires some dexterity to exhibit the property. The method is this: the diamond is held up to the eye, and a needle point or a small hole pierced in a card is looked at. If the object is seen double, as if there were two needle points or two holes, then the stone examined is not a diamond, as but one aperture should be seen. With the aid of the tourmaline tongs, however, we have an easier method--subject to a few exceptions--of detecting the properties of refraction of all gems even when they have been cut. The transparent tourmaline, when cut in thin plates parallel to the axes of its natural crystals, possesses the strange and extraordinary power of extinguishing or causing to disappear one of the rays of polarized light, while the other is preserved. Therefore, when a body possessing single refraction, although perfectly transparent, is placed between the two thin slices of tourmaline composing the tourmaline tongs or polariscope, no light passes through; the instrument does not afford the least glimmer of a ray of light. But when the transparent body interposed in the polariscope is of double refraction, light passes through as if by magic.