When the gem has been cut for ornamental purposes it is often difficult to adjust the faces of it to the plates of the polariscope. Then recourse may be had to the use of a glass cell containing a fluid of a high refractive power, like the oil of cassia, turpentine, sulphuret of carbon, etc. The stone immersed in these fluids admits the light in all directions, and is then easily viewed through the plates of the instrument. These fluids, however, are not of sufficiently high refractive powers to do justice to the diamond and the zircon, but they answer admirably for all gems of a less refractive power, like the varieties of sapphire, spinel, topaz, garnet, tourmaline, emerald, etc. The tourmaline tongs furnish the expert with a rapid and easy method of detecting many of the gems by reason of the phenomena of their refraction. But the experimentalist must be on his guard lest he pronounce substances to be of double when they really possess but single refraction. Glass has a tendency to crystalline regularity when heated and cooled suddenly, and may therefore acquire the property of polarizing the ray that passes the first plate of tourmaline and dispose of a part of that which passes the second. Certain minerals of the cubic system produce the same result by reason of a certain rare but forced arrangement; and some crystals, like the topaz, when cut in a certain direction to their optical axes cease to exhibit the phenomena of double refraction. One of the chief tests used by the jewellers of olden times to distinguish the diamond was the test of the tincture. This tincture was a varnish made of ivory black and mastic, and when applied to the back of the diamond seemed to increase its lustre, while other gems were impaired in their natural effects. Modern investigators, however, have shown that this procedure is a fallacious one, and is in reality one of the absurd traditions which have been attached to the gem from early times, like some of the supposed spiritual properties. A perfect diamond must stand the tests for purity, faultlessness, and transparency, and when these are carefully applied to the stone perfect gems will be found to be very rare.