Sir David Brewster, in conducting his famous experiments in optics, was of the opinion that the white sapphire, on account of its structure and its refractive power, was superior to all other transparent minerals for lenses for the microscope. The diamond, which one would naturally suppose to be the most perfect material for the purpose, on account of its high refractive power and apparent clearness, is really faulty, and comparatively worthless on account of its internal structure. If the white sapphire is of such excellence in this respect, on account of its compactness and refractive power, why will not the white zircon prove far superior as a lens, as it is the most compact, transparent, hard mineral known, and its refractive power is much greater than that of the sapphire? Black sapphires are now and then mentioned by authors, but we are inclined to believe that they are very rare; for Davy declares that he met with but two or three specimens in his travels in India or Ceylon. Blue stones of very deep hue appear sometimes quite black; but when they are placed in a strong light, and viewed in another direction, the blue tinge clearly appears. The violet sapphire of perfect hue is a very rare gem, and may be regarded as an accidental stone, being formed of an admixture of the blue and the red. Davy, in all his extended researches in Ceylon, found but two specimens of violet sapphire; and in our examinations of the rough gems from the mines, we are inclined to think the purple spinel is often supposed to be a purple sapphire. Romè de L’Isle found that this variety of sapphire is oftener ruby-violet than sapphire-violet, or that the red tinge prevailed more distinctly than the blue. The lilac-blue are exceedingly rare, and are eagerly sought for by amateurs. There are some sapphires which exhibit a double play of colors when viewed by natural and then by artificial light. For instance, they may display a decided blue color by day and an amethystine tint by night. D’Auguy possessed a stone that showed in the daylight a beautiful, clear, and sparkling blue, but by candlelight it changed to a royal purple. The cause of this phenomenon is perhaps due to an excess of latent red in the stone, which, however, is not visible in the daytime; but which is called forth by the difference in the illuminating lights, as is shown in their spectra. The hyacinth sapphire is seldom seen, and when perfect is regarded as among the marvels of the species. Dutens possessed a fine one which had been engraved upon by the Greeks.