MOTHER-OF-PEARL, or NACRE, is the hard, silvery, internal layer of several kinds of shells, especially oysters, the large varieties of which in the Indian seas secrete this coat of sufficient thickness to render the shell an object of manufacture. The genus of shell-fish, _Pentadinæ_, furnishes the finest pearls as well as mother-of-pearl: it is found round the coasts of Ceylon, near Ormus in the Persian Gulf, at Cape Comorin, and in some of the Australian seas. The dealers in pearl-shells consider the Chinese from Manilla to be the best: they are fine, large, and very brilliant, with yellow edges. Fine large shells of a dead white are supplied by Singapore. Common varieties come from Bombay and Valparaiso, from the latter place with jet black edges. South Sea pearl-shells are common, with white edges. The beautiful dark-green pearl-shells called _ear-shells_ or _sea-ears_ are more concave than the others, and have small holes round the margin: they are the coverings of the _Haliotis_, which occurs in the Californian, South African, and East Indian seas. In the Indian collection of the Great Exhibition in London, specimens of the finest pearl-shells were shown, known in commerce as _flat-shells_, _ear-shells_, _green snail-shells_, _buffalo-shells_, _Bombay shells_. It is stated that the shores of the Sooloo Islands afford the finest shells. The beautiful tints of mother-of-pearl depend upon its structure; the surface being covered with a multitude of minute grooves, which decompose the reflected light. Sir David Brewster, who was the first to explain these chromatic effects, discovered, on examining the surface of mother-of-pearl with a microscope, "a grooved structure, like the delicate texture of the skin at the top of an infant's finger, or like the section of the annual growths of wood as seen upon a dressed plank of fir. These may sometimes be seen by the naked eye; but they are often so minute that 3,000 of them are contained in an inch." It is remarkable that these iridescent hues can be communicated to other surfaces as a seal imparts its impress to wax. The colors may be best seen by taking an impression of the mother-of-pearl in black wax; but "a solution of gum-arabic or of isinglass (white glue), when allowed to indurate upon a surface of mother-of-pearl, takes a most perfect impression from it, and exhibits all the communicable colors in the finest manner, when seen either by reflection or transmission. By placing the isinglass between two finely-polished surfaces of good specimens of mother-of-pearl, we obtain a film of artificial mother-of-pearl, which, when seen by single lights, such as that of a candle, or by an aperture in the window, will shine with the brightest hues."