These colors, as brilliant but more evasive than the hues of the rainbow, are not due to the presence of a pigment; they arise from a phenomenon of light and form one of the most wonderful illustrations of the ease with which our senses play tricks upon judgment and understanding. It is the striated surface and the very thin transparent plates of nacre, which cause a double interference and produce the beautiful iridescence peculiar to the lining of these shells. "Interference," as it is called, is an optical phenomenon arising from two causes. When light falls upon a sufficiently thin transparent surface covering a denser substratum not exactly parallel with it, part of the light is at once reflected. Of that which passes through to the under surface a part also is in turn reflected through the first surface, and the confusion of rays or "interference" resulting, produces to the eye the sensation of color. [Illustration: VENEZUELAN PEARL-SHELL, WITH PEARL ATTACHED] A familiar illustration is seen when a thin film of oil is spread over water. The other way in which iridescence by interference is produced in shells, may be demonstrated by drawing fine lines close together on glass with a diamond. Light falling upon them will make the surface iridescent. Melted wax dropped upon this striated surface would, upon removal, show a like iridescence, reproduced with the impression of the fine lines. The outer markings of the large Australian shell are similar to the small Venezuelan. The mother-of-pearl interior is not so iridescent. Pearls and the shells in which they grow are composed almost entirely of calcium carbonate or lime. A small percentage of organic matter and water are the other ingredients. As pearls are accidental and the result of a misdirection of normal processes, a general knowledge of those processes is necessary to an insight into the nature and genesis of the pearl, and as pearl shells and the pearls in them are constructed on the same general plan, a knowledge of the former will assist to a better understanding of the gem and its eccentricities. The mother-of-pearl shell is built up of a series of calcium carbonate plates or prisms set in organic matter. In the material of the inner shell, the calcium carbonate greatly preponderates; on the outside of the shell, the organic matter is largely in excess. In the building of its shell, the animal deposits the finest material and does the best and most compact work where the house is in touch with itself, the walls becoming coarser in construction and quality as they approach the outer surface. In the inside of the shell, the calcium carbonate plates are very fine and transparent, and the animal membrane in which they are set is of extreme tenuity. In the middle shell these plates become more chalky and less compact; in the exterior shell they are set in a thicker binding of organic matter and terminate outside in rough, horny fringes, completely covering the shell.