The pearl ; its story, its charm, and its value . he very thin transparent plates of nacre, whichcause a double interference and produce thebeautiful iridescence peculiar to the lining ofthese shells. Interference, as it is called, is an opticalphenomenon arising from two causes. Whenlight falls upon a sufficiently thin transparentsurface covering a denser substratum notexactly parallel with it, part of the light is atonce reflected. Of that which passes throughto the under surface a part also is in turn re-flected through the first surface, and the con-fusion of rays or interference resulting
The pearl ; its story, its charm, and its value . he very thin transparent plates of nacre, whichcause a double interference and produce thebeautiful iridescence peculiar to the lining ofthese shells. Interference, as it is called, is an opticalphenomenon arising from two causes. Whenlight falls upon a sufficiently thin transparentsurface covering a denser substratum notexactly parallel with it, part of the light is atonce reflected. Of that which passes throughto the under surface a part also is in turn re-flected through the first surface, and the con-fusion of rays or interference resulting, pro-duces to the eye the sensation of color. A familiar illustration is seen when a thinfilm of oil is spread over water. The other wayin which iridescence by interference is pro-duced in shells, may be demonstrated by draw-ing fine lines close together on glass with adiamond. Light falling upon them will makethe surface iridescent. Melted wax droppedupon this striated surface would, upon removal,show a like iridescence, reproduced with the130. VENEZUELAN PEARL-SHELL, WIIH ATTACHED GENESIS OF PEARLS impression of the fine lines. The outer markingsof the large Australian shell are similar to thesmall Venezuelan. The mother-of-pearl interioris not so iridescent. Pearls and the shells in which they grow arecomposed almost entirely of calcium carbonateor lime. A small percentage of organic matterand water are the other ingredients. As pearls are accidental and the result of amisdirection of normal processes, a generalknowledge of those processes is necessary to aninsight into the nature and genesis of the pearl,and as pearl shells and the pearls in them areconstructed on the same general plan, a knowl-edge of the former will assist to a better under-standing of the gem and its eccentricities. Themother-of-pearl shell is built up of a series ofcalcium carbonate plates or prisms set in organicmatter. In the material of the inner shell, thecalcium carbonate greatly preponder
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