114 PROCEEDINGS OF THE ACADEMY OF [Feb., 



use in cutting a cabochon ruby to bring out its brilliant color to best 

 advantage. 



There are other minerals beside opal which show colored reflections 

 due to successive laminae. The "moon stone" from Delaware Co., 

 Penna., consists of triclinic feldspar, albite or oligoclase, and the 

 opalescence is shown on the face parallel to the laminae due to repeated 

 twinning, and only on such specimens in which the laminae are ex- 

 tremely fine. The reflections from this mineral vary from a pearly- 

 white to sky-blue, but in the closely related labradorite they assume 

 all tints, although never as pure as in the case of the opal, probably 

 owing to the less uniform spacing of the laminae, although these are 

 thin enough to account for the colors. Labradorite sometimes like- 

 wise shows entirely different reflections corresponding to those of 

 "sun stone" and due to the same cause, interference resulting from 

 single thin films of included minerals. 



The chatoyance of Ceylon "moon stone," frequently cut as a gem, 

 cannot be accounted for in a similar manner, as it consists of adularia, 

 a variety of orthoclase, which does not contain twin laminations, but 

 on examining a section at right angles to the opalescent face, with 

 polarized light and comparatively high powers, a very fine micro- 

 perthite structure, due to intergrowth with a small percentage of 

 another feldspar, becomes apparent. The micro-spectroscope is of 

 no help here, as the spectra show no characteristic appearances, but 

 the fact that the reflected blue light from one of these "moon 

 stones" is partially polarized raises the question whether it may not 

 be due, at least in part, to the scattering of light such as causes the 

 blue color of a clear sky, and the microscopic texture of the mineral 

 seems better calculated to produce this effect than it does to produce 

 the color by any known form of interference. 



If authorities be consulted as to the cause of the luster of pearls, 

 the explanation given will be generally found to be that it is due to 

 the breaking up of the light by reflections from minute corrugations 

 with which the surface of the pearl is covered. This explanation, 

 originally given by Sir David Brewster, has been copied by all the 

 writers in whose works I have been able to find a reference to the 

 subject. Brewster made an extended investigation of the phenomena 

 shown by pearl shell in the form of plates and sections, and finding 

 that light from a contracted source produced undoubted diffraction 

 effects when reflected from the surfaces of such pieces of pearl, con- 

 cluded that the fine wavy or'serrated parallel lines which he discovered 

 were largely responsible forlthe pearly luster, without apparently 



