1911.] NATURAL SCIENCES OF PHILADELPHIA. 115 



giving due consideration to the facts that this luster was likewise 

 present in diffused light and that the lines described might not neces- 

 sarily be present in an undamaged pearl, but were due to cutting a 

 section through the lamina? which produced them. Brewster's own 

 description of his investigations proves that he did not overlook the 

 light reflected from the laminae of the pearl, but he does not appear 

 to have regarded it as differing from ordinary reflected light, and 

 paid most attention to the effects of diffraction, which he distinguishes 

 from the reflected light by the term "communicable colors" because 

 they could be communicated by pressure to another surface of softer 

 material. Later writers, however, seem to have regarded only the 

 lines whose influence on the true luster of a pearl is negligible. 



I have one shell of the so-called pearl oyster, Meleagrina margar- 

 tifera, apparently in its natural condition, which in places shows 

 comparatively coarse parallel lines on its surface, but have examined 

 numerous pearls without finding any such lines present, although 

 under high powers of the microscope the surface is by no means 

 smooth, but as soon as a small facet was ground on the pearl the 

 typical wavy lines appeared and could be seen side by side with the 

 unabraded surface in same field of view. 



Furthermore, the brilliancy of a pearl section is increased by mount- 

 ing in balsam, instead of being suppressed, as would be the case were 

 it due to corrugations on the surface, and if a nearly diametrical 

 section of a round pearl is observed by reflected light, the pearly 

 luster will be confined to a small spot near the center, where the 

 laminae are approximately parallel with the surface of the section. 

 Brewster mentions that the distance between the grooves varies 

 from a two hundredth to a five thousandth part of an inch, while 

 Carpenter states that they may be as close as a seventy-five hundredth 

 of an inch, both measurements evidently made on oblique sections, 

 as I have carefully counted the laminae in a section of Unio pearl at 

 right angles to the surface and found them to range between 54,000 

 and 57,000 to the inch. This brings the luster of pearls within the 

 range of Lord Rayleigh's explanation of colors due to repeated laminae, 

 which deduction may be confirmed by the micro-spectroscope. 



In round pearls used as gems the patches of color are generally 

 too small and intermingled to permit of satisfactory investigation in 

 this manner, but in sections of pearl shell places can readily be found 

 where the laminae are nearly parallel to the surface and sufficiently 

 flat to exhibit an area of uniform color large enough to produce a 

 good spectrum, and the single bands of color shown are sometimes 



