60 EVENINGS AT THE MICROSCOPE 



and these I will cite from the abstract translation of them 

 made by Mr. Dallas. 



"The surface of pearls is not perfectly smooth, but cov- 

 ered with very fine microscopic elevations and depressions. 

 These are more or less irregular in their altitude, but ap- 

 proach most nearly to equality in pearls of the finest water. 

 In pearls which exhibit a certain iridescence, and which, 

 when turned in different directions toward the eye, present 

 even very faint bluish, greenish, and reddish tints, the sur- 

 face is found to present delicate irregular curved furrows} 

 which either run tolerably parallel to each other, or form 

 small irregular closed curves. This is due to the mode of 

 growth of the pearl, in which thin layers of nacre, of small 

 dimensions, have been laid over each other. There is no 

 continuous layer over the pearl, but a number of small 

 portions which sometimes overlie the margins of the sub- 

 jacent layers, and sometimes leave them uncovered. This 

 structure is seen most distinctly in the pearl shell, where 

 the conditions are rendered more simple by the layers be- 

 ing deposited on a flat, or but slightly curved, surface. 

 The distance of the furrows from each other is not always 

 the same; sometimes they may be recognized with the 

 simple lens, while on other parts they approach within 

 T^th of an inch of each other. That the iridescence of 

 nacre, or the nacreous color, as distinguished from pearly 

 lustre, is caused by the interference of the light reflected 

 from these furrows and the intervening edges of the strata 

 is proved by the circumstance, ascertained by Brewster, 

 that impressions of mother-of-pearl taken in red or black 

 sealing-wax exhibit the same phenomena of color distinctly. 

 In pearls, in consequence of their spherical form, the dif- 

 ferent masses of colored light are so diffused that they 



