44 EVENINGS AT THE MICROSCOPE. 



obvious that their presence depends upon something pe- 

 culiar in the texture of this substance, and not upon any 

 mere superficial arrangement. When a piece of nacre is 

 carefully examined, it becomes evident that the lines are 

 produced by the cropping out of lamina? of shell, situated 

 more or less obliquely to the plane of the surface. The 

 greater the dip of these lamina?, the closer will their edges 

 be ; whilst the less the angle which they make with the 

 surface, the wider will be the interval between the lines. 

 When the section passes for any distance in the plane of 

 a lamina, no lines will present themselves on that space. 

 And thus the appearance of a section of nacre is such, 

 as to have been aptly compared by Sir J. Herschel to the 

 surface of a smoothed deal board, in which the woody 

 layers are cut perpendicularly to their surface in one part, 

 and nearly in their plane in another." * 



Those beautiful objects, — so much prized for personal 

 adornment, — pearls, are concretions accidentally formed 

 within the shells of such mollusks, and are wholly com- 

 posed of the inner layer. Drs. Kelaart and Mobius have 

 recently published some highly interesting observations 

 on the causes both of the iridescence and of the pearly 

 lustre ; and these I will cite from the abstract trans- 

 lation of them made by Mr. Dallas. 



" The surface of pearls is not perfectly smooth, but 

 covered with very fine microscopic elevations and depres- 

 sions. These are more or less irregular in their altitude, 

 but approach most nearly to equality in pearls of the 

 finest water. In pearls which exhibit a certain iridescence, 

 and which, when turned in different directions towards 

 the eye, present even very faint bluish, greenish, and red- 

 dish tints, the surface 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 



* Carpenter, "The Microscope," 5<H. 



