58 EVENINGS AT THE MICROSCOPE 



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. Sir D. Brewster appears to sup- 

 pose that nacre consists of a multitude of layers of carbon- 

 ate of lime, alternating with animal membrane, and that 



SECTION OF NACRE FROM PEARL OYSTER. 



the presence of the grooved lines on the most highly pol- 

 ished surface is due to the wearing away of the edges of 

 the animal laminae, while those of the hard calcareous 

 laminae stand out. . . . There is one shell, however, the 

 well-known Haliotis splendens, which affords us the oppor- 

 tunity of examining the plaits without any disturbance of 

 the arrangement, and thus presents a clear demonstration 

 of the real structure of nacre. This shell is for the most 

 part made up of a series of plates of animal matter, resem- 

 bling tortoise-shell in its aspect, alternating with thin lay- 

 ers of nacre; and if a piece of it be submitted to the ac- 



