43 



EVENINGS AT THE MICKOSCOPE. 



by any amount of polishing, it is obvions tliat tlieir 

 presence depends upon sometliing peculiar in the tex- 

 ture of this substance, and not upon any mere super- 

 ficial arrangement. When a piece of nacre is carefully 

 examined, it becomes evident that the lines are pro- 

 duced by the cropping out of laminae of shell, situated 

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

 greater the dip of these laminae, the closer will their 





/ ■ 



iMtii^ii 



BF.CTIOS OF SACr.C rr.OM TEAEL OTSTEE. 



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 til at space. And thus the appearance of a section 

 of nacre is such as to have been aptly compared by Sir 

 J. Ilerschel to the surface of a smoothed deal board, in 

 which the woody layers are cut perpendicidarly to their 

 surface in one part, and nearly in their plane in an- 

 other. Sir D. Brewster appears to suppose that nacre 



