88 Pearls. 



results appeared in the Edinburgh Philosophical 

 Journal in 1820. 



An elaborate examination of the microscopic 

 structure of shells was undertaken many years ago 

 by the late Dr. W. B. Carpenter, and some valuable 

 results obtained as to the nature of the nacreous 



varieties. 



Microscopic examination of a thin section of 

 nacre shows that the surface is traversed by nu- 

 merous delicate lines ; in some cases almost straight, 

 while in others they are crumpled and corrugated. 

 These lines are produced by the outcrop of thin 

 laminae running more or less obliquely to the surface. 

 It appears that Sir David Brewster regarded them as 

 the edges of hard calcareous layers, alternating with 

 softer membranous laminae ; the latter being more 

 readily worn away, would naturally form grooves, 

 while the former being hard, would stand out as 

 alternating ridges. Dr. Carpenter, however, regarded 

 the lines on the nacre as the plaited edges of a 

 single membranous layer. 



Whatever be the exact nature of the lineation, 

 it is certain that the iridescence is due to the 

 interference of light reflected from the undulations 

 of the delicately-grooved surface. 



When a piece of Mother-of-Pearl is digested in 



