Mothcr-of- Pearl Shell. 87 



due to the presence of any material pigment or 

 colouring matter, as might at first sight be supposed, 

 but is referable to the action of light on its deli- 

 cate structure. Like m.ost other shells, the Mother- 

 of-Pearl consists of carbonate of lime, with a little 

 animal matter serving as a connective tissue or 

 frame-work. In Dr. Watts' Dictionary of CJiemistiy, 

 the Mother-of- Pearl is said to consist of 66 per cent, 

 of carbonate of lime, 2.5 of organic matter, and 

 31.5 of water. The chemical composition, however, 

 offers no explanation of the cause of the exquisite 

 pearly lustre, and science is indebted to the late 

 Sir David Brewster for the discovery of the real 

 cause of this phenomenon. He was the first to 

 point out that the iridescence of Mother-of-Pearl 

 does not reside in the shell, nor depend on the 

 chemical nature of its substance, but is due to the 

 delicate striatio'ns of its surface. 



As far back as 18 14, Brewster published the 

 result of his researches on the structure of Mother- 

 of-Pearl, in a letter addressed to Sir Joseph Banks, 

 and presented to the Royal Society. His views on 

 this subject were also set forth in his well-known 

 "Treatise on Optics," contributed in 1831 to Lardner's 

 Cabinet Cyclopcedia. 



The late Sir John Herschel also applied himself 

 to the optical study of Mother-of-Pearl, and his 



