232 THE MECHANICAL FUNCTIONS. 



ance is caused by the peculiar thinness, transpa- 

 rency, and regularity of arrangement of the outer 

 layers of the membrane, which, in conjunction 

 with the particles of carbonate of lime, enter into 

 the formation of that part of the surface of the 

 shell. The surface, which has thus acquired a 

 pearly lustre, was formerly believed to be a pe- 

 culiar substance, and was dignified with the ap- 

 pellation of mother of pearl, from the notion that 

 was entertained of its being the material of which 

 pearls are formed. It is true, indeed, that pearls 

 are actually composed of the same materials, and 

 have the same laminated structure as the mem- 

 branous shells ; being formed by very thin con- 

 centric plates of membrane and carbonate of 

 lime, disposed alternately, and often surrounding 

 a central body, or nucleus : 

 but Sir David Brewster has 

 satisfactorily shown that the 

 iridescent colours exhibited 

 by these surfaces are wholly 

 the effect of the parallel 

 grooves consequent upon the 

 regularity of arrangement in 

 the successive deposits of 

 shell.* The appearance of these grooves or striae 

 when highly magnified is shown in Fig. 106. f 



* Philosophical Transactions for 1814, p. 397. 



-f- See also a paper on this subject by Herschel in the Edin- 

 burgh Philosophical Journal, ii. 114, from which the annexed 

 fiinire is taken. 



