MOLLUSCA : THEIR SHELLS. 45 



of nacre, of small dimensions, have been laid over each 

 other. There is no continuous layer over the pearl, but a 

 number of small portions which sometimes overlie the 

 margins of the subjacent layers, and sometimes leave 

 them uncovered. This structure is seen most distinctly 

 in the pearl shell, where the conditions are rendered more 

 simple by the layers being deposited on a flat, or but 

 slightly curved surface. The distance of the furrows from 

 each other is not always the same ; sometimes they may 

 be recognised with the simple lens, while on other parts 

 they approach within jxnnr^ °f an i 110 * 1 °^ eac ^ other. 

 That the iridescence of nacre, or the nacreous colour, as 

 distinguished from pearly lustre, is caused by the inter- 

 ference of the light reflected from these furrows and the 

 intervening edges of the strata, is proved by the circum- 

 stance, ascertained by Brewster, that impressions of 

 mother-of-pearl taken in red or black sealing-wax exhibit 

 the same phenomena of colour distinctly. In pearls, in 

 consequence of their spherical form, the different masses 

 of coloured light are so diffused that they unite to form 

 white light ; and this takes place with the greater perfec- 

 tion in proportion as the furrows are lost, and become 

 converted into a surface of fine elevations and depres- 

 sions. 



" For their lustre, pearls are indebted to their being 

 composed of fine layers, which allow light to pass through 

 them ; whilst the numerous layers, lying one under the 

 other, disperse and reflect the light in such a manner that 

 it returns and mixes with that which is directly thrown 

 back from the outer surface. It is the co-operation of 

 light reflected from the surface, with light dispersed and 

 reflected in the interior, that gives rise to lustre ; for this 

 reason the knots of window-glass exhibit pearly lustre, and 

 the membranes of pearls deprived of their line are almost 

 as lustrous as solid pearls, except that their whiteness is 

 destroyed. ' The two masses of light entering the eye, act 



