mollusca: their shells. 61 



Biiriple by the layers being deposited on a Hat, or but 

 slightly curved, surface. The distance of the furrows 

 from each other is not always the same ; sometimes 

 the} may be recognised Avitli the simple lens, whilst on 

 other parts they approach Mithin 3 oV o^h of an inch of 

 each other. That the iridescence of nacre, or the na- 

 creous colour, as distinguished from pearly lustre, is 

 caused by the interference of the light reflected from 

 these furrows and the intervening edges of the strata, 

 is proved by the circumstance, ascertained by Brewster.^ 

 that impressions of mother-of-pearl taken in red or 

 black sealing-wax exhibit the same phenomena of col- 

 our distinctly. In pearls, in consequence of their spher- 

 ical form, the difi'erent masses of coloured light are so 

 diifused that, they imite to form white light ; and this 

 takes place with the greater perfection in proportion 

 as the furrows are lost, and become converted into a 

 surface of fine elevations and depressions. 



" 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 de- 

 prived of their lime are almost as lustrous as solid 

 pearls, excej)t that their whiteness is destroyed. ' Tlie 

 two masses of light entering the eye act upon it from 

 difierent distances. ISTow, as it adapts itself to the 

 body seen through the transparent layer, it cannot dis- 



