THE OTSTEE. 87 



lustre, is the abortive egg of an oyster enveloped in its 

 own nacre, who will not be struck with wonder and 

 astonishment?" "Wonder and astonishment are words 

 which scarcely exist now. Science has shown so many 

 wonders that we are hardly astonished at anything ; but 

 Sir Everard's assertion admits of proof. A pearl cut in 

 two exhibits the concentric layers like an onion, as may 

 be seen through a strong lens ; and in the centre is a 

 round hole, very minute it may be, but wherein the 

 ovum has been deposited. 



Sometimes the ovum, or sand, or enclosed substance 

 has attached itself to the shell, and has then been 

 covered with mucus, forming a pearl which cannot be 

 separated from the shell. There are several specimens 

 of such pearls in the British Museum. 



The great beauty in pearls is their opalescence, and -a 

 lustre which, as we have before observed, however clever 

 the imitation, has never yet been given to artificial 

 pearls. Sir Everard Home supposes that this lustre 

 arises from the highly polished coat of the centre shell, 

 the pearl itself being diaphanous. Sir David Brewster 

 accounts for it by the pearl and mother-of-pearl having 

 a grooved substance on its surface resembling the 

 minute corrugations often seen on substances covered 

 with oil, paint, or varnish. Philosophers are some- 

 times not very explanatory. Sir David means to say 

 that beneath the immediate polish of the pearl there are 

 certain wavelets and dimples from which the light is 

 reflected. '^The direction of the grooves," again to 

 quote Sir David, ''is in every case at right angles to 

 the line joining the coloured image; hence, in iiTegu- 



