the Pearl Oyster and on the Production of Pearls. 93 



lustrous as solid pearls, except that their whiteness is destroyed. 

 " The two masses of light entering the eye, act upon it from 

 different distances. Now as it adapts itself to the body seen 

 through the transparent layer, it cannot distinctly see the light 

 reflected from the surface, and the consciousness of this infinitely 

 perceptible reflexion produces the phsenomenon of lustre'^ *. 

 The thinner and more transparent the layers of which the pearl 

 consists, the more beautiful is its lustre, and in this respect the 

 sea-pearls excel those of our river-mollusks. 



Besides the furrows on the natural or cut surfaces of mother- 

 of-pearl, fine sections or laminae of cleavage exhibit a second 

 system oi fine dark lines. These, however, are never so distinct 

 as the superficial furrows, but always appear somewhat cloudy 

 and very finely undulated. In several specimens in the posses- 

 sion of Dr. Mobius they follow the same direction, whilst that 

 of the superficial furrows varies frequently; so that the two 

 systems are sometimes parallel and sometimes intersect each 

 other at all angles. These minute lines also remain at the same 

 distance apart (yy^^^th of an inch, according to Herschelf). 

 This structure of the nacreous layers is probably of great im- 

 portance in the dispersion of the light which is reflected by 

 mother-of-pearl. 



In pearls these fine dark lines usually run round the globe 

 in the direction of circles passing through its poles. Sometimes, 

 however, they run in very various directions, and in a pearl 

 from Unio margaritifer from which Dr. Mobius had dissolved 

 the carbonate of lime by acetic acid, the lines of the different 

 strata of membrane crossed each other at various angles. 



In thin sections of pearls a quantity of fine lines run round 

 the central point in the layer of nacre, but these rarely form 

 closed curves, most of them losing themselves again without 

 completing the circuit, and thus showing, like the superficial 

 furrows, that the pearl does not grow by complete spherical 

 coats, but by the superposition of small segments. Sometimes 

 layers occur which run almost all round, and show that they 

 were formed at one time, by their uniformly darker colour. 

 These concentric lines of the nacre are intersected by two systems 

 of extremely fine lines at angles of about 10-15°, whilst the 

 latter intersect each other at angles of about 20-30°. These 

 lines appear to be due to an intimate structure of the nacreous 

 layer, probably to the unequal inclination of its particles towards 

 the cut surface. When a portion of the layers are cut at an 

 acute angle, transitions are seen to take place from the super- 

 ficial furrows to these lines. It is also probable that the above- 



* Dove, Farbenlehre und optische Studien, p. 117. 

 t Edinbuif^h Phil. Journ. ii. p. 114, 1820. 



