176 PEARLS AND PEARLING 



the shell and has the appearance of being composed of a 

 large number of very thin wave plates slightly over- 

 lapping each other and set in animal membrane. 



The layers or strata of the pearl are not always regu- 

 lar. Pearls are often found in which there is quite a 

 variation in the stratification. Some layers are thicker 

 than others or have a spiral formation as if they had 

 been made by rolling the pearl in the nacreous material. 



Pear pearls are often pearls that were once round, but 

 which have become elongated by the irregular deposit of 

 nacreous material. Button pearls are somewhat flattened 

 on account of being pressed against the shell, but if these 

 become loose in the mantle they gradually become round. 



The normally formed ball pearl is composed of a series 

 of layers very similar to the structure of an onion. 



In Fig. 43 cross section views of an onion and a pearl 

 are shown to illustrate their similarity. 



The arrangement of the transluscent strata of the 

 pearl are such as to cause the phenomenon of interfer- 

 ence, but not in just the same manner as it is produced 

 by the edges of the strata in the adductor scar of the 

 shell. 



In the drawing of the pearl, the vertical dotted line is 

 intended to represent a ray of light entering the trans- 

 luscent layers of the pearl. 



There is a difference in the circumference of the vari- 

 ous strata of the pearl, and, therefore, the ray of light 

 falls more obliquely upon some layers than others. As 

 the light strikes the strata at various angles, it becomes 

 bent or diffracted, and, if the rays meet after diffraction, 



