174 PEARLS AND PEARLING 



rangement of the reflecting surfaces of the curved edges 

 presents an endless variety of beautiful tints and colors. 

 The wealth of the colors in the arched edges artistically 

 portrays a magnificent rain-bow or reminds one of the 

 lavish display of rich colors often seen among small 

 clouds at sunrise; and if the shell is moved slightly, a 

 pretty representation of the aurora borealis may be seen 

 in the unique arrangement of the laminae of the adductor 

 scar. 



Now the fact is that while the shell has the appear- 

 ance of being composed of a series of richly colored lay- 

 ers of nacre, the various layers are about the same and 

 the beautiful series of colors is an optical illusion, which 

 is known as the phenomenon of interference. The word 

 interference is the term employed to express the. effect 

 which rays of light, after being bent or diffracted, pro- 

 duce on each other. The large number of narrow, curved 

 surfaces which are in the adductor scar furnish one of 

 the best examples of the phenomenon of interference. 



The pearl is also composed of a series of nacreous lay- 

 ers interstratified with animal membrane, but the pearl 

 is made up of concentric layers, each new layer com- 

 pletely surrounding those already made. There is also 

 another difference in their structure. While the shell is 

 composed of three distinct parts, the conchioline epider- 

 mis, the inner prismatic formation, and the nacreous lin- 

 ing, only one of these three materials is used ordinarily 

 in the formation of the pearl, this one being the nacreous 

 material which is used in making the lining of the shell. 

 This material is the finest that is used in the formation of 



