90 Pearls. 



with hardened steel dies upon which the dehcate 

 lines had been engraved. Nothing can better illus- 

 trate the nature of the optical phenomena presented 

 by nacre, or Mother-of-Pearl, than their artificial 

 reproduction by Barton's method. 



It is therefore demonstrated that the iridescent 

 colours of nacre are produced by the light reflected 

 from the corrugated surface, resulting in that 

 beautiful appearance by which the very atoms 

 of the substance seem as if lit up by colour; 

 now at one end of the solar spectrum where violet, 

 blue and green rays predominate, and now at the 

 other end where red, orange and yellow unite in 

 such harmonious blending as to produce the most 

 pleasing ofFects. 



This nacre then composes the whole interior 

 of the shell, and is the same secretion which in the 

 Pearl has assumed a more or less globular form : 

 between nacre and Pearls, therefore, there is virtually 

 only the difference of the form of deposition. 



The Mother-of-Pearl shell lies on the sea bottom, 

 usually inclined at an angle of 20 degrees, with the 

 flat valve downwards, although it is frequently found 

 reversed. In young shells, more particularly the 

 black-edged variety, there is a byssiLs or bunch of 

 strong fibres which passes through the hinge or 



