Mother-of-Pearl Shell. 89 



weak acid, the calcareous matter is removed ; yet 

 the decalcified shell retains the iridescence as long 

 as it remains undisturbed, but loses its lustre when 

 pressed flat. It is, therefore, evidently the striated 

 structure which gives rise to the nacreous lustre. 

 A cast taken from a piece of Mother-of-Pearl in 

 wax, mastic, or fusible metal, displays similar iri- 

 descence, in consequence of its having received the 

 microscopic rippling on its surface. According to 

 Brewster — "A solution of gum-arabic or of isin- 

 glass, when allowed to indurate upon a surface of 

 Mother-of-Pearl, takes a most perfect impression 

 from it, and exhibits all the communicable colours 

 in the finest manner, when seen either by reflexion 

 or transmission." 



It is worth noting that the production of 

 rainbow-like effects from delicately grooved surfaces 

 produced artificially, is well illustrated by the or- 

 naments called " Barton's Buttons." Many years 

 ago the late Mr. John Barton, of the Royal Mint, 

 succeeded in cutting grooves upon steel at a 

 distance of from the 2,000th to the 1 0,000th of 

 an inch apart. The metal surfaces thus treated 

 displayed the most brilliant colours in consequence 

 of the " interference " of the reflected undulations 

 of light. His "iris ornaments" consisted of buttons 

 and other objects wrought in brass and stamped 



