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 iri­descence, in consequence of its having received the microscopic rippling on its surface. According to Brewster — "A solution of gum-arabic or of isin­glass, 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 or­naments 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 10,000th 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