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
