They
show, at the same time, how the light must necessarily undergo very
different modifications in the two cases, and why cut mother-of-pearl
can never have the same optical effects as the pearl.
Although
pearl molluscs exist in all parts of the world, there are but few
places where their gathering has become an industry. One of these
places was formerly the Red Sea, which, in the time of the Ptolemies,
produced an abundance of pearls. But the beds are probably exhausted;
at any rate they are no longer worked. The two regions which for a long
time have produced the most beautiful pearls are the Persian Gulf, and
the Straits of Manaar which separate Ceylon from the peninsula of India.
More
recently great quantities of pearl-oysters have been discovered in
America, particularly in the Gulf of Mexico, upon the coasts of
California, and in the vicinity of Panama.
There
have been experiments made to determine the time necessary to the
development of a pearl. No very certain results have been obtained; but
it has been proved that at least two or three years are necessary for
the formation of a pearl of any value.
Hitherto
the pearl shells have been gathered by divers, who, practising the
pursuit from their earliest years, end by being able to stay nearly six
