the
lapping of these thin transparent waves, and the minute undulations of
the layer edges reflecting through the transparent plates, which
produce the soft luster peculiar to the linings of the shells and the
surface of pearls, and which is known as "pearly."
The
wave edges do not usually produce iridescence, but if the waves are
very thin and transparent the undulating lines of many under waves
following close upon each other appear on the surface, under the
microscope, as dark lines when the light is passed through the skin, or
silvery lines if the light be thrown upon it from above; to the naked
eye this becomes the tempered brilliancy of the pearl's orient. Under
the microscope these waves appear to be constructed of minute
hexagonal plates or prisms set in animal membrane.
A
set of waves forming a plate, when broken at right angles to the trend
of the wave, shows under the microscope a rough irregular edge, and the
small plates of which they are composed sometimes appear separated
individually from the mass though more often they are dislodged in
clusters or strips. Broken with the trend of
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