our
senses play tricks upon judgment and understanding. It is the striated
surface and the very thin transparent plates of nacre, which cause a
double interference and produce the beautiful iridescence peculiar to
the lining of these shells.
"Interference,"
as it is called, is an optical phenomenon arising from two causes. When
light falls upon a sufficiently thin transparent surface covering a
denser substratum not exactly parallel with it, part of the light is at
once reflected. Of that which passes through to the under surface a
part also is in turn reflected through the first surface, and the
confusion of rays or "interference" resulting, produces to the eye
the sensation of color.
A
familiar illustration is seen when a thin film of oil is spread over
water. The other way in which iridescence by interference is produced
in shells, may be demonstrated by drawing fine lines close together on
glass with a diamond. Light falling upon them will make the surface
iridescent. Melted wax dropped upon this striated surface would, upon
removal, show a like iridescence, reproduced with the
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