If the second medium be very dense in
proportion, the ray will be both refracted and reflected; and the object
from which it proceeds, will assume a variety of grotesque and
extraordinary shapes, and it will sometimes appear as in a reflection
from a concave mirror, dilated in size, and changed in situation.
The following striking effects are known to proceed from this simple
cause.
The first is the mirage, seen in the desert of Africa. M. Monge, a
member of the National Institute, accompanied the French army into
Egypt. In the desert, between Alexandria and Cairo, the mirage of the
blue sky was inverted, and so mingled with the sand below, as to impart
to the desolate and arid wilderness an appearance of the most rich and
beautiful country. They saw, in all directions, green islands,
surrounded with extensive lakes of pure and transparent water. Nothing
could be conceived more lovely and picturesque than this landscape. On
the tranquil surface of the lakes, the trees and houses, with which the
islands were covered, were strongly reflected with vivid hues, and the
party hastened forward to enjoy the cool refreshments of shade and
stream, which these populous villages preferred to them.
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