The mirage is an optical phenomenon in which images of distant objects
are seen, reflected beneath, or suspended in the heavens above.
Occasionally, also, objects are seen double, being repeated laterally
instead of vertically.
The mirage is caused by the refraction and reflection of light as it
passes from denser to rarer strata of air. It is most frequent in arid
plains, where the soil, exposed to the burning rays of the sun, becomes
intensely heated, and, in consequence, the strata of air near the ground
are less dense than those above.
In this case rays of light passing from any distant object, as a tree,
to the ground, are refracted more and more towards the horizontal, until
finally they are reflected from a horizontal layer of the heated air,
and reach the eye from beneath. Then an image of the object is seen as
if mirrored in the tranquil waters of a lake.
[Illustration: =THE MAGNIFICENT CURTAINS OF LIGHT THAT FORM THE AURORA
BOREALIS=]