THE mirage is an optical phenomenon, produced by the refractive
power of the atmosphere. The appearance presented is that of the
double image of an object in the air; one of the images being in the
natural position, and the other inverted, so as to resemble a natural
object and its image in the water. The mirage is commonly vertical,
or upright, that is, presenting the appearance, above described, of
one object over another, like a ship above its shadow in the water.
Sometimes, however, the image is horizontal, or upon the water, and at
other times, it is seen on the right or left hand of the real object,
or on both sides.
All the effects of the mirage may be represented artificially to the
eye. For this purpose, provide a glass tumbler two-thirds full of
water, and pour spirit of wine upon it; or pour into a tumbler some
syrup, and fill it up with water: as the water and spirit, or the
syrup and water incorporate, they will produce a refractive power;
then, by looking through the mixed or intermediate liquids at any
object held behind the tumblers, its inverted image may be seen. The
same effect, Dr. Walloston has shown, may be produced, by looking along
the side of a red-hot poker at a word or object ten or twelve feet
distant. At a distance less than three-eighths of an inch from the line
of the poker, an inverted image was seen; and within and without that,
an erect image.
The above phenomena may likewise be illustrated, by holding a heated
iron above a tumbler of water, until the whole becomes changed; then
withdraw the iron, and, through the water, the phenomena of the mirage
may be seen in the finest manner.
Or, look directly above the flame of a candle, or over the glass of a
lighted lamp, and a tremulous motion may be observed; because the warm
air rises, and its refracting power being less than that of the colder
air, the currents are rendered visible by the distortion of objects
viewed through them. The same effect is observable over chimney pots,
and slated roofs which have been heated by the sun.