This name is given to similar illusions which are seen in various parts of the world, and more particularly to the illusive appearance of water, which is frequent in the sandy deserts of the East. You shall hear what the celebrated traveller, Dr. Clarke, says of it.
"We procured asses for our party, and, setting out for Rosetta, began to re-cross the desert, which appeared like an ocean of sand, but flatter and firmer as to its surface, than before. The Arabs, uttering their harsh guttural language, ran chattering by the side of our asses; until some of them calling out 'Raschid!' we perceived its domes and turrets, apparently upon the opposite side of an immense lake or sea, that covered all the intervening space between us and the city. Not having in my own mind, at the time, any doubt as to the certainty of its being water, and seeing the tall minarets and buildings of Rosetta, with all its groves of dates and sycamores, as perfectly reflected by it as by a mirror, insomuch that even the minutest detail of the architecture, and of the trees, might have been thence delineated, I applied to the Arabs to be informed in what manner we were to pass the water. Our interpreter, although a Greek, and therefore likely to have been informed of such a phenomenon, was as fully convinced as any of us that we were drawing near to the water's edge, and became indignant when the Arabs maintained, that within an hour we should reach Rosetta, by crossing the sands in the direct line we then pursued, and that there was no water. 'What,' said he, giving way to his impatience, 'do you suppose me an idiot, to be persuaded contrary to the evidence of my senses?' The Arabs, smiling, soon pacified him, and completely astonished the whole party, by desiring us to look back at the desert we had already passed, where we beheld a precisely similar appearance. It was, in fact, the Mirage, a prodigy to which every one of us were then strangers, although it afterwards became more familiar. Yet upon no future occasion did we ever behold this extraordinary illusion so marvellously displayed. The view of it enabled us to imagine the horrible despondency to which travellers must sometimes be exposed, who, in traversing the interminable desert, destitute of water, and perishing with thirst, have sometimes this deceitful prospect before their eyes."
Another traveller adds a particular which is well worthy of notice.
"The most singular quality of this vapour is its power of reflection. When a near observer is a little elevated, as on horseback, he will see trees and other objects reflected as from the surface of a lake. The vapour, when seen at a distance of six or seven miles, appears to lie upon the earth like an opaque mass; and it certainly does not rise many feet above the ground, for I observed that, while the lower part of the town of Abusheher was hid from the view, some of the more elevated buildings, and the tops of a few date-trees, were distinctly visible."
CHAPTER X.
HOW PARLEY SUPPOSES THESE APPEARANCES TO BE PRODUCED.
If you wish to understand the manner in which it is most probable that these illusions are produced, you must closely attend to what I am going to tell you. I will make my explanation as simple as I can, but I shall be able to teach you nothing, unless you do your part by paying attention.
You know that vision is produced by rays of light passing from the object seen, to the eye of the spectator. These rays have a tendency to form straight lines, and they would be perfectly straight, if they had only to pass through empty space; but this cannot be the case near the surface of the earth, as they there have to go through air, and frequently through other transparent substances.