[Illustration: A MIRAGE IN THE DESERT.] We must remember, moreover, that the immensity, uniformity, and vacuity of the Desert, singularly contribute to render optical illusions frequent. The very serenity of the air assists in destroying the perspective to which we are accustomed in temperate climates, which are always more or less misty. Objects appear much nearer than they are in reality, because they are more distinctly visible, and also because nothing intervenes between them and the observer. Their dimensions, too, become arbitrary, for want of standards of comparison by which to measure them. So the trees and the mountains where the weary traveller hopes to obtain a temporary repose and a passing shelter from the Pythian's fiery arrows, seem constantly to recede before him, like the rainbow when pursued by the ignorant peasant; and, until experience has taught him to rectify the apparent testimony of his senses, he is doomed, like Tantalus, to be the victim of continual deceptions,-- "Ev'n in the circling floods refreshment craves, And pines with thirst amidst a sea of waves; When to the water he his lip applies, Back from his lip the treacherous water flies."[64] Nor is this all; hunger, thirst, weariness, and especially the action of the solar heat upon the brain, determine a peculiar pathological condition, a species of mental intoxication or delirium which powerfully predisposes the victim to hallucinations, and deprives the mind of that self-control which would enable it to chase away the phantoms that haunt it. To this affection, whose symptoms are frequently but erroneously confounded with those of the mirage, the Arabs have given a specific name. They call it _Ragle_. A distinguished French traveller has described it with exhaustive fulness,[65] and he attributes it to fatigue, excessive heat, and want of sleep. It shows itself most commonly at night, and in dreams, attacks of nightmare, and a somnambulism of which the sufferer is perfectly conscious, without being able to throw it off. By day strange hallucinations affect the sight, the hearing, and even, though less powerfully, the senses of taste and smell. The aberration extends, as far as the sight is concerned, to the objects which we are in the habit of seeing; a small stone, for instance, expands into a rock; the rut of a carriage-wheel enlarges into the furrow of a freshly ploughed field; a tuft of grass or a bush will assume the grand proportions of a forest; and, what is remarkable, these objects seem always close at hand. Another frequent error is the elevation of horizontal surfaces; the horizon becomes a wall or a mountain. "It has happened to myself," says M. d'Escayrac, "to meet with walls constantly reappearing before me. My extended arm has plunged into the masonry, but my body never encountered any obstacle; the rampart opened to give me a free passage."