Pliny mentions the Scythian regions within Mount Imaus, and Pomponius Mela those of Mauritania, behind Mount Atlas, as peculiarly subject to these spectral appearances. Diodorus Siculus likewise refers to the regions of Africa, situated in the neighborhood of Cyrene, as another chosen site:—“Even,” says he, “in the severest weather, there are sometimes seen in the air certain condensed exhalations that represent the figures of all kinds of animals; occasionally they seem to be motionless, and in perfect quietude; and occasionally to be flying; while immediately afterward they themselves appear to be the pursuers, and to make other objects fly before them.” Milton might have had this passage in his eye when he penned the allusion to the same apparitions:— “As when, to warn proud cities, war appears Waged in the troubled sky, and armies rush To battle in the clouds; before each van Prick forth the airy knights, and couch their spears, Till thickest legions close, with feats of arms From either side of heaven the welkin rings.” [Illustration: The Mirage of the Desert.] The mirage is the most familiar form of optical illusion. M. Monge, one of the French savans, who accompanied Buonaparte in his expedition to Egypt, witnessed a remarkable example. In the desert between Alexandria and Cairo, in all directions green islands appeared, surrounded by extensive lakes of pure, transparent water. Nothing could be conceived more lovely or picturesque than the landscape. In the tranquil surface of the lakes, the trees and houses with which the islands were covered were strongly reflected with vivid and varied hues, and the party hastened forward to enjoy the refreshments apparently proffered them. But when they arrived, the lake on whose bosom they floated, the trees among whose foliage they arose, and the people who stood on the shore inviting their approach, had all vanished; and nothing remained but the uniform and irksome desert of sand and sky, with a few naked huts and ragged Arabs. But for being undeceived by an actual progress to the spot, one and all would have remained firm in the conviction that these visionary trees and lakes had a real existence in the desert. M. Monge attributed the liquid expanse, tantalizing the eye with an unfaithful representation of what was earnestly desired, to an inverted image of the cerulean sky, intermixed with the ground scenery. This kind of mirage is known in Persia and Arabia by the name of _Serab_ or miraculous water, and in the western deserts of India by that of _Tehittram_, a picture. It occurs as a common emblem of disappointment in the poetry of the orientals. [Illustration: Atmospheric Illusion.]