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.”
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.
Atmospheric Illusion.
In the Philosophical Transactions for the year 1798, an account is given by W. Latham, Esq., F.R.S., of an instance of lateral refraction observed by him, by which the coast of Picardy, with its more prominent objects, was brought apparently close to that of Hastings. On July the 26th, about five in the afternoon, while sitting in his dining-room, near the sea-shore, attention was excited by a crowd of people running down to the beach. Upon inquiring the reason, it appeared that the coast of France was plainly to be distinguished with the naked eye. Upon proceeding to the shore, he found, that without the assistance of a telescope, he could distinctly see the cliffs across the Channel, which, at the nearest points, are from forty to fifty miles distant, and are not to be discovered, from that low situation, by the aid of the best glasses. They appeared to be only a few miles off, and seemed to extend for some leagues along the coast. At first the sailors and fishermen could not be persuaded of the reality of the appearance, but they soon became thoroughly convinced, by the cliffs gradually appearing more elevated, and seeming to approach nearer, that they were able to point out the different places they had been accustomed to visit, such as the Bay, the Old Head, and the Windmill at Boulogne, St. Vallery, and several other spots. Their remark was that these places appeared as near as if they were sailing at a small distance into the harbor. The apparition of the opposite cliffs varied in distinctness and apparent contiguity for nearly an hour, but it was never out of sight, and upon leaving the beach for a hill of some considerable height, Mr. Latham could at once see Dungeness and Dover cliffs on each side, and before him the French coast from Calais to near Dieppe. By the telescope the French fishing-boats were clearly seen at anchor, and the different colors of the land on the heights, with the buildings, were perfectly discernible. The spectacle continued in the highest splendor until past eight o’clock, though a black cloud obscured the face of the sun for some time, when it gradually faded away. This was the first time within the memory of the oldest inhabitants, that they had ever caught sight of the opposite shore. The day had been extremely hot, and not a breath of wind had stirred since the morning, when the small pennons at the mast-heads of the fishing-boats in the harbor had been at all points of the compass. Professor Vince witnessed a similar apparent approximation of the coast of France to that of Ramsgate, for at the very edge of the water he discerned the Calais cliffs a very considerable height above the horizon, whereas they are frequently not to be seen in clear weather from the high lands above the town. A much greater breadth of coast also appeared than is usually observed under the most favorable circumstances. The ordinary refractive power of the atmosphere is thus liable to be strikingly altered by a change of temperature and humidity, so that a hill which at one time appears low, may at another be seen towering aloft; and a city in a neighboring valley, may from a certain station be entirely invisible, or it may show the tops of its buildings, just as if its foundations had been raised, according to the condition of the aerial medium between it and the spectator.
