THE following description of a certain class of wonderful and romantic scenes reported by travellers is from the pen of Thomas Milner, M. A. A series of curious and interesting phenomena, involving the apparent elevation and approach of distant objects, the production of aerial images of terrestrial forms, of double images, their inversion, and distortion into an endless variety of grotesque shapes, together with the deceptive aspect given to the desert-landscape, are comprehended in the class of optical illusions. Different varieties of this singular visual effect

constitute the "mirage" of the French, the "fata mor gana" of the Italians, the "looming" of our seaman, and the "glamur" of the highlanders. It is not peculiar to any particular country, though more common in some than others, and most frequently observed near the margin of lakes and rivers, by the sea-shore, in mountain districts, and on level plains. These phantoms are perfectly explicable upon optical principles, and though influenced by local combinations, they are mainly referable to one common cause, the refractive and reflective properties of the atmosphere, and inequalities of refraction arising from the intermixture of strata of air of different temperatures and densities. But such appearances in former times were really converted by the imagination of the vulgar into supernatural realities; and hence many of the goblin stories with which the world has been rife, not yet banished from the discipline to which childhood is subject,

"As when a shepherd of the Hebrid Isles

Placed far amid the melancholy main,
(Whether it be lone fancy him beguiles,

Or that aerial beings sometimes deign
To stand, embodied, to our senses plain)
Sees on the naked hill, or valley low,
The whilst in ocean Phoebus dips his wain,

A vast assembly moving to and fro,

Then all at once in air dissolves the wondrous show."

Pliny mentions the Scythian regions within Mount Imaus, and Pomponius Mela those of Mauritania, behind Mount Atlas, as peculiarly subject to these spectral ap

pearances. 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 afterwards 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 apparition:

"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
Till thickest legions close, with feats of arms
From either side of heaven the welkin rings."

spears,

The mirage is the most familliar form of optical illusion. M. Monge, one of the French savans, who accompanied Bonaparte 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