feet broad, which encompassed all England as it were. There went out semicircles from the side of it, at whose intersection the four mock suns were situated, the true sun being in the east, and the air very clear. And because this monstrous prodigy cannot be described by words, I have represented it by a scheme, which shows immediately how the heavens were circled. The appearance was painted in this manner by many people, for the wonderful novelty of it."

3. Paraselena. Mock moons, depending upon the causes which produce the solar image, or several examples of it, as frequently adorn the arctic sky. On the 1st of December 1819, in the evening, while Parry's expedition was in Winter Harbour, four paraselenæ were observed, each at the distance of 211° from the true moon. One was close to the horizon, the other perpendicular above it, and the other two in a line parallel to the horizon. Their shape was like that of a comet, the tail being from the moon, the side of each towards the real orb being of a light orange colour. During the existence of these paraselenæ, a halo appeared in a concentric circle round the moon, passing through each image. On the evening of March 30, 1820, about ten o'clock, the attention of Dr. Trail at Liverpool was directed by a friend to an unusual appearance in the sky, which proved to be a beautiful display of paraselenæ. The moon was then 35° above the southern horizon. The atmosphere `was nearly calm, but rather cloudy, and obscured by a slight haze. A wide halo, faintly exhibiting the prismatic colours, was described round the moon as a centre, and had a small portion of its circumference cut off by the horizon. The circular band was intersected by two small segments of a larger circle, which if completed would have passed through the moon, and parallel to the horizon. These segments were of a paler colour than the first mentioned circle. At the points of intersection appeared two pretty well defined luminous discs, equalling the moon in size, but less brilliant. The western paraselenæ had a tail or coma, which was directed from the moon, and the eastern also, but much less clearly defined.

4. The Rainbow. The most glorious vision depending upon the decomposition, refraction, and reflection of light, by the vapour of the atmosphere reduced to fluid drops, is the well-known arch projected during a shower of rain upon a cloud opposite to the sun, displaying all the tints of the solar spectrum. The first marked approximation to the true theory of the rainbow occurs in a volume entitled De Radiis Visus et Lucis, written by Antonius de Dominis, Archbishop of Spalatro, published in the year 1611 at Venice. Descartes pursued the subject, and correctly explained some of the phenomena; but upon Newton's discovery of the different degrees of refrangibility in the different coloured rays which compose the sunbeam, a pencil of white or compounded light, the cause of the coloured bands in the rainbow, of the order of their position, and of the breadth they occupy, was at once apparent. The bow is common to all countries, and is the sign of the covenant of promise to all people, that there shall no more be such a wide-spread deluge as that which the sacred narrative records.

"But say, what mean those coloured streaks in heaven
Distended, as the brow of God appeased?

Or serve they, as a flowery verge to bind
The fluid skirts of that same watery cloud,

Lest it again dissolve, and shower the earth?

To whom the Archangel: Dexterously thou aim'st;

So willingly doth God remit his ire

That he relents, not to blot out mankind;

And makes a covenant never to destroy

The earth again by flood; nor let the sea
Surpass its bounds; nor rain to drown the world,
With man therein, or beast; but when he brings

O'er the earth a cloud, will therein set

His triple-coloured bow, whereon to look,

And call to mind his covenant."

It is happily remarked by Mr. Prout, in his Bridgewater treatise, that no pledge could have been more felicitous or satisfactory; for, in order that the rainbow may appear, the

clouds must be partial, and hence its existence is absolutely incompatible with universal deluge from above. So long, therefore, as "He doth set his bow in the clouds," so long have we full assurance that these clouds must continue to shower down good, and not evil, to the earth.

When rain is falling, and the sun is on the horizon, the bow appears a complete semicircle, if the rain-cloud is sufficiently extensive to display it. Its extent diminishes as the solar altitude increases, because the coloured arch is a portion of a circle whose centre is a point in the sky directly opposite to the sun. Above the height of 45° the primary bow is invisible, and hence, in our climate, the rainbow is not seen in summer about the middle of the day. In peculiar positions a complete circle may be beheld, as when the shower is on a mountain, and the spectator in a valley; or when viewed from the top of a lofty pinnacle, nearly the whole circumference may sometimes be embraced. Ulloa and Bouguer describe circular rainbows, frequently seen on the mountains, which rise above the tableland of Quito. When rain is abundant, there is a secondary bow distinctly seen, produced by a double reflection. This is exterior to the primary one, and the intervening space has been observed to be occupied by an arch of coloured light. The secondary bow differs from the other, in exhibiting the same series of colours in an inverted order. Thus the red is the uppermost colour in the interior bow, and the violet in the exterior. A ternary bow may exist, but it is so exceedingly faint from the repeated reflections, as to be scarcely ever perceptible. The same lovely spectacle may be seen when the solar splendour falls upon the spray of the cataract and the waves, the shower of an artificial

fountain, and the dew upon the grass. There is hardly any other object of nature more

pleasing to the eye, or soothing to the mind, than the rainbow, when

distinctly developed a familiar sight in all regions, but most common in mountainous districts, where the showers are most frequent. Poetry has celebrated its beauty, and to convey an adequate representation of its soft and variegated tints, is the highest achievement of the painter's art. While the Hebrews called it the Bow of God, on account of its association with a divine promise, and the Greeks the Daughter of Wonder, the rude inhabitants of the North gave expression to a fancy which its peculiar aspect might well create, styling it the Bridge of the Gods, a passage connecting heaven and earth.

The principles which account for the formation of the rainbow explain the appearance of beautiful irridescent arches which have occasionally been observed during the prevalence of mist and sunshine. Mr. Cochin describes a spectacle of this kind, noticed from an eminence that overlooked some low meadow grounds, in a direction opposite to that of the sun, which was shining very brightly, a thick mist resting upon the landscape in front. At about the distance of half a mile from each other, and incurvated, like the lower extremities of the common rainbow, two places of peculiar brightness were seen in the mist. They seemed to rest on the ground, were continued as high as the fog extended, the breadth being nearly half as much more as that of the rainbow. In the middle, between these two places, and on the same horizontal line, there was a coloured appearance, whose base subtended an angle of about 12°, and whose interior parts were thus variegated. The centre was dark, as if made by the shadow of some object resembling in size and shape an ordinary sheaf of corn. Next this centre there was a curved space of a yellow flame colour. To this succeeded another curved space of nearly the same dark cast as the centre, very evenly bounded on each side, and tinged with a faint blue green. The exterior exhibited a rainbow circlet, only its tints were less vivid, their boundaries were not so well defined, and the whole, instead of forming part of a perfect circle, appeared like the end of a concentric ellipsis, whose transverse axis was perpendicular to the horizon. The mist lay thick upon the surface of the meadows; the observer was standing near its margin, and gradually the scene became fainter, and faded away, as he entered into it. A similar fog-bow was seen by Captain Parry during his attempt to reach the North Pole by means of boats and sledges, with five arches formed within the main one, and all beautifully coloured.

The iris lunaris, or lunar rainbow, is a much rarer object than the solar one. It

frequently consists of a uniformly white arch, but it has often been seen tinted, the colours differing only in intensity from those caused by the direct solar illuminations. Aristotle states that he was the first observer of this interesting spectacle, and that he only saw two in the course of fifty years; but it must have been repeatedly witnessed, without a record having been made of the fact. Thoresby relates an account received from a friend, of an observation of the bow fixed by the moon in the clouds, while travelling in the Peak of Derbyshire. She had then passed the full about twenty-four hours. The evening had been rainy, but the clouds had dispersed, and the moon was shining very clearly. This lunar iris was more remarkable than that observed by Dr. Plot, of which there is an account in his History of Oxford, that being only of a white colour, but this had all the hues of the solar rainbow, beautiful and distinct, but fainter. Mr. Bucke remarks upon having had the good fortune to witness several, two of which were perhaps as fine as were ever witnessed in any country. The first formed an arch over the vale of Usk. The moon hung over the Blorenge; a dark cloud was suspended over Myarth; the river murmured over beds of stones, and a bow, illumined by the moon, stretched from one side of the vale to another. The second was seen from the castle overlooking the Bay of Carmarthen, forming a regular semicircle over the river Towy. It was in a moment of vicissitude; and the fancy of the observer willingly reverted to the various soothing associations under which sacred authority unfolds the emblem and sign of a merciful covenant.

5. Spectral Illusions.-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 morgana of the Italians, the looming of our seamen, and the glamour 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 readily 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 appearances. Diodorus Siculus likewise refers to the regions of Africa, situated in the neighbourhood 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 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

The mirage, adverted to in noticing the sandy deserts of the globe, 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, tantalising 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 Tchittram, a picture. It occurs as a common emblem of disappointment in the poetry of the orientals.

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