illustrated by the common experiment of putting a piece of money into an empty cup, retiring till the edge of the cup barely hides the money from the sight, and then causing a friend to pour water over the money, which will again bring it into view. The experiment may also be made with the flower, the landscape, or whatever else happens to be painted on the cup. It will follow, that pouring water into the cup, makes it appear less deep than it is in reality. This informs us, also, why a clear stream of water seems less deep than it is; for the refraction of the light causes the bottom to appear higher than it is; the ignorance of which has more than once betrayed the unwary to venture into water beyond their depth to the hazard of their lives.

Many a young life,' says Dr. Arnott, has been sacrificed to this error. A person looking from a boat directly down upon objects at the bottom of water, sees them in their true places and at their true distances, but if he view them more and more obliquely, the appearance is more and more deceiving, until at last it represents them as at less than half of their true depth.

The ship in which the author sailed once in the China sea, before danger was apprehended, had entered by a narrow passage into a horseshoe enclosure of coral rocks. When the alarm was given, the predicament had become truly terrific. On every side, in water most singularly transparent, as the wave swelled, the rocks appeared to be almost at the surface of the water, and the anchor, which in the first moments had been let go to limit the danger, appeared to be lifted with them. It was judged that if the ship, then drawing 24 feet, or the depth of a two-storied house, had moved but a little way in almost any direction, she must have met her certain destruction. On sending boats around to sound and to search, the place of entrance was again discovered, and was safely traversed a second time as an outlet from that terrible prison.

'On account of this bending of light from objects under water, there is more difficulty in hitting them with a bullet or spear. The aim by a person not directly over a fish must be made at a point apparently below it, otherwise the weapon will miss it by flying too high. The spear is sometimes used in this country for killing salmon, but is a common weapon among the islanders of the Atlantic and Pacific Oceans for killing the albacore; the use of it, like that of the fly-hook in England, affording to the fishermen good sport as well as profit. The author once with much interest witnessed at St. Helena this employment of the spear. A small fish, previously half-killed, that it might not try to escape, was every minute or two thrown upon the water as a bait, in the sight of perhaps a hundred great albacores, greedily waiting for it at one side below, and knowing the danger to which they exposed themselves by darting across to seize it. Some albacore, bold enough, soon made at the mouthful, apparently with the speed of lightning, but yet with speed which did not save him, for every now and then the thrown spear met the adventurer, and held him writhing there in a cloud of his death-blood. After a victim so destroyed, the scene of action was changed.

The bending of light when passing obliquely from water, is also the reason of the following facts. A straight rod or stick, of which a portion

is immersed in water, appears crooked or broken at the surface of the water, the portion immersed seeming to be bent upwards. That part of a ship or boat, visible under water, appears much flatter or shallower than it really is. A deep-bodied fish, seen near the surface of the water, appears almost a flat-fish. A round body there appears oval. A gold fish in a vase may appear as two fishes, being seen as well by light bent through the upper surface of the water, as by straight rays passing through the side of the glass. To see bodies under water, in their true places, and of their true proportions, the eye must view them through a tube, of which the distant end, closed with plate-glass, is held in the

water.

'Certain states of the atmosphere depending upon its humidity, warmth, &c., change very considerably its ordinary refractive power; hence in one state of it, a certain hill or island may appear low and scarcely rising above the intervening heights or ocean, while in another state, the same object shall be seen towering above: and from a certain station, a city in a neighbouring valley may be either entirely visible, or it may shew only the tops of its steeples, as if the bed on which it rested had sunk deeper into the earth. In days of ignorance and superstition, such appearances have sometimes excited a strange interest.

A beautiful phenomenon is observable in a day of warm sunshine, owing to the bending of light in passing through media of different densities. Black or dark-coloured substances, by absorbing much light and heat from the sun's rays, and warming the air in contact with them, until it dilates and rises in the surrounding air, as oil rises in water, cause the light, from more distant objects, reaching the eye through the rarefied medium, to be bent a little; and owing to the heated air rising irregularly under the influence of the wind and other causes, these objects acquire the appearance of having a tremulous or a dancing motion. In a warm clear day, the whole landscape at last appears to be thus dancing.

The same phenomenon is to be observed at any time, by looking at an object beyond the top of a chimney from which hot air is rising. An illicit distillery was once discovered by the exciseman happening thus to look across a hole used as the chimney, although charcoal was the fuel, and there was no vestige of smoke.

This bending of light by the varying states of the atmosphere, makes precaution necessary in making very nice geometrical observations :—as in measuring base-lines for the construction of maps or charts.

'As it is the obliquity between the passing ray and the surface, which in any case of refraction determines the degree of bending, a body seen through a medium of irregular surface appears distorted according to the nature of that surface. It is because the two surfaces of common windowglass are not perfect planes, and not perfectly parallel to each other, as in the case of plate-glass, that objects seen through the former appear generally more or less out of shape: and hence comes the elegance and beauty of plate-glass windows: and hence the singular distortion of things viewed through that swelling or lump of glass which remains where the glass-blower's instrument was attached, and which appears at the centre of certain very coarse panes.'-vol. ii. pp. 184-188.

We have mentioned above, that, in consequence of the timelight,

takes to travel, we do not see the heavenly bodies exactly where they are, but where they were some minutes before, when the light left them. We have now to add, that in consequence of the principle of refraction, their apparent place is considerably changed. The air only extends a few miles above the earth, and beyond is an unknown region, supposed to be filled with something as much thinner than air, as air is thinner than water. The light of the sun and stars, therefore, when it passes from this rare medium into the thicker air is bent; but though they be really set, they will appear in view, in the same way as the money in the cup comes into view when the water is poured over it. We may, with equal justice, refer many of our erroneous judgments to particular states of the reflection of light. The obscure white light, for example, reflected in the twilight from a pond, or other narrow piece of water, or from a white post or a birch-tree, may, by an apprehensive fancy, very readily be imagined to be a sheeted ghost escaped from the grave. We recollect an instance of a whole village having been thrown into consternation at the supposed appearance of a ghost among a row of tall beech-trees, adjacent to the manor house. This ghost was reported to have been repeatedly seen in the form of a headless woman, dressed in white, climbing the trees with one hand, and brandishing a pale, glimmering torch in the other. On this apparition, however, being observed by some persons less timid, and less fanciful, it was found to be nothing more than the reflected light of a miner's lantern, gleaming among the white trees, on his going to his labour.

'It is remarkable,' says Dr. Arnott, when the imagination is once excited by some beautiful or striking view, how readily any visual hint produces clear and strong impressions. One day in the cosmorama, a school-boy visitor exclaimed, with fearful delight, that he saw a monstrous tiger coming from its den among the rocks;-it was a kitten belonging to the attendant, which by accident had strayed among the paintings. And another young spectator was heard calling that he saw a horse galloping up the mountain side;-it was a minute fly crawling slowly along the canvas. There is in this department a very fine field yet open to the exercise of ingenuity, for the contemplation of pictures representing motion or progressive events, may be made the occasion of mental excitement the most varied and intense. For instance, there are few scenes on earth calculated to awaken more interesting reflections on the condition of human nature than that beheld by a person who sails along the river Thames from London to the sea, a distance of about sixty miles, through the wonders which on every side there crowd on the sight-the forest of ships from all parts of the world-the glorious monuments of industry, of philanthropy, of science-the marks of the riches, the high civilization, and the happiness of the people. Now this scene was last year, in one of our theatres, strikingly pourtrayed by what was called a moving panorama of the southern bauk of the Thames. It was a very long painting, of which a part only was seen at a time gliding slowly across the stage, and the impression made on the spectators was, that of their viewing the

realities while sailing down the river in a steam boat. In the same manneri the whole coast of Britain might be most interestingly represented-or any other coast, or any line of road, or even a line of balloon flight. There was another moving panorama exhibited about the same time at Spring Gardens, aiming at an object of still greater difficulty, viz. to depict a course of human life; and the history chosen was that of the latter part of Buonaparte's career. Scenes representing the principal events were, in succession, and apparently on the same canvas, made to glide across the field of view, so designed that the real motion of the picture gave to the spectator the feeling of the events being only then in progress, and with the accompaniments of clear narration and suitable music, they produced on those who viewed them the most complete illusion. The story began with the blow struck at Buonaparte's ambition in the battle of Trafalgar, and to mark how completely, by representations of various moments and situations of the battle, the spectators were in imagination made present to it, the author of this work may mention, that on the occasion of his visiting the exhibition, a young man seeing a party of British preparing to board an enemy's ship, started from his seat with a hurra, and seemed quite surprised when he found that he was not really in the battle. To the first views there succeeded many others, similarly introduced and explained, in each of which the hero himself appeared: there were, his defeat at Waterloo-his subsequent flight-his delivery of himself to the British Admiral-his appearing at the gangway of the Bellerophon to thousands of spectators, waiting in boats around, while he was in Plymouth harbour, previous to his departure for ever from the shores of Europe-his house and habits during his exile, with various views of St. Helena;-and last of all, that solemn procession, in which the bier, with his lifeless corpse, appeared moving slowly on its way to grave under the willow-tree. The exhibition now spoken of might have been made much better in all respects, yet in its mediocrity it served to prove how admirably adapted such unions of painting, music, and narration, or poetry, are to affect the mind, and therefore to become the means of conveying most impressive lessons of historical fact and moral principle.' pp. 280-282.

the

Mr. Hughes, we think it was, one of the contributors to the Spectator, once wrote an Essay upon "Nothing," as a contrast to the German Professor's treatise on "All Things and Some Other Things." Were we, in the same way, to comment upon darkness as some philosophers comment on light, we think we could reason as plausibly as they can for the real existence of darkness, which is said to be a mere privation of light, as cold is said to be a privation of heat: for the reverse seems to rest upon arguments equally probable. They say, for example, without a shadow of proof, that light is a fluid, and heat is a fluid; now were we to assert that darkness is a fluid, and cold a fluid, on what grounds could we be contradicted which would not apply as strongly to the philoso+ phers just alluded to? If they say that they can prove the velocity of light, we say that we can equally prove the velocity of darkness -for if we were to extinguish the candles by whose light we are writing, darkness will dart through every corner of the room as

rapidly as light will when they are relit: and when the sun sets, darkness travels as rapidly as light does when the sun rises. If they say that light penetrates glass, we may the same of darkness; for if we shut out the light, darkness will immediately make its way to occupy its place. Many experiments and arguments might be brought to prove this view of the subject; we merely throw out these hints to put our readers on their guard against taking every thing they find in books for unquestionable truth. We were once asked by a stickler for the fluidity of electricity, how we proved that it was not a fluid, and we answered him by asking how he proved that it was not a cabbage or a piece of ice, to which it seems to bear about as much resemblance as it does to water, oil, or any other known fluid. By maintaining the fluidity of cold and cf darkness we may always, by a reductio ad absurdum, put an end to such fancies, which put on the air and the strut of philosophy, while they are at bottom altogether baseless and air-drawn-without a tangible fact or a decent probability to support their flimsiness. Dr. Arnott's book is more free from such absurdities than any scientific work-popular or profound-with which we have ever met. The conclusion of the part before us is written with considerable eloquence, while it abounds with good sense, sound philosophy, and unaffected piety. Our readers cannot fail, we think, to be pleased to see this excellent passage:

'The investigations in progress respecting the phenomena of light, are furnishing new proofs of the extreme simplicity of nature, amidst the boundless extent and infinite variety. When men thought of the sense of touch only as it exists at the tips of the fingers, or on the general surface of the body, they were far from suspecting that the sense of hearing had the near relation to it which subsequent discoveries have proved, and still less did they think, that the sense of sight was similarly related; but step by step they ascertained, 1st, of sound coming to the ear through the air that air was a material fluid as much as water, consisting of the same or similar particles, only more distant among themselves-that a motion or trembling in the air, by affecting nerves exposed in the ear, produced the sensation of sound, as the trembling in a log of wood caused by the action of a saw produces a peculiar sensation of touch in a hand laid on the log, and, finally, that common sound in all its varieties, is merely such trembling of the air, affecting a structure of nerve so exposed in the ear, as to be as much more readily excitable than the nerves in the fingers, and elsewhere in the skin, as the action or impulse of moving air is more delicate than that of common solids and liquids. And now, in the investigations respecting light, this kind of comparison is carried a step further, for it is become matter almost of certainty that the sensation of light is produced in a suitable nervous tissue in the eye, by a trembling motion in another fluid than air, which fluid pervades all space, and in rarity or subtlety of nature surpasses air vastly more than air does water or solids; and while in sound, different tones or notes depend on the number of vibrations in a given time, so in light do different colours depend on the extent of the single vibrations. Can human imagination