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• The fine gothic pile of Westminster Abbey may break upon the view in some situation where nearer edifices, and perhaps some minor imitations of its beauties, already fill and dazzle the eye with their brightness, but the misty or less distinct outline of the former warn the approaching stranger of its true magnitude, and prepare him for the enjoyment which a nearer inspection of its grandeur and perfection is to afford.
"A small yacht or pleasure-boat may be built from the same model or of the same comparative dimensions as a first-rate vessel of war, and may be in view from the shore at the same time, only so much nearer than the ship, that boih shall form images of the same magnitude on the retina of a spectator. In such a case, to an unpractised eye, it might be difficult to detect the difference, but to another, the bright lights of the little vessel, contrasted with the softer or more misty appearance of the larger, would leave no room for doubt. A haziness occurring in the atmosphere between the little vessel and the eye, might considerably disturb the judgment. .In a fleet of ships, if the sun's direct rays fall upon one here and there through openings among the clouds, while the others remain in shade, the former immediately start in appearance towards the spectator. Similarly the mountains of an unknown coast, if the sun-shine fall upon them, appear comparatively near, but if clouds again intervene, they recede and mock the awakened hope of the approaching mariner.
"A conflagration at night, however distant, appears to spectators generally, as if very near, and inexperienced persons often run towards it with hope of arriving immediately, but find after miles travelled that they hae made but a little part of their way.
"A person ignorant of astronomy deems the heavenly bodies vastly nearer to the earth than they are, merely because of their being so bright or luminous. The evening star, for instance, seen in a clear sky over some distant hill-top, appears as if a dweller on the hill might almost reach it-for the most intense artificial light that could be placed on the height would be dim in comparison with the beauteous star, yet to a dweller on the bill it appears just as distant as 10 one on the plain ; nay, at tbousands of miles nearer, the appearance would still be nearly the same.
* The concave of the starry heavens appears Aattened above, or nearer to the earth in its zenith than towards its horizon, because the light from above having to pass through only the depth or thickness of the atmosphere, is little obstructed, while of that which darts towards any place horizontally through hundreds of miles of dense vapour-loaded air, only a small part arrives.
• The sun and moon appear larger at rising and setting than when midway in heaven, partly, as already explained, because while below they can easily be compared with other objects, of which the size is known, but partly, also, because of their less light in the former situation, while their diameters are always the same.
· A fog or mist is said to magnify objects seen through it. The truth is, that by reason of the diminished intensity of light, it makes them appear further distant without lessening the visual angles subtended by them; and because an object at two miles, subtending the same angle as an object at one mile, must be twice as large. The conclusion is drawn that the dim object is large. Thus a person in a fog may believe that he is approaching a great tree, fifty yards distant, when the next step throws him into the bush which had deceived him.—Two
friends meeting in a fog have often mutually mistaken each other for persons of much greater stature.—A row of foxglove flowers on a neighbouriog bank, has been mistaken for a company of scarlet-clad soldiers on the more distant face of the bill. There are, for similar reasons, frequent misjudgings in late twilight and early dawn.—The purpose and effect of a thin gauze screen interposed between the spectators in a theatre and some person or object meant to appear distant, are intelligible on the same principle: a boy near, so screened, will appear to be a man at a distance. The art of the painter uses sombre colours when his object is to produce in his picture the effect of distance.-- On the alarming occasion of a very dense fog coming on at sea, wbere the ships of a fleet are near to each other, without wind, and where there is considerable swell or rolling of the sea, much damage is often done, but it is to be remarked in such a case that the size of ships approaching to the shock is always in idea exaggerated.
• The celebrated Spectre of the Brocken, among the Hartz Mountains, is a good illustration of our present subject. On a certain ridge, just at sud-rise, a gigantic figure of a man had often been observed walking, and extraordinary stories were related of it. About the year 1800 a French philosopher went with a friend to watch the phenomenon ; but for many mornings they had paraded on an opposite ridge in vain. At last, however, they discovered the monster, but he was not alone; he had a companion, and singularly he and his companion aped all the motions and attitudes of the observer and his companion : in fact, the spectres were merely shadows of the observers, formed by the horizontal rays of the rising sun falling on the morning fog which hovered over the valley beyond; but because the shadows were very faint, they were deemed distant, and therefore seemed men walking on the opposite ridge, and because a comparatively small figure seen near, but supposed distant, appears of gigantic dimensions, these shadows were accounted giants.'-vol. ii. pp. 260—264.
Dr. Arnott has most wisely avoided the common slang of philosophy which talks of light, heat, electricity, galvanism, and magnetism, as caused by peculiar fluids. Philosophers of this imaginative school, describe light as consisting of particles as a fluid of inconceivable tenuity; and as a substance which can Deither be weighed in scales, nor felt by the touch. All these descriptions appear to us to be exceedingly inaccurate ; for the words particle, fluid, and substance, have a meaning altogether different in their usual applications from what is here given them. If light be called a fluid, it is surely but reasonable we should expect it to be like water, or oil, or some other fluid with which we are acquainted, at least, in having this property of fluidity. Light, however, does not appear to have any such property. If it be said, that by a fluid is meant a vapour or gas, light is also deficient in one essential property of either of these, since they can be kept in vessels which light cannot ; for though light has been beaming into a room during the whole of a bright summer's day, the instant the window shutters are closed, the light disappears. Besides gas or vapour can be weighed, and can by particular management be perceived by touch ; when it is, for example, forced through a pair of bellows; but this will not hold with respect to light.
The opinion that light is composed of particles, is founded on the assumption for which we have not a shadow of proof, and not even an analogy to adduce in its support, though Niewenty most preposterously undertook to compute the number of particles of light given out by a candle in a given time! That light is a material substance, meaning by that, a thing which can be weighed or touched, or its bulk measured, is equally unproved, and is, indeed, contrary to the universal experience of mankind. To say, then, that light is a material fluid composed of particles, is not only giving us a string of meaningless words, which is too often done in books. of philosophy ;-it is actually leading us into erroneous notions, and substituting fiction and falsehood for ascertained facts. · We may ask, therefore, in what class of the things which be, must light be ranked, since it is not a fluid, nor a vapour, nor a gas, nor composed of material particles? Evidently, as we think, it must be classed with heat, electricity, galvanism, and magnetism, which though they agree in some properties, are as different in species (if we may so term it) from one another as iron, and wood, and water. Light has the property, with other things of the same class, of penetrating very hard bodies,-hence called transparent, such as glass and the diamond, the hardest of all known bodies. The assumed notion that light was made up of particles, led, consequently, to the assertion, that there are innumerable pores, or minute holes, in glass and diamonds, through which the particles of light pass. Nobody, of course, ever saw those holes or pores, and their existence remains to be proved.
Concerning the supposed particles of light, it is farther asserted, that they have no force, or, as it is technically called, momentum ; that is, when light strikes our face, it is not felt as the wind is felt, nor does it pain the eye as sand thrown into it would do. This assertion, though perhaps it is a little better founded than some of the preceding, is not quite correct; for the eye really does feel pain, very similar, also, to pain arising from pressure, when it is exposed to a strong beam of light, as in looking at the sun, in which case not only the curtain of the eye-lids is let down to protect it, but the iris besides, in order to protect the retina, instinctively contracts so much that the pupil almost disappears. What is more remarkable, the iris does not thus contract in any other case when the eye-lids are shut as a protection against injury; in such cases, on the contrary, the iris expands. This may be readily proved by the simple experiment of threatening to injure a person's eye, and observing the state of the iris immediately before and after the circumstance. Of the several colours, red seems to have the strongest effect upon the eye, if we may judge from the eagerness of children and savages for stuffs of this colour. It is
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the most brilliant and splendid of all the colours, and has, in almost all countries, been chosen for the robes of princes ; among the ancients, the statues of Jupiter, on high festivals, were painted res. In diseased states of the nerves, red seems to give more pain to the eye than any other colour. M. Richeraud says he has seen maniacs thrown into a rage, and others, supposed to have been long cured, whose madness came afresh at the sight of scarlet cloth. Bulls, turkeys, and other animals, are also enraged at the sight of scarlet. Green, on the other hand, is opposite to Ted in this respect, being the softest and least vivid of the colours. It adds plausibility to these views of the red rays of light striking or infringing most forcibly upon the eye, that they seem to have greater power in penetrating dense media, than any of the others; when the sun, for instance, shines through “ the horizontal misty air, shorn of” all “his ” other“ beams," the red alone penetrates the fog, and makes the king of day appear like a plate of red hot iron. These hypothetical conjectures of what may be called the pressure or infringement of light, is somewhat countenanced by the coloured rings produced by external pressure on the closed eye-lids; for the hardest pressure produces red or luminous flashes; the slightest pressure produces violet, indigo, and blue, exactly in the order of the refrangibility of the colours. When the pressure, however, is made on the centre of the ball, or on the two corners of the eye at once, no flash nor colour is seen; a circumstance caused, perhaps, by the retina not being affected in the point of vision.
This leads us to consider the velocity of light, which is so great that, if we believe the doubtful calculations of astronomers, (see Monthly Review, for August, 1829, p. 522), it takes only eight minutes and a half to come to us from the sun, a supposed distance of 95,000,000 miles. It must follow, from this, that we never see the sun exactly where he is, but where he was eight minutes and a half before. We cannot form a plausible conjecture how far the fixed stars may have receded from the place where we apparently see them, for we know not their distance; but astronomers have fancied that there may be some of them so very distant that their light has been travelling to the earth from the time of the creation itself for these six thousand years, and has not yet come into view. The following is Dr. Arnott's account of the velocity of light:
· The extraordinary precision with which the astronomical skill of modern times enables men to foretel the times of remarkable appearances or changes among the 'heavenly bodies, has served for the detection of the fact, that light is not an instantaneous communication between distant objects and the eye, as was formerly believed, but a messenger which requires time to travel ; and the rate of travelling has been ascertained in the same way.
• The eclipses of the satellites or moons of the planet Jupiter had been
carefully observed for some time, and a rule was obtained which foretold the instants in all future time when the satellites were to glide into the shadow of the planet, and disappear, or again to emerge into view. Now it was found, that these appearances took place 16} minutes sooner when Jupiter was near the earth, or on the same side of the sun with the earth, than when it was on the other side; that is to say, more distant from the earth by one diameter of the earth's orbit, and at all intermediate stations the difference diminished from 163 minutes, in exact proportion to the less distance from the earth. This proves then that light takes 161 minutes to travel across the earth's orbit, and 84 minutes for half that distance, or to come down to us from the sun.
• The velocity of light, ascertained in this way, is such, that in one second of time, viz. during a single vibration of a common clock pendulum, it would go from Loudon to Edinburgh, and back, 200 times, and the distance between these is 400 miles. This velocity is so surprising, that the philosophic Dr. Hooke, when it was first asserted that light was thus progressive, said he could more easily believe the passage to be absolutely instantaneous, even for any distance, than that there should be a progressive movement so inconceivably swift. The truth, however, is now put quite beyond a doubt by many collateral facts bearing upon it.
*As regards all phenomena upon earth, they may be regarded as happening at the very instant when the eye perceives thein ; the difference of time being too small to be appreciated: for, as shown in the preceding paragraph, if our sight could reach from London to Edinburgh, we should perceive a phenomenon there in the four-hundredth part of a second after its occurrence.
• It is hence usual, and not sensibly incorrect, when we are measuring the velocity of sound, as when a cannon is fired, by observing the time between the Aash and the report, to suppose that the event takes place at the very moment when it is perceived by the eye.
• In using a telegraph, no sensible time is lost on account of light requiring time to travel. A message can be sent from London to Portsmouth in a minute and a half; and at the same rate a communication might pass to Rome in about half an hour, to Constantinople in forty minutes, to Calcutta in a few hours, and so on. A telegraph is any object which can be made to assume different forms or appearances, at the will of an attendant, and so that the changes may be distinguished at a distance. A pole with moveable arms is the common construction, each position standing for a letter, or cypher, or word, or sentence, as may be agreed upon. Telegraphic signals between ships at sea are generally made by a few fays, the meanings of each being varied by the mast on which it is hoisted, and by its combination with others.'-pp. 172-- 174.
The superiority of Dr. Arnott's manner of illustrating the principles of philosophy cannot be better exemplified, than in his elucidation of the refraction of light, or the bending of its rays in passing into a different medium, as from air into water, into glass, or any other transparent substance of different density from itself. A little consideration will lead to the inference that it is this which causes an oar to appear as if broken, when one part of it is in water and the other in the air. The principle can be strikingly