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such explanatory power with reference to fire, are seen at the same moment by specthe more intricate recently discovered pro-tators, however their relative distances may perties of light, as to have received the differ, fifty, a hundred, and a thousand sanction of the great majority of modern miles, being travelled by light in perfectly philosophers. Sir J. Herschel has styled inappreciable time. It follows from this it, in allusion to its facility of explanation, wonderful velocity, adopting the corpuscu
one succession of facilities,” and assum- lar theory, that the luminous particles must ing it not to be the truth of nature, one have an inconceivable minuteness, for it of the happiest fictions that the genius ot has been calculated, that a molecule having man has yet invented.”
the sensible magnitude and weight of a Both theories recognise conclusions found-single grain, would be equal in its effect, ed upon the prodigious velocity with which owing to its momentum, to a cannon ball light is transmitted -a fact restiny on sen- of 150 pounds, discharged at the rate of sible evidence—which are astonishing even 1000 feet a second. In such circumstances, to those who are accustomed to contemplate the agent of so much good to man would the power of natural agencies. That time be the instrument of his destruction, meetis required for its propagation in space, ing the organs of vision like a charge of shot was first shown by observation of the from the barrel of a gun, and the globe he eclipses and emersions of Jupiter's satellites inhabits would as surely perish as a house taking place sooner or later according as of clay under the action of a park of arthe earth is at its least or greatest dis- tillery. But no sensible effect has ever tance from the planet, the difference of been produced upon the most delicate aptime being fourteen minutes, and the dif- paratus, by millions of molecules concenference of distance the diameter of the trated by mirrors and lenses at a single earth's orbit. Planetary light, therefore, point. How utterly beyond conception, whether viewed as a projection or an undu- therefore, the tenuity of the component lation, occupies that time in travelling over parts? The other theory involves equally the space in question, which gives it a ve- overwhelming results, in the excessive locity of about 192,000 miles a second. smallness and frequency of the ethereal viThis has been confirmed by subsequent ac- brations, as calculated from the known curate astronomical determinations, based velocity of light, exhibiting, with referupon different data. A shell shot from a ence to the extreme violet ray 59,750 mortar, supposing it to proceed onwards undulations in an inch of
space, and retaining its ordinary initial velocity, would 727,000,000,000,000 in second of require something like ten years to accom-time. plish a distance equal to that which light describes in eight minutes in reaching us 11. Cowper, who noted down things the from the sun. The greatest average velo- most common as he strolled in the quiet valley city of ponderable matter with which we of the Ouse, and invested them with inteare acquainted, the gallop of Mercury in rest, has celebrated the sun's slanting ray. his orbit, does not much exceed thirty miles a second, which only amounts to yoooth of
"From every herb and every spiry blade, that of light. Descartes, while maintain
Stretching a length of shadow o'er the field,
Mine spindling into longitude immense." ing the doctrine of instantaneous transmission, perceived the mathematical conse- This familiar circumstance shows that light quences of an opposite opinion, namely, moves in straight lines so long as it remains that if the motion of light is progressive, in the same medium, for the forms of shathe celestial bodies are not seen in their dows correctly represent the outlines of the true places, which he thought contrary to objects that produce them, as seen from the observation, and therefore an argument on direction of the luminous body. Upon being his side. This is one of the best estab- intercepted in its course, a portion is relished facts in astronomy.
Looking at fracted or returned from the surface of Uranus, at any given instant, we do not see the object, while another portion entering it where it actually is, but where it was up- it, is either wholly absorbed, or, if only wards of two hours before, the sight of the partly so, the rest is transmitted through planet taking that time to pass the interval it. The quantity of light reflected depends between us. Owing to the enormous rapidity on the nature of the bodies upon which it of transmission, luminous objects at the falls, on the character of their surfaces, and surface of the earth, a rocket or a signal the degree of inclination with which the rays
impinge. The amount is greatest in the stopped by the particles of the body, assimiinstance of smooth and polished surfaces, lated to its substance, remaining within it and, generally speaking, at small angles of in the form of imponderable matter. A incidence, but pure mercury, one of the fine example of the absorptive power of the most perfect of reflectors, does not return air, is afforded by the greater lustre and vimore than 721 out of 1000 rays under the vacity of the stars, as seen from the summost favorable circumstances. It appears mits of high mountains, than when bebeld from experiments by Bonguer, that in fuids, at a lower level through an increased votransparent solids, and some metals, as wa- lume of the atmosphere, a number also comter, glass, and mercury, the quantity reflect- ing into view which are not visible from the ed increases with the angle of incidence reck- plains below. As substances differ in their oning it from the perpendicular, while in capacity to absorb light, so likewise a few white opaque bodies, as silver and plaster, act equally upon all the color rays, preit decreases with the angle of incidence. The senting a perfectly white image of the sun, reflected light always forms an angle with while other media copiously absorb the blue the reflecting substance equal to that of the and transmit the red. Hence the
gorgeous incident, the ray proceeding in a straight golden or glowing red hue which marks the line as before the reflection, while the me- sunset-the natural appearance of sky and dium continues unaltered. But, besides ocean in the direction of the descending this process of regular reflection, a portion luminary, so vigorously painted in the Apoof the incident light is dispersed and scat-calypse, “ a sea of glass mingled with fire,” tered in all directions by irregular reflection, arises from the horizontal passage of the both of which operate in giving us the gene- solar light, embracing a larger tract of the rally diffused twilight and the illumination atmosphere, and the densest portion, which of day. We should have a sudden mid-absorbs the blue rays, the red and yellow night darkness with the setting sun, but variously modified by reflected light forcing that his beams for some time reach the their passage to the eye. From the same higher region of the atmosphere, and are re- cause the sun, as seen from a diving-bell a flected by the vapors and minute particles considerable depth under water, appears a floating in it, and perhaps by the atoms of fiery globe, the one class of rays piercing the air itself; and night would return at the superincumbent fluid, the others being mid-day with every passing cloud obscuring absorbed by it, or reflected from the surface. the face of the luminary, hills, woods, and Passing from one medium into another streams, out of the direct sunshine, ceasing of a different density, or the same medium to be visible, but for the reflection and scat-changing its density, the rays of light are tering of the solar rays by the atmosphere. diverted from a rectilinear course at the
The substances upon which light falls, af- junction of the media, except when their ter reflecting a portion, absorb or transmit direction is perpendicular. The common the rest. Hence the distinction of bodies illustration of this refraction is the apparent into opaque and transparent; but the most reflection of a walking stick from a right line opaque may be thinned into transparency, obliquely placing part of it in water, the and the most transparent are rendered bending commencing at the point where the opaque by being sufficiently thickened. medium changes. As the lower regions of the Thus gold and silver, among the densest sub- atmosphere are the densest arising from the stances, hammered out into thin films, exhi- pressure of the higher, and from terrestrial bit a beautiful green and blue hue, showing exhalations, the refraction of light in trathe transmission of some light through the versing it follows from this variation of the metals, while the most transparent bodies, medium ; but its rays falling obliquely upthe clearest crystal, and the purest air or on it, are not bent at once into another water, absorbo a great quantity. In the right line, as apparently happens with the case of pure water, half the light that en-walking-stick in the illustration, because of ters it is lost at the depth of seven feet, the very gradual change of density in and objects in the bed of a stream become the atmospheric medium. They are graless and less visible as the depth increases, dually deflected more and more into the till they wholly disappear. It is an ob- form of curves bending towards the perscure point in what manner light is arrested pendicular. The resulting phenomena are by the absorbing body, and how it is dis highly interesting and important, for as we posed of, but the general opinion is, see objects in the direction in which the that by some unknown power it is adtually rays of light meet the eye, none of the bea
venly bodies, not in the zenith, appear in as to be intelligible to those who are not fatheir true places, but are apparently lifted miliar with the subject. Its distinguishing nearer to it, have their altitudes increased, feature from common light is, that when it the refraction operating in that direction. falls upon a reflector, a plate of glass for Hence morning and evening, when the en- example, at an incident angle of 56° il', tire body of the sun is actually below the it is almost completely reflected in one posihorizon, the refractive power of the atmo- tion of the glass, and scarcely at all in ansphere Igreatest at the earth's surface, brings other. Suppose the glass to be vertical and him above it by the extent of his own reflection to ensue, there is no reflection upon diameter, causing a sensible prolongation of the glass being turned so as be to horizontal, the day. To unusual fits of refraction, 00- the angle of incidence remaining the same. casioned by great and sudden changes in the More popularly, the peculiarity may density of the air through variations of pressed by supposing a ray of polarized light temperature, sometimes so local that two to present itself to a plate of glass so as to contiguous strata are in opposite states, those be reflected, the north side of the ray, as extraordinary optical appearances are due, for mere illustration we may call it, meetupon which the eye of ignorance has turned ing the glass; the same reflection will enwith wonder and alarm, coasts looming in sue upon the glass being turned round so as the air, ships sailing high out of the water, to meet the south side of the ray; but none the mirage and the celebrated fata morga- whatever upon its meeting the east and west na of the Messina Straits.
sides. Thus a ray of polarized light exThe refractive power of different media posed to a reflecting surface, will be reflecttransmitting light is very various, but in ed if it falls upon the surface on either of general it is in proportion to their density, the opposite sides, but will not be reflected though this is far from being a universal if it falls on either of the other two, at right rule, alcohol, ether, and olive-oil, which are angles with the former. This remarkable lighter than water, possessing it more property, termed polarity from its analogy strongly. Some substances exhibit the pro- to magnetism, impressed upon light not only perty of a double refraction. This was first by double refraction, but by simple refleoobserved in Iceland spar a carbonate of tion from various substances, provided the lime widely diffused, occurring in crystals light is incident to the surface at a certain of various shapes and in large masses, in angle, called the polarizing angle, which vaboth of which states, however, the mineral ries with different bodies. The phenomecan always be split into the particular shape non shows undoubtedly that a change or called rhombohedron, a solid bounded by modification takes place in the physical nasix equal surfaces. In looking at an object, ture of common light as the effect of the such as a black line on a piece of paper, processes referred to. through a rhomb of this spar, in a certain The late Captain Basil Hall, ever prompt position, two parallel lines separated by a and fertile in expedients, turned his knowdistinct interval are visible, showing that ledge of optical science to good account, in light, in passing through the crystal, has the pic-nic party expedition to the great been divided into two portions, one of cave of Elephanta, the relation of which which is found to have obeyed the ordinary forms some interesting chapters of his fraglaw of refraction, and the other to have ments of adventure, and led to Mr. Ersbeen extraordinarily refracted. This pro- kine's accurate description of the temple experty of giving a double image of objects cavation in the Bombay Transactions :belongs to many crystallized substances, in The scientific heads of the company were fact to all crystals, the original form of which put in requisition to devise methods for ilis neither a cube nor a regular octahedron. luminating the dark parts of the temple. But the most striking fact to be noticed, The first and most obvious plan was to stick which has led to all the brilliant optical dis- a number of little bits of wax taper all coveries of the present age, in which our over and round those portions of the sculpcountryman Sir David Brewster has reaped tures which were under immediate investia large harvest of honor, is that light trans- gation. But this was found to be troublesome, mitted through doubly refracting substances in more respects than one. The wax meltsuffers that remarkable change in its physi-ed and ran down, and the corner of the cal properties which the term Polarization cave in which we were working either bedenotes. It is difficult to characterize by came too choky by the smoke and heat, or words the phenomena of polarized light so the lights turned down and required to be
shifted. This plan therefore was only re-| the seven colors to result from three prisorted to when the other methods I am mary rays—the red, the yellow, and the about to describe failed in effecting the pur- blue. Where these colors
rays pose. The sun at no time of day shone are concentrated, but spreading more full into the cave, which faces due north, less over the spectrum, the others are probut we found that by borrowing the look- duced by the intermixture, red and yellow ing-glasses from the ladies' tent, we could composing the orange, blue and yellow the catch his rays and send them to the very green, and red with blue, and å tinge of back of the excavation, and thence, by yellow, forming the violet. Light is the means of other mirrors, could polarize our great beautifier of nature; and truly did light in such a way as even to make it turn the Divine Artist remark of some of its corners, and fall on spots where probably humblest productions, that “Solomon in sunlight never rested before. The ecstasy all his glory was not arrayed like one of of the natives on beholding the success of these.” That endless variety and combinathis maneuvre, was so great, that some of tion of tints displayed by the flowers of the them expressed themselves highly flattered field, the rich hues of the autumnal woods, by the honors paid to their long degraded and the gorgeous plumage of tropical birds deities—another device of the same kind -in short, the colors of all objects, whether assisted our researches not a little, and was opaque bodies or transparent media, arise of still greater service to us in dissipating from their varying capacity of absorbing or nearly all the gloom of the cave, thus help- reflecting certain rays. The reflection of ing to keep up that air of cheerfulness all the rays causes white, and the absorpwhich is of such vast importance to the suc- tion of all black. Further analysis of the cess of every undertaking in this world, solar beam, by the elder Herschel, Wollasgreat or small. The tea-urn having been ton, and Frauenhofer, unveiled additional capsized on the breakfast-table one morn- phenomena, and recognised, distinct from ing, the servants naturally spread the table- the luminous rays, the calorific excitatory cloth in the sun on the shrubs before the heat, and the chemical, which excite neither cave. The immediate effect of this mass heat nor light, but produce peculiar chemiof white, was to lighten up everything cal changes in certain substances exposed within; and the hint once given, we lost no to their action, as the white chloride of siltime in expanding it, by hoisting half a do- ver, which is blackened in a few minutes by zen other cloths, at the proper angles, till being placed in the sunshine. The appaa bright yet soft glow of light was thrown rently magical photographic processes, by upon the principal figure of all, at the top which true, delicate, and beautiful images of the great division of the cave.” are instantaneously produced, are founded
From Newton's analysis of the sunbeam on the action of the chemical rays. admitted into a dark chamber, through a hole in the window-shutter, and subjected III. For more than thirty years attention to a prism, he inferred white light to be a has been called, at times, to experiments compound of seven differently colored rays, tending to establish the relation of light and called colorific and primary, because cach magnetism, and to the same experiments, single ray was in capable of separation by in other hands, failing to produce the effects the prism. These tints of the solar spec- described. The Italian philosopher Moritrum-red, orange, yellow, green, blue, chini was the first to announce the magnetizindigo, and violet—are finely expressed, by ing power of the solar rays, succeeding in the pencil of Nature, in the rainbow-the magnetizing steel with the violet rays colappropriate pledge against another general lected in the focus of a convex lens. Mrs. deluge occurring, at least from above, as Somerville repeated the experiment, coverits appearance is incompatible with a sky ing one half of a sewing needle with paper, completely mantled with clouds. The com- and exposing the other half to the violet position of white light may be experimentally rays, when, in about two hours, the exposed illustrated by mixing powders tinted after end had acquired magnetism. Nearly the the spectrum, in proportionate quantity, when same effect was produced by the indigo rays; the resulting color will be a greyish white, by the blue and green, also, in a less defrom the impossibility of securing perfectly gree; but none whatever by the yellow, accurate tints and quantities. Newton's orange, and red. Other experimentalists, analysis of the spectrum has been shown to however, signally failed in arriving at a like be imperfect by Brewster, who has proved result; and hence the question of the relation of light and magnetism remained in netic in the force of light; by the term magnetic, I doubt till very recently decisively estab- include here either of the peculiar exertions of the lished by the independent methods of Dr. power of a magnet, whether it be that which is Faraday, who stands at the head of an- bodies. The phrase • illumination of the lines of
manifest in the magnetic or diamagnetic class of analytical physical inquirers. It was on magnetic force has been understood to imply that Nov. 27, 1845, that his discovery was I had rendered them luminous. This was not withformally laid before the scientific world, in in my thought. I intended to express that the a paper read to the Royal Society of Lon- line of magnetic force was illuminated as the earth don, from which, as since published in its is illuminated by the sun, or the spider's web illu“ Transactions,” the following interesting minated by the astronomer's lamp. Employing a
ray of light, we can tell, by ihe eye, the direction prefatory passage is extracted :
of the magnetic lines through a body: and by the
alteration of the ray and its optical effect on the “I have long held an opinion, almost amount eye, can see the course of the lines just as we can ing to conviction, in common, I believe, with see the course of a thread of glass, or any other many other lovers of natural knowledge, that the transparent substance, rendered visible by the various forms under which the forces of matter light; and this was what I meant by illumina. are made manifest have one common origin, or, in tion, as the paper fully explains.- December 15, other words, are so directly related and mutually 1845. M. F." dependent, that they are convertible as it were, into one another, and possess equivalents of power We will now state, as briefly as the subin their action. In modern times, the proofs of ject will allow, and as clearly as its intritheir convertibility have been accumulated to a very considerable extent, and a commencement the fundamenal experiment was performed,
cacy will admit, the conditions' under which made of the determination of their equivalent forces. This strong persuasion extended 10 the which revealed a link of connexion between powers of light, and led, on a former occasion, 10 two great departments of nature, premising many exertions, having for their object the dis- that it is but one of a series illustrating the covery of the direct relations of light and electric general fact. Three agents were employed city, and their mutual action in bodies subject in the discovery. jointly to their power, but the results were nega- 1. A ray of light from an Argand lamp, tive. These ineffectual exertions, and many others polarized by reflection from a glass mirror. never published, could not remove my strong persuasion, derived from philosophical considerations;
2. Magnetism derived from an electroand, therefore, I recently resumed the inquiry, by magnet of power sufficient to sustain from experiment in a most strict and searching manner : 28 lbs. to 54 lbs. This instrument consists I have at last succeeded in magnetizing and elec: of a piece of soft iron, an inch in diameter, trifying a ray of light, and illuminating a mag- bent into the form of a horseshoe, around netic line of force."
which copper wire covered with silk is
coiled. Upon connecting the ends of the This paper is entitled, “On the Magnetiza-wire with a galvanic battery, the electric tion of Light, and the Illumination of Mag- current instantly renders the soft iron magnetic Lines of Force,” which appears to netic, the force of which is proportionately have occasioned some misconception ; and increased with the number of the coils, and all writers severely feel the imperfection of the intensity of the current. Upon disconlanguage in relation to the higher branches necting the wires with the battery, thus of modern science. The author, therefore, breaking the current, the charm is dissolved, communicated the following note to the and the magnetic power of the iron ceases. “Philosophical Magazine :">
Electro-magnets have supported 2063 lbs.,
or nearly a ton weight. Dr. Faraday em“ The title of this paper has, I understand, led ployed one at the Royal Institution, last many to a misapprehension of its contents, and I therefore take the liberty of appending this ex- year, of which he related, to give an idea planatory note.
of its force, that once, while he was in the the hypothesis of an ether, or the corpuscular, of laboratory, an iron candlestick which hapany other view that may be entertained of the pened to be standing on the table near its nature of light; and, as far as I can see, nothing poles, instantly flew to them, attracted with being really known of a ray of light more than of such violence as to displace or break everya line of magnetic or electric force, or even of a thing in its way. line of gravitating force, except as it and they are
3. A non-magnetic, or what we must now manifest in and by substances; I believe that, in the experiments I describe in the paper, light has call a dia-magnetic substance, was the third been magnetically affected, i. e. that that which agent concerned. This was a piece of heavy is magnetic in the forces of matter has been affected, transparent glass, composed of silicated and in turn has affected that which is truly mag-1 borate of lead, made and described several