ed upon 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 corpuscuone 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 of 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, the prodigious velocity with which owing to its momentum, to a cannon ball light is transmitted-a fact resting 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 a 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. from the sun. The greatest average velocity of ponderable matter with which we are acquainted, the gallop of Mercury in his orbit, does not much exceed thirty miles a second, which only amounts to both of that of light. Descartes, while maintaining 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 11. Cowper, who noted down things the most common as he strolled in the quiet valley of the Ouse, and invested them with interest, has celebrated the sun's slanting ray. "From every herb and every spiry blade, Stretching a length of shadow o'er the field, Mine spindling into longitude immense." stopped by the particles of the body, assimilated to its substance, remaining within it in the form of imponderable matter. A fine example of the absorptive power of the air, is afforded by the greater lustre and vivacity of the stars, as seen from the summits of high mountains, than when beheld at a lower level through an increased vo ing into view which are not visible from the plains below. As substances differ in their capacity to absorb light, so likewise a few act equally upon all the color rays, presenting a perfectly white image of the sun, while other media copiously absorb the blue and transmit the red. Hence the gorgeous golden or glowing red hue which marks the sunset-the natural appearance of sky and ocean in the direction of the descending luminary, so vigorously painted in the Apocalypse, "a sea of glass mingled with fire," arises from the horizontal passage of the solar light, embracing a larger tract of the atmosphere, and the densest portion, which absorbs the blue rays, the red and yellow variously modified by reflected light forcing their passage to the eye. From the same cause the sun, as seen from a diving-bell a considerable depth under water, appears a fiery globe, the one class of rays piercing the superincumbent fluid, the others being absorbed by it, or reflected from the surface. impinge. The amount is greatest in the instance of smooth and polished surfaces, and, generally speaking, at small angles of incidence, but pure mercury, one of the most perfect of reflectors, does not return more than 721 out of 1000 rays under the most favorable circumstances. It appears from experiments by Bonguer, that in fluids, transparent solids, and some metals, as wa-lume of the atmosphere, a number also comter, glass, and mercury, the quantity reflected increases with the angle of incidence reckoning it from the perpendicular, while in white opaque bodies, as silver and plaster, it decreases with the angle of incidence. The reflected light always forms an angle with the reflecting substance equal to that of the incident, the ray proceeding in a straight line as before the reflection, while the medium continues unaltered. But, besides this process of regular reflection, a portion of the incident light is dispersed and scattered in all directions by irregular reflection, both of which operate in giving us the generally diffused twilight and the illumination of day. We should have a sudden midnight darkness with the setting sun, but that his beams for some time reach the higher region of the atmosphere, and are reflected by the vapors and minute particles floating in it, and perhaps by the atoms of the air itself; and night would return at mid-day with every passing cloud obscuring 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, absorb 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, and objects in the bed of a stream become less and less visible as the depth increases, till they wholly disappear. It is an obscure point in what manner light is arrested by the absorbing body, and how it is disposed of, but the general opinion is, that by some unknown power it is actually the very gradual change of density in the atmospheric medium. They are gradually deflected more and more into the form of curves bending towards the perpendicular. The resulting phenomena are highly interesting and important, for as we see objects in the direction in which the rays of light meet the eye, none of the hea venly bodies, not in the zenith, appear in their true places, but are apparently lifted nearer to it, have their altitudes increased, the refraction operating in that direction. Hence morning and evening, when the entire body of the sun is actually below the horizon, the refractive power of the atmosphere, greatest at the earth's surface, brings him above it by the extent of his own diameter, causing a sensible prolongation of the day. To unusual fits of refraction, occasioned by great and sudden changes in the density of the air through variations of temperature, sometimes so local that two contiguous strata are in opposite states, those extraordinary optical appearances are due, upon which the eye of ignorance has turned with wonder and alarm, coasts looming in the air, ships sailing high out of the water, the mirage and the celebrated fata morgana of the Messina Straits. The refractive power of different media transmitting light is very various, but in general it is in proportion to their density, though this is far from being a universal rule, alcohol, ether, and olive-oil, which are lighter than water, possessing it more strongly. Some substances exhibit the property of a double refraction. This was first observed in Iceland spar a carbonate of lime widely diffused, occurring in crystals of various shapes and in large masses, in both of which states, however, the mineral can always be split into the particular shape called rhombohedron, a solid bounded by six equal surfaces. In looking at an object, such as a black line on a piece of paper, through a rhomb of this spar, in a certain position, two parallel lines separated by a distinct interval are visible, showing that light, in passing through the crystal, has been divided into two portions, one of which is found to have obeyed the ordinary law of refraction, and the other to have been extraordinarily refracted. This property of giving a double image of objects belongs to many crystallized substances, in fact to all crystals, the original form of which is neither a cube nor a regular octahedron. But the most striking fact to be noticed, which has led to all the brilliant optical discoveries of the present age, in which our countryman Sir David Brewster has reaped a large harvest of honor, is that light transmitted through doubly refracting substances suffers that remarkable change in its physical properties which the term Polarization denotes. It is difficult to characterize by words the phenomena of polarized light so as to be intelligible to those who are not familiar with the subject. Its distinguishing feature from common light is, that when it falls upon a reflector, a plate of glass for example, at an incident angle of 56° 11', it is almost completely reflected in one position of the glass, and scarcely at all in another. Suppose the glass to be vertical and reflection to ensue, there is no reflection upon the glass being turned so as be to horizontal, the angle of incidence remaining the same. More popularly, the peculiarity may be expressed by supposing a ray of polarized light to present itself to a plate of glass so as to be reflected, the north side of the ray, as for mere illustration we may call it, meeting the glass; the same reflection will ensue upon the glass being turned round so as to meet the south side of the ray; but none whatever upon its meeting the east and west sides. Thus a ray of polarized light exposed to a reflecting surface, will be reflected if it falls upon the surface on either of the opposite sides, but will not be reflected if it falls on either of the other two, at right angles with the former. This remarkable property, termed polarity from its analogy to magnetism, impressed upon light not only by double refraction, but by simple reflection from various substances, provided the light is incident to the surface at a certain angle, called the polarizing angle, which varies with different bodies. The phenomenon shows undoubtedly that a change or modification takes place in the physical nature of common light as the effect of the processes referred to. The late Captain Basil Hall, ever prompt and fertile in expedients, turned his knowledge of optical science to good account, in the pic-nic party expedition to the great cave of Elephanta, the relation of which forms some interesting chapters of his fragments of adventure, and led to Mr. Erskine's accurate description of the temple excavation in the Bombay Transactions :The scientific heads of the company were put in requisition to devise methods for illuminating the dark parts of the temple. The first and most obvious plan was to stick a number of little bits of wax taper all over and round those portions of the sculptures which were under immediate investigation. But this was found to be troublesome, in more respects than one. The wax melted and ran down, and the corner of the cave in which we were working either became too choky by the smoke and heat, or the lights turned down and required to be are concentrated, but spreading more or less over the spectrum, the others are produced by the intermixture, red and yellow composing the orange, blue and yellow the green, and red with blue, and a tinge of yellow, forming the violet. Light is the great beautifier of nature; and truly did the Divine Artist remark of some of its humblest productions, that "Solomon in all his glory was not arrayed like one of 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 appear, the rays pose. The sun at no time of day shone full into the cave, which faces due north, but we found that by borrowing the looking-glasses from the ladies' tent, we could catch his rays and send them to the very back of the excavation, and thence, by means of other mirrors, could polarize our light in such a way as even to make it turn corners, and fall on spots where probably sunlight never rested before. The ecstasy of the natives on beholding the success of these." That endless variety and combinathis manœuvre, 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 of still greater service to us in dissipating nearly all the gloom of the cave, thus helping to keep up that air of cheerfulness which is of such vast importance to the success of every undertaking in this world, great or small. The tea-urn having been capsized on the breakfast-table one morning, the servants naturally spread the tablecloth in the sun on the shrubs before the cave. The immediate effect of this mass of white, was to lighten up everything within; and the hint once given, we lost no time in expanding it, by hoisting half a dozen other cloths, at the proper angles, till a bright yet soft glow of light was thrown upon the principal figure of all, at the top of the great division of the cave." opaque bodies or transparent media, arise from their varying capacity of absorbing or reflecting certain rays. The reflection of all the rays causes white, and the absorption of all black. Further analysis of the solar beam, by the elder Herschel, Wollaston, and Frauenhofer, unveiled additional phenomena, and recognised, distinct from the luminous rays, the calorific excitatory heat, and the chemical, which excite neither heat nor light, but produce peculiar chemical changes in certain substances exposed to their action, as the white chloride of silver, which is blackened in a few minutes by being placed in the sunshine. The apparently magical photographic processes, by which true, delicate, and beautiful images are instantaneously produced, are founded on the action of the chemical rays. From Newton's analysis of the sunbeam 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 each magnetism, and to the same experiments, single ray was incapable 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 rela tion 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 prefatory passage is extracted: see the course of a thread of glass, or any other transparent substance, rendered visible by the light; and this was what I meant by illumina tion, as the paper fully explains.- December 15, 1845. M. F.” ray of light, we can tell, by the eye, the direction 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 many other lovers of natural knowledge, that the various forms under which the forces of matter are made manifest have one common origin, or, in other words, are so directly related and mutually dependent, that they are convertible as it were, into one another, and possess equivalents of power in their action. In modern times, the proofs of their convertibility have been accumulated to a very considerable extent, and a commencement made of the determination of their equivalent forces. This strong persuasion extended to the powers of light, and led, on a former occasion, to many exertions, having for their object the discovery of the direct relations of light and electricity, and their mutual action in bodies subject jointly to their power, but the results were negative. These ineffectual exertions, and many others never published, could not remove my strong persuasion, derived from philosophical considerations; and, therefore, I recently resumed the inquiry, by experiment in a most strict and searching manner: I have at last succeeded in magnetizing and elec trifying a ray of light, and illuminating a magnetic line of force." This paper is entitled, "On the Magnetization of Light, and the Illumination of Magnetic Lines of Force," which appears to have occasioned some misconception; and all writers severely feel the imperfection of language in relation to the higher branches of modern science. The author, therefore, communicated the following note to the "Philosophical Magazine:" We will now state, as briefly as the subject will allow, and as clearly as its intricacy will admit, the conditions under which the fundamenal experiment was performed, which revealed a link of connexion between two great departments of nature, premising that it is but one of a series illustrating the general fact. Three agents were employed in the discovery. 1. A ray of light from an Argand lamp, polarized by reflection from a glass mirror. 2. Magnetism derived from an electromagnet of power sufficient to sustain from 28 lbs. to 54 lbs. This instrument consists of a piece of soft iron, an inch in diameter, bent into the form of a horseshoe, around which copper wire covered with silk is coiled. Upon connecting the ends of the wire with a galvanic battery, the electric current instantly renders the soft iron magnetic, the force of which is proportionately increased with the number of the coils, and the intensity of the current. Upon disconnecting the wires with the battery, thus breaking the current, the charm is dissolved, and the magnetic power of the iron ceases. 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 of its force, that once, while he was in the planatory note. Neither accepting nor rejecting laboratory, an iron candlestick which happened to be standing on the table near its poles, instantly flew to them, attracted with such violence as to displace or break everything in its way. the hypothesis of an ether, or the corpuscular, or any other view that may be entertained of the nature of light; and, as far as I can see, nothing being really known of a ray of light more than of a line of magnetic or electric force, or even of a line of gravitating force, except as it and they are manifest in and by substances; I believe that, in the experiments I describe in the paper, light has been magnetically affected, i. e. that that which is magnetic in the forces of matter has been affected, and in turn has affected that which is truly mag 3. A non-magnetic, or what we must now call a dia-magnetic substance, was the third agent concerned. This was a piece of heavy transparent glass, composed of silicated borate of lead, made and described several |