quiry into the character of an age, which has dwelt on the popular mind for seven generations, linked with the emphatic phrase, "the golden days of good Queen Bess"-a sovereign, to whom a greater ruler, even the great Protector, bore honorable testimony, as "that lady, that great queen-Elizabeth of famous memory; we need not be ashamed to call her so."-(Carlyle's Cromwell, vol. ii., p. 240.) แ From Lowe's Magazine. LIGHT AND MAGNETISM: FARADAY'S DISCOVERIES. IF, after having been manifested to the these burning lenses to combustion, were senses of men for near six thousand years, certainly known to the ancients, even if we unveiling the home landscape with the far suppose the oft-told tale of Tzetzes to be off sublimities of the creation, and win a fiction, respecting the destruction of the ning universal homage to the sentiment so Roman ships in this way at the siege of beautifully expressed by the royal teacher, Syracuse. "When," says he, "the fleet truly light is sweet, and a pleasant thing of Marcellus was within bowshot, the old it is for the eyes to behold the sun;" and man, Archimedes, brought an hexagonal if, after the loftiest intellects have been mirror, which he had previously prepared, employed in the endeavor to search it out, at a proper distance from which he also the nature of the grand element still remains placed other smaller mirrors of the same somewhat enigmatical, a large amount of kind that moved in all directions on hinges, knowledge has been acquired respecting its which, when placed in the sun's rays, diproperties and connexions, not only of the rected them upon the Roman fleet, whereby highest interest, but of positive utility in it was reduced to ashes." Buffon, who the economy of human life. Some new elaborately canvassed this curious story, views have also been recently presented of succeeded in igniting beech wood at a disits more recondite relations, founded upon tance of 150 feet, with the faint rays of the direct experiment, which we propose to no- sun in the month of March, by means of tice, and which seem to intimate the con- a combination of 168 plane mirrors; and clusion that the mighty influences of the M. Villette melted copper ore in eight material world, the main forces of the ma- seconds, iron ore in twenty-four seconds, chine, electricity, magnetism, and light, are silver in seven seconds, and seconds, and tin in not only bound together by fine affinities, three seconds, by a lens condensing the and mutually dependent, but so convertible solar rays 17,257 times, generating a degree as to proclaim their common unity of source of heat 490 times greater than that of an and actual identity. First, however, it ordinary fire. It is probable, from the simay not be unnecessary to a class of read-lence of the older historians relative to the ers, or uninteresting to any, cursorily to re- particular feat of the Syracusan-a man cur to the progress of discovery, and to the possessing the inquiring and inventive simpler truths of optical science, illustra- genius in the highest degree-that he extive of the great agent upon which vision, perimentalized on various occasions with vegetation, color, and an endless series of burning mirrors, and also fired the enemy's beneficial phenomena, depend-an agent ships by machines constructed to throw consecrated by inspired appropriation as lighted materials into them, later writers the symbol of Divinity, and the emblem of erroneously connecting the two circumthe condition of humanity glorified-the stances together. However this may be, importance of which is emphatically declared by the magnificent command inaugurating its visible activity in the present cycle of terrestrial history-" Let there be light, and there was light." I. The concentration of the sun's rays by concave mirrors, and the application of * Sir D. Brewster, however, states that Kircher, who first observed the efficacy of a union of plane mirrors to form a powerful burning instrument, went with his pupil Scheiner to Syracuse to examine the position of the hostile fleet; and they were both satisfied that the ships of Marcellus could not have been more than thirty paces distant from Archimedes. One of the largest burning lenses ever con Two theories have chiefly divided the suffrages of the scientific world respecting the nature and propagation of light, the undulatory and the corpuscular or Newtonian. The former had precedence of the later, but was almost completely supplanted for upwards of a century, when it was revived in 1801, and gradually obtained the ascendency. The corpuscular hypothesis supposes light to consist of excessively minute material particles projected from luminous bodies, the sun, fixed stars, and incandescent substances. It was advanced the Romans as well as the Greeks were philosophical inquiry has gone off with unconversant with the laws of the reflection flagging pace in new directions, reaching of objects from the surface of polished by delicate investigations and exact memetals; and at Rome, we are told, if the thods of research, occult and unsuspected sacred fire in the temple of Vesta went out properties of the subtle, benign, and beautithrough the carelessness of the attendant, ful element. the chief pontiff scourged the vestal-virgin in the dark, the city mourned, and the fire could only be re-ignited by the pure rays of the sun. While holding a false and perfectly gratuitous hypothesis upon the subject of vision, the Platonists, three centuries before the Christian era, had arrived at two remarkable properties of light-its propagation in straight lines, and the equality of the angles of incidence and reflection when it falls on bright and polished surfaces, which are assumed as axioms in the optical treatise ascribed to Euclid. The effect of the atmosphere in changing the direction of the rays of light was also known to Ptolemy, who, though unable to appreciate the amount of its refractive power, clearly perceived its influence on the altitude of celestial objects, was aware of its increase with the distance from the zenith, and assigned it as the cause of the greater magnitude of the solar and lunar discs at the horizon. These statements comprise the knowledge upon the science of light possessed by the ancients, who seem to have been as inapt generally in the various departments of natural philosophy, as they were expert in pure geometry and abstract reasoning. The successful cultivation of this branch of physics is entirely of modern origin, and dates from the dawn of the seventeenth century. It opened with Kepler explaining the true theory of vision, and suggesting the astronomical telescope, which gave an impulse to the study of the optical properties of light in consequence of their application to that instrument, and with Snellius discovering the true law of refraction. It closed with the grand conclusion of Newton, that a beam of light, white as emitted from the sun, was not homogeneous, but consisted of seven different colors possessing different degrees of refrangibility; and with Huyghens observing the polarization. Since that period, some views have been renounced, others modified, while structed was made of flint glass, by Mr. Parker of Fleet Street, 212 pounds in weight, 3 feet in diameter, 34 inches thick at the centre, exposing a surface of 330 square inches to the sun's light. It melted platina, gold, silver, copper, tin, quartz, agate, jasper, flint, topaz, garnet, asbestos, in a few seconds, cost £709, and passed into the possession of the Emperor of China. as an objection, that no sensible diminution has been observed of the solar mass and volume since the epoch of observation commenced, notwithstanding the presumed incessant projection of luminous particles from every part of his surface. But calculation shows that no perceptible waste could have transpired. If we take the sun's apparent diameter to be 2000, which it is nearly, a second of a degree, at his distance, will correspond to about 460 miles. Now supposing the solar substance to undergo a daily waste of two feet-an amount in excess, considering the vast magnitude of the body, and the extreme rarity of light-it would require a period of 3000 years for a diminution of 460 miles, or 1" of apparent diameter to occur. But this loss in the lapse of even thirty centuries would still not be apprehensible, so small a variation as 1" being scarcely appreciable by our best instruments. The undulatory hypothesis supposes a highly attenuated medium, or ether, to pervade all space, so rare as not to offer perceptible resistance to the movements of the planets, and not to be cognisable by our senses while at rest, light resulting from luminous bodies setting its particles in motion, propagating waves or vibrations through it in all directions, as sonorous bodies propagate vibrations through the atmosphere, conveying sound. The theory of emission, while more open to popular appprehension, and affording an easy and intelligible explanation of many observed facts, completely fails to account for an extensive class of phenomena. On the other hand, the theory of undulations, though less apparently natural, and not so readily embraced by the mind, possesses 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, ed upon 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 volume of the atmosphere, a number also coming 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 water, 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, 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, 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 dually 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 1847.] FARADAY'S DISCOVERIES. 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 own him above it by the extent of his 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 proThis was first perty of a double refraction. 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 357 as to be intelligible to those who are not fa- The late Captain Basil Hall, ever prompt |