LIGHT

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LIGHT is that which renders objects perceptible to our sense of seeing. It is one of the most interesting subjects that fall under the contemplation of the philosopher : at the same time it must be acknowledged to be one that is as little understood, and upon which opinions are as much divided, as any of the most abstruse subjects of philosophical inquiry. Some consider light as a fluid per se ; while others consider it merely as a principle, and attribute to it a sort of pression, or vibration propagated from the luminous body through a subtile, ethereal medium. The ancients believed it to be propagated from the sun and other luminous bodies instantaneously; but the observations of the moderns have shown that this was an erroneous hypothesis, and that light, like any other projectile, employs a certain time in passing from one part of space to another, though the velocity of its motion is truly astonishing, as has been manifested in various ways. And first, from the eclipses of Jupiter's saj^Sg^s; it was observed by Roemer, that" me eclipses ofby the tables of them, and that the observation was before or after the computed times, according as the earth was nearer to or farther from Jupiter than the mean distance. Hence it was concluded that this circumstance depended on the distance of Jupiter from the earth ; and that, to account for it, we must suppose that the light is 14 minutes in crossing the earth's orbit. The original observations have received some corrections, and it is now found that, when the earth is exactly between Jupiter and the sun, his satellites are seen eclipsed about eight minutes and a quarter sooner than they could be according to the tables ; but when the earth is nearly in the opposite point of its orbit, these eclipses happen about eight minutes and a quarter later than the tables predict them. Hence, then, it is certain that the motion of light is not instantaneous, but that it takes up about 16| minutes of time to pass over a space equal to the diameter of the earth's orbit, which is nearly 190,000,000 of miles in length, or at the rate of 200,000 miles per seconda conclusion which, it may be added, is placed beyond doubt, by the aberration of the stars discovered by the celebrated doctor Bradley. Upon the subject of the materiality of light, doctor Franklin observes, in expressing his dissent from the doctrine that light consists of particles of matter continually driven off from the sun's surface, with such enormous swiftness " Must not the smallest portion conceivaable have, with such a motion, a force exceeding that of a 24 pounder discharged from a cannon ? Must not the sun diminish exceedingly by such a waste of matter, and the planets, instead of drawing nearer to him, as some have feared, recede to greater distances, through the lessened attraction ? Yet these particles, with this amazing motion, will not drive before them or remove the least and slightest dust they meet with, and the sun appears to continue of his ancient dimensions, and his attendants move in their ancient orbits." He therefore conjectures that all the phenomena of light may be more properly solved, by supposing all spacefilled with a subtile elastic fluid, not yisible when at rest, but which, by its vibrations, affects that fine sense in the eye, as those of the air affect the grosser organs of the ear ; and even that different degrees of vibration of this medium may cause the appearances of different colors. And the celebrated Euler has maintained sides those of doctor Franklin above alluded to. Newton first discovered that certain bodies exercise on light a peculiar attractive force. When a ray passes obliquely from air into any transparent liquid or solid surface, it undergoes, at its entrance, an angular flexure, which is called refraction. The variation of this departure from the rectilineal path for any particular substance, depends on the obliquity of the ray to the refracting surface ; so that the sine of the angle of refraction is to that of the angle of incidence in a constant ratio. Newton, having found that unctuous or inflammable bodies occasioned a greater deviation in the luminous rays than their attractive mass, or density, gave reason to expect, conjectured, that both the diamond and water contained combustible matter a conjecture which was verified by subsequent discovery. Doctor Wollaston invented a very ingenious apparatus, in which, by means of a rectangular prism of flint glass, the index of refraction of each substance is read off at once by a vernier, the three sides of a movable triangle performing the operations of reduction in a very compendious manner. (Phtt.Trans., 1802.) But transparent media occasion not merely a certain flexure of the white sunbeam, called the mean refraction : they likewise decompose it into its constituent colors. This effect is called dispersion. Now, the mean refractive and dispersive powers of bodies are not proportional to each other. In some refracting media, the mean angle of refraction is smaller, whilst the angle of dispersion is larger. From the refractive power of bodies, we may, in many cases, infer their chemical constitution. For discovering the purity of essential oils, an examination with doctor Wollaston's instrument is of great utility, on account of the smallness of the quantity requisite for trial. This idea of doctor Wollaston has been happily prosecuted by M. Biot with regard to gaseous compounds ; and we now have accurate tables of the refractive power of all transparent gaseous, liquid and solid bodies. Carburet of sulphur exceeds all fluid substances in refractive power, surpassing even flint glass, topaz and tourmalin ; and in dispersive power, it exceeds every fluid substance except oil of cassia. Rays of light, in traversing the greater number of crystallized bodies, are commonly split into two pencils ; one of which, called the ordinary ray, follows the common laws of refraction, agreeably to the tables alludedto, whilst the other, called the extraordinary ray, obeys very different laws. This phenomenon is produced in all transparent crystals, whose primitive form is neither a cube nor a regular octahedron. The division of the beam is greater or less, according to the nature of the crystal, and the direction in wdiich it is cut; but, of all known substances, that which produces this phenomenon in the most striking manner, is the crystallized carbonate of lime, called Iceland spar. If the white sunbeam, admitted through a small hole of a windowshutter into a darkened room, be made to pass through a triangular prism of glass, it will be divided into a number of splendid colors, which may be thrown upon a sheet of paper. Newton ascertained that if this colored image, or spectrum, as it is called, be divided into 360 parts, the red will occupy 45, the orange 27, the yellow 48, the green 60, the blue 60, the indigo 40, and the violet 80. The red rays, being least bent by the prism from the directionof the white beam,are said to be least refracted, or the least refrangible, while the violet rays, being always at the other extremity of the spectrum, are called the most refrangible. If these differently colored rays of light be now concentrated on one spot, by a lens, they will reproduce colorless light. Newton ascribes the different colors of bodies to their power of absorbing all the primitive colors, except the peculiar one which they reflect, and of which color they therefore appear to our eye. The different colored rays possess veiy different powers of illumination. The lightest green, or deepest yellow, which are near the centre, throw more light on a printed page than any of the rays towards either side of the spectrum. The rays of the prismatic spectrum differ from one another also in their heating power, as was first noticed by Herschel. In viewing the sun, by means of large telescopes, through differently colored darkening glasses, he sometimes experienced a strong heat, attended with very little light, and, at other times, he had a strong light with a little heat. This observation led to his well known researches upon this subject, from which he concluded that the maximum heat is just without the spectrum, beyond the red ray. Others have found the greatest heat in the red ray itself; but the recent observations of M. Seebeck have shown that the point of greatest heat was variable, according to the kind of prism which was employed for refracting the rays. When a prism of fine flint gjass is used, the greatest heat is constantly beyond the red ; when a prism of crown glass, the greatest heat is in the red itself. It has long been known, that the solar light is capable of producing powerful chemical changes. One of the most striking instances of it is its power of darkening the white chloride of silveran effect which takes place slowly in the diffused light of day,but in the course of two or three minutes by exposure to the sunbeam. This effect was formerly attributed to the influence of the luminous rays; but it appears, from the observations of Ritter and Wollaston, that it is owing to the presence of certain rays, that excite neither heat nor light, and which, from their peculiar agency, are termed chemical rays. It is found that the greatest chemical action is excited just beyond the violet ray of the prismatic spectrum, and that the spot next in energy is occupied by the violet ray itself, and that the property gradually diminishes as we advance to the green, beyond which it seems wholly wanting. The sunbeams, in traversing a colored glass, produce similar effects to those caused by the differently colored portions of the spectrum. Thus the chloride of silver acquires a black tint behind a blue or violet glass, but does not blacken behind a red or orange glass; on the other hand, it becomes red behind a red glass, and that much more quickly than even in the solar spectrum. Light produced by coal and oil gases, or by olefiant gas, even when concentrated so as to produce a sensible degree of heat was found, by Mr. Braude, to occasion no change in the color of muriate of silver, nor in mixtures of chlorine and hydrogen; while the light emitted by electrized charcoal speedily affected the muriate, and caused these gases to unite, and sometimes with explosion. The concentrated light of the moon, like that of the gases, produced no change. The importance of light to plants is well known: deprived of it, they become white, and contain an excess of saccharine and aqueous parti cles; and flowers owe the variety and in tensity of their hues to the influence of th*> solar beams. Even animals require th& presence of the rays of the sun, and theii colors seem materially to depend upon the chemical influence of these rays. A comparison between the polar and tropical animals, and between the parts of theirbodies exposed, and those not exposed to light, shows the correctness of this opinion. (For an o account of the physical affections, and other chemical effects of light, see Optics, Phosphorescence, and Polarization of Light.) Light, Aberration of. (See Abenation.) Ldght, Diffusion of its Particles. (See Divisibility.)