SECTION II.-LIGHT. ANALYSIS OF THE SECTION. Light is an emanation from the SUN and other self-luminous bodies, becoming less intense as it spreads, and by falling on other bodies, and being reflected from them to the eye, renders them visible. Its absence is called darkness. It moves with great velocity, and in straight lines where there is no obstacle, -leaving shadows where it cannot fall. It passes readily through some bodies which are therefore called transparent, but when it enters or leaves their surfaces obliquely, it suffers a degree of bending or REFRACTION. A beam of white light thus refracted does not bend equally, but is divided or resolved into beams of the different colours seen in the rainbow. These colours, on being again blended, become white light as before. A transparent substance, like glass, may be so formed as, by the power of refraction due to its shape, to cause all the rays which pass through it from any point on one side of it, to bend and meet again in a corresponding point beyond it called a focus, and after so meeting and crossing, to pass on as before-the body then, because in form somewhat resembling a flat bean or lentil, being called a LENS. When the light thus proceeding from every point of an object placed before a lens, is collected at corresponding points behind it, a distinct image of the object is there produced, visible from any situation on a white screen placed to receive it, or in the air, if viewed from behind. The most important optical instruments, and even the living eye, are merely different arrangements of parts for producing and examining such images as those now described. When the image is received upon a suitable white surface or screen in a dark room, the arrangement is called, according to minor circumstances, a CAMERA OBSCURA (that is, a dark chamber), a MAGIC LANTERN, or a SOLAR MICROSCOPE. And even the living EYE is, in fact, but a small camera obscura, enabling the mind to judge of external objects, by the size, brightness, colour, &c., of the very minute but perfect images or pictures formed at its back part, on the smooth screen of nervous matter or sensitive membrane called the retina. The art of painting aims at producing on a larger scale such a picture as is formed on the retina, which when afterwards held before the eye, and reproducing itself in miniature upon this membrane, may excite nearly the same impression as the original objects. When the image beyond a lens, formed as above described, is viewed in the air, by looking at it from behind, in the line of the axis of the lens, there then exists the arrangement of parts constituting a TELESCOPE or a MICROSCOPE. Rays of light falling on very smooth or polished plane surfaces, are reflected so exactly in the order in which they fall, as to appear to the eye receiving them to be coming directly from the objects originally emitting them—and such surfaces are called plane mirrors. Mirrors may be plane, convex, or concave; and certain concave forms concentrate light, to produce images by reflection, just as lenses produce them by refraction; so that there are. reflecting telescopes, and microscopes, as there are refracting instruments of the same names. Light, again, falling on bodies having rough or irregular surfaces, or which have other peculiarities, is so modified as to produce cll the phenomena of colour and varied brightness seen among natural bodies, and giving to them their distinctive characters and beauty. The decomposition of light into the colours seen in the rainbow, was first experimentally studied by Newton; and since his day the subject has been developed into the extensive science of SPECTRUM ANALYSIS, an important auxiliary to the modern chemist and astronomer. By applying a telescope to examine the prismatic band or spectrum of a flame, we form a SPECTROSCOPE; and by means of this instrument we can determine, however distant the light may be, whether it is the flame of a burning gas or whether it is a solid in a state of incandescence; and by its particular lines of colour, we are able in some cases to identify the substance. Thus, the astronomer knows what metallic vapours are incandescent in the solar atmosphere (or photosphere); and can even detect the presence of well-known chemical elements in the inconceivably distant fixed stars. According to the WAVE THEORY of light, different colours are simply different lengths of ethereal undulations, just as different musical notes are different lengths of aërial pulsations; and this undulatory theory has received remarkable confirmation from a large class of phenomena referred to under the names, DIFFRACTION, INTERFERENCE, and POLARIZATION LIGHT. OF Sources of Light. Light from the Sun-from Combustion. Phosphorescence. 779. The phenomena of light and vision are of surpassing interest, on account of their beauty and their utility. Their beauty is seen in the verdure of fields and forests, among the beds of the flowergarden, in the plumage of birds, in the richly coloured clouds around the rising and the setting sun, and in the circles of the rainbow. Their utility is such that, if man had needed to supply his wants by groping in utter and unchangeable darkness, even with all the knowledge now existing in the world, he could scarcely have secured The Sun the Chief Source of Light. 557 his existence for a single day. Eternal night would have been equivalent to universal death. Light is at once the beauteous garb of nature, and the absolutely necessary medium of communication between living creatures and the universe around them. The rising sun converts a wilderness of darkness which, to a child, not yet aware of the regularity of nature's changes, is so full of horror, into a visible and lovely paradise. No wonder, then, if in the early ages of ignorance, man bowed the knee to worship, in the glorious Sun, the God of Nature! 780. The eye receives from every illuminated object around, nay, from every point in every object, and at every moment of time, a messenger of light to tell what is there, and in what condition. Had we the power of flitting from place to place with the speed of thought, we could not be more promptly informed. The sense of sight contributes to our knowledge of the phenomena of the world around us, more than all our other senses put together. With the aid of the microscope, it reveals to us a miniature world of life and activity, even in the tiny water-drop, and a most wonderful complexity of structure in the minutest objects of nature. With the aid of the telescope, it gives us intelligence from the utmost bounds of space, telling us of suns and systems before which our terrestrial globe dwindles into insignificance. The Sun is the chief source of light. 781. This truth is most strikingly impressed upon us in tropical countries, where the sun rises and sets almost perpendicularly, not allowing the long dawn and twilight of temperate latitudes, and where the change from perfect darkness to the overpowering effulgence of day, and the contrary change in the evening, take place suddenly. We may contrast this tropical effulgence with the darkness and desolation of an arctic winter. In the arctic regions the inhabitants lose the sun for many months together, and are obliged to resort to artificial light. On what particular conditions solar light depends is not well understood; but, to use the common mode of expression, rays of heat and light are emitted from this luminary together, and they reach the earth in about the same period of time. The light is probably a result of intense heat, not arising from any cause similar to combustion on the earth, but, so far as spectroscopic examination will enable us to form a judgment, from the intense incandescence of the vapours of many metals, as well as of gases constituting the 558 Light from Combustion. luminous atmosphere or photosphere, of the sun. To the questions, what causes this incandescence, and what maintains it with such apparent uniformity for ages, science at present can furnish no answer. The polariscope, to be afterwards explained, shows that the sun is a self-luminous body. Light emanates from the STARS, and here, also, an examination of these remote bodies by the spectroscope and polariscope proves that, like the sun, they are self-luminous, and that their brilliant light, without sensible heat, is dependent on the incandescence of metallic vapours and gases. Owing to their enormous distance, the heat-rays which they emit do not reach the earth, but the rays of light are sufficiently penetrating to admit of optical examination and definition. 782. The artificial production of light from combustion is a subject more within the reach of experiment. In the preceding section combustion, as a source of heat, has been fully examined. now be necessary to consider it as a source of light. It will The quantity of light emitted from a burning substance in general depends, first, on the intensity of the heat produced; and, secondly, on the presence of some solid non-volatile matter which is capable of receiving the heat without undergoing any change in its physical condition, and of emitting it as light. With regard to the effect of heat in producing light, some remarks have been elsewhere made (Art. 719, p. 507). The first effect is to render a solid visibly red, and this point is reached at about 1000°; the substance then emits the less refrangible rays of light, and appears red-hot. As the heat is increased, it passes through the stages of orange and yellow heat, and finally, by the emission of all the rays of light, it appears as a dazzling white heat. Hence it might be said that light is nothing more than visible heat, and heat, invisible light, their difference being only in the degree of certain qualities. It is further assumed that they are convertible into each other. Experiment shows, however, that they admit of separation to a certain extent, and that they act differently in causing chemical combinations, and in decomposing chemical compounds. 783. When combustion takes place at a low red heat, as in the platinum-wire lamp of Sir H. Davy, the metal emits light, visible only in the dark, the metallic substance receiving and radiating the heat. Hydrogen burns with intense heat, but there is scarcely any light; indeed it may be said that the flame of pure hydrogen is invisible by daylight. This disproportion between the light and The Drummond Light. 559 heat emitted is explained by the fact that there is no solid matter which admits of being heated to incandescence. Watery vapour is the only product, and this cannot absorb and emit the heat as light. If platinum-wire, or particles of charcoal, lime, asbestos, or ironfilings, are introduced into burning hydrogen, they receive the heat of combustion, and emit it as a very bright light. 784. The Drummond or Lime Light.-Lime was first used as a source of light with the nearly invisible oxyhydrogen jet, by Lieutenant Drummond in 1826. He employed it in the triangulation survey, and successfully connected the opposite shores of England and Ireland at Holyhead, a distance of sixty-four miles. In Scotland he obtained successful results on the summits of Ben Lomond and Knock Layd, a distance of ninety-five miles. Dr. Miller states that the Drummond light has been seen at a distance in a right line of 112 miles. It is probably the most powerful of all artificial lights. 785. The intense white light produced by the combustion of coalgas, or the vapours of paraffine or rock oil, is entirely owing to the carbon atoms contained in the gas or vapour being intensely heated by the burning hydrogen, and emitting this heat as light. The same substance emits a feeble light when the products of combustion are gaseous, and a powerful light when they are solid. Thus, in burning phosphorus in chlorine, a gaseous chloride results, and a dull flame scarcely emitting any light, is seen. If, while thus burning, the phosphorus is raised from the bell-jar into the air, it combines with oxygen and produces solid white phosphoric acid, which, being strongly heated, emits an intensely white light. The difference in illuminating power is rendered still more conspicuous, by plunging the ladle with the burning phosphorus into a bell-jar of pure oxygen. A thin slip of zinc or magnesium, introduced into a Bunsen's smokeless flame, burns with dazzling splendour, more marked in the magnesium than the zinc In either case a white infusible oxide is produced, which receives and emits the heat of combustion as white light. From the photo-chemical researches of Bunsen and Roscoe, it was calculated that the light of the sun's disc was 524 times greater than that of burning magnesium. So intense is the light emitted by magnesium, that a wire of only the hundredth of an inch in diameter was found to produce as much light as seventyfour stearine candles! The brightness of the light, therefore, arising from combustion, depends not only on the combustible, but on the |