Constitution of Light according to Sir Isaac Newton—Absorption of Light—Colours of Bodies—Constitution of Light according to Sir David Brewster—New Colours—Fraunhofer’s Dark Lines—Dispersion of Light—The Achromatic Telescope—Homogeneous Light—Accidental and Complementary Colours—M. Plateau’s Experiments and Theory of Accidental Colours.

It is impossible thus to trace the path of a sunbeam through our atmosphere without feeling a desire to know its nature, by what power it traverses the immensity of space, and the various modifications it undergoes at the surfaces and in the interior of terrestrial substances.

Sir Isaac Newton proved the compound nature of white light, as emitted from the sun, by passing a sunbeam through a glass prism (N.195), which, separating the rays by refraction, formed a spectrum or oblong image of the sun, consisting of seven colours, red, orange, yellow, green, blue, indigo, and violet—of which the red is the least refrangible, and the violet the most. But, when he reunited these seven rays by means of a lens, the compound beam became pure white as before. He insulated each coloured ray, and, finding that it was no longer capable of decomposition by refraction, concluded that white light consists of seven kinds of homogeneous light, and that to the same colour the same refrangibility ever belongs, and to the same refrangibility the same colour. Since the discovery of absorbent media, however, it appears that this is not the constitution of the solar spectrum.

We know of no substance that is either perfectly opaque or perfectly transparent. Even gold may be beaten so thin as to be pervious to light. On the contrary, the clearest crystal, the purest air or water, stops or absorbs its rays when transmitted, and gradually extinguishes them as they penetrate to greater depths. On this account objects cannot be seen at the bottom of very deep water, and many more stars are visible to the naked eye from the tops of mountains than from the valleys. The quantity of light that is incident on any transparent substance is always greater than the sum of the reflected and refracted rays. A small quantity is irregularly reflected in all directions by the imperfections of the polish by which we are enabled to see the surface; but a much greater portion is absorbed by the body. Bodies that reflect all the rays appear white, those that absorb them all seem black; but most substances, after decomposing the white light which falls upon them, reflect some colours and absorb the rest. A violet reflects the violet rays alone and absorbs the others. Scarlet cloth absorbs almost all the colours except red. Yellow cloth reflects the yellow rays most abundantly, and blue cloth those that are blue. Consequently colour is not a property of matter, but arises from the action of matter upon light. In fact, the law of action and reaction obtains in light as in every other department of nature, so that light cannot be reflected, refracted, much less absorbed, by any medium without being reacted upon by it. Thus a white riband reflects all the rays, but, when dyed red, the particles of the silk acquire the property of reflecting the red rays most abundantly and of absorbing the others. Upon this property of unequal absorption the colours of transparent media depend; for they also receive their colour from their power of stopping or absorbing some of the colours of white light, and transmitting others. As, for example, black and red inks, though equally homogeneous, absorb different kinds of rays; and, when exposed to the sun, they become heated in different degrees; while pure water seems to transmit all rays equally, and is not sensibly heated by the passing light of the sun. The rich dark light transmitted by a smalt-blue finger-glass is not a homogeneous colour like the blue or indigo of the spectrum, but is a mixture of all the colours of white light which the glass has not absorbed. The colours absorbed are such as mixed with the blue tint would form white light. When the spectrum of seven colours is viewed through a thin plate of this glass, they are all visible; and, when the plate is very thick, every colour is absorbed between the extreme red and the extreme violet, the interval being perfectly black; but, if the spectrum be viewed through a certain thickness of the glass intermediate between the two, it will be found that the middle of the red space, the whole of the orange, a great part of the green, a considerable part of the blue, a little of the indigo, and a very little of the violet, vanish, being absorbed by the blue glass; and that the yellow rays occupy a larger space, covering part of that formerly occupied by the orange on one side and by the green on the other: so that the blue glass absorbs the red light, which when mixed with the yellow constitutes orange; and also absorbs the blue light, which when mixed with the yellow forms the part of the green space next to the yellow. Hence, by absorption, green light is decomposed into yellow and blue, and orange light into yellow and red: consequently the orange and green rays, though incapable of decomposition by refraction, can be resolved by absorption, and actually consist of two different colours possessing the same degree of refrangibility. Difference of colour, therefore, is not a test of difference of refrangibility, and the conclusion deduced by Newton is no longer admissible as a general truth. By this analysis of the spectrum, not only with blue glass but with a variety of coloured media, Sir David Brewster, so justly celebrated for his optical discoveries, is of opinion that the solar spectrum consists of three primary colours, red, yellow, and blue, each of which exists throughout its whole extent, but with different degrees of intensity in different parts; and that the superposition of these three produces all the seven hues according as each primary colour is in excess or defect. That since a certain portion of red, yellow, and blue rays constitute white light, the colour of any point of the spectrum may be considered as consisting of the predominating colour at that point mixed with white light. Consequently, “by absorbing the excess of any colour at any point of the spectrum above what is necessary to form white light, such white light will appear at that point as never mortal eye looked upon before this experiment, since it possesses the remarkable property of remaining the same after any number of refractions, and of being capable of decomposition by absorption alone.” This analysis of light has been called in question by Professor Challis, of Cambridge, who does not admit of any resolution by absorbing media different from that by the prism, though he admits that a mixture of blue and yellow solar light produces green. Professor Stokes, on the contrary, does not allow that a mixture of blue and yellow solar light produces green, although that mixture produces green when the light is from other sources, for he found the gradation from sunlight to pass from yellow through diluted yellow, white, diluted blue to blue; so he does not admit of Sir David Brewster’s analysis of the spectrum; however, there appears to be still a doubt as to the real character of the phenomena presented by certain absorbing substances.

In addition to the seven colours of the Newtonian spectrum, Sir John Herschel has discovered a set of very dark red rays beyond the red extremity of the spectrum which can only be seen when the eye is defended from the glare of the other colours by a dark blue cobalt glass. He has also found that beyond the extreme violet there are visible rays of a lavender gray colour, which may be seen by throwing the spectrum on a sheet of paper moistened by the carbonate of soda. The illuminating power of the different rays of the spectrum varies with the colour. The most intense light is in the mean yellow ray, or, according to M. Fraunhofer, at the boundary of the orange and yellow.

When the prism is very perfect and the sunbeam small, so that the spectrum may be received on a sheet of white paper in its utmost state of purity, it presents the appearance of a riband shaded with all the prismatic colours, having its breadth irregularly striped or subdivided by an indefinite number of dark, and sometimes black lines. The greater number of these rayless lines are so extremely narrow that it is impossible to see them in ordinary circumstances. The best method is to receive the spectrum on the object-glass of a telescope, so as to magnify them sufficiently to render them visible. This experiment may also be made, but in an imperfect manner, by viewing a narrow slit between two nearly closed window-shutters through a very excellent glass prism held close to the eye, with its refracting angle parallel to the line of light. The rayless lines in the red portion of the spectrum become most visible as the sun approaches the horizon, while those in the blue extremity are most obvious in the middle of the day. When the spectrum is formed by the sun’s rays, either direct or indirect—as from the sky, clouds, rainbow, moon, or planets—the black bands are always found to be in the same parts of the spectrum, and under all circumstances to maintain the same relative positions. Similar dark lines are also seen in the light of the stars, in the electric light, and in the flame of combustible substances, though differently arranged, each star and each flame having a system of dark lines peculiar to itself. Dr. Wollaston and M. Fraunhofer, of Munich, discovered these lines deficient of rays independently of each other. M. Fraunhofer found that their number extends to nearly six hundred, but they are much more numerous. There are bright lines in the solar spectrum which also maintain a fixed position. Among the dark lines, M. Fraunhofer selected seven of the most remarkable, and determined their distances so accurately, that they now form standard and invariable points of reference for measuring the refractive powers of different media on the rays of light, which renders this department of optics as exact as any of the physical sciences. These lines are designated by the letters of the alphabet, beginning with B, which is in the red near the end of the spectrum; C is farther advanced in the red; D is in the orange; E in the green; F in the blue; G in the indigo; and H in the violet. By means of these fixed points, M. Fraunhofer has ascertained from prismatic observation the refrangibility of seven of the principal rays in each of ten different substances solid and liquid. The refraction increased in all from the red to the violet end of the spectrum. The rays that are wanting in the solar spectrum, which occasion the dark lines, were supposed to be absorbed by the atmosphere of the sun. But the annular eclipse which happened on the 15th of May, 1836, afforded Professor Forbes the means of proving that the dark lines in question cannot be attributed to the absorption of the solar atmosphere; they were neither broader nor more numerous in the spectrum formed during that phenomenon than at any other time, though the rays came only from the circumference of the sun’s disc, and consequently had to traverse a greater depth of his atmosphere.

Sir David Brewster found that in certain states of the atmosphere the obscure lines become much broader, and some of them deeply black; and he observed also, that, at the time the sun was setting in a veil of red light, part of the luminous spectrum was absorbed, whence he concluded that the earth’s atmosphere had absorbed the rays of light which occupied the dark bands. By direct experiments also the atmosphere was observed to act powerfully upon the rayless lines.

When a lens is used along with a prism, longitudinal dark lines of different breadths are seen to cross the bands, already described, at right angles; these M. Ragona-Scina and M. Babinet believe to be lines of interference which exist in light that has passed through a convex lens.

The lines are different both in kind and number in the spectra of gases and flames. In a highly-magnified spectrum from light passed through nitrous acid gas, Sir David Brewster counted 2000 dark bands. In the spectrum of a lamp, and generally of all white flames, none of the defective lines are found; so all such flames contain rays which do not exist in the light of the sun or stars. Brilliant red lines appear in the spectrum produced by the combustion of nitre upon charcoal; and in all artificial flames dark and bright bands exist, sometimes corresponding in position with those in the solar spectrum, and sometimes not.

A sunbeam received on a screen, after passing through a small round hole in a window-shutter, appears like a round white spot; but when a prism is interposed, the beam no longer occupies the same space. It is separated into the prismatic colours, and spread over a line of considerable length, while its breadth remains the same with that of the white spot. The act of spreading or separation is called the dispersion of the coloured rays. Dispersion always takes place in the plane of refraction, and is greater as the angle of incidence is greater. It varies inversely as the length of a wave of light, and directly as its velocity: hence towards the blue end of the spectrum, where the undulations of the rays are least, the dispersion is greatest. Substances have very different dispersive powers; that is to say, the spectra formed by two equal prisms of different substances, under precisely the same circumstances, are of different lengths. Thus, if a prism of flint-glass and one of crown-glass of equal refracting angles be presented to two rays of white light at equal angles, it will be found that the space over which the coloured rays are dispersed by the flint-glass is much greater than the space occupied by that produced by the crown-glass: and as the quantity of dispersion depends upon the refracting angle of the prism, the angles of the two prisms may be made such that, when the prisms are placed close together with their edges turned opposite ways, they will exactly oppose each other’s action, and will refract the coloured rays equally, but in contrary directions, so that an exact compensation will be effected, and the light will be refracted without colour (N.195). The achromatic telescope is constructed on this principle. It consists of a tube with an object-glass or lens at one end to bring the rays to a focus, and form an image of the distant object, and a magnifying-glass at the other end to view the image thus formed. Now it is found that the object-glass, instead of making the rays converge to one point, disperses them, and gives a confused and coloured image: but by constructing it of two lenses in contact, one of flint and the other of crown-glass of certain forms and proportions, the dispersion is counteracted, and a perfectly well-defined and colourless image of the object is formed (N.196). It was thought to be impossible to produce refraction without colour, till Mr. Hall, a gentleman of Worcestershire, constructed a telescope on this principle in the year 1733; and twenty-five years afterwards the achromatic telescope was brought to perfection by Mr. Dollond, a celebrated optician in London.

By means of Mr. Fraunhofer’s determination of the refraction of the principal rays in substances, their dispersive powers may be found (N.197).

A perfectly homogeneous colour is very rarely to be found; but the tints of all substances are most brilliant when viewed in light of their own colour. The red of a wafer is much more vivid in red than in white light; whereas, if placed in homogeneous yellow light, it can no longer appear red, because there is not a ray of red in the yellow light. Were it not that the wafer, like all other bodies, whether coloured or not, reflects white light at its outer surface, it would appear absolutely black when placed in yellow light.

After looking steadily for a short time at a coloured object, such as a red wafer, on turning the eyes to a white substance, a green image of the wafer appears, which is called the accidental colour of red. All tints have their accidental colours: thus the accidental colour of orange is blue; that of yellow is indigo; of green, reddish white; of blue, orange-red; of violet, yellow; and of white, black; and vice versÂ. When the direct and accidental colours are of the same intensity, the accidental is then called the complementary colour, because any two colours are said to be complementary to one another which produce white when combined.

From experiments by M. Plateau of Brussels, it appears that two complementary colours from direct impression, which would produce white when combined, produce black, or extinguish one another, by their union, when accidental; and also that the combination of all the tints of the solar spectrum produces white light if they be from a direct impression on the eye, whereas blackness results from a union of the same tints if they be accidental; and in every case where the real colours produce white by their combination, the accidental colours of the same tints produce black. When the image of an object is impressed on the retina only for a few moments, the picture left is exactly of the same colour with the object, but in an extremely short time the picture is succeeded by the accidental image. M. Plateau attributes this phenomenon to a reaction of the retina after being excited by direct vision, so that the accidental impression is of an opposite nature to the corresponding direct impression. He conceives that when the eye is excited by being fixed for a time on a coloured object, and then withdrawn from the excitement, it endeavours to return to its state of repose; but in so doing, that it passes this point, and spontaneously assumes an opposite condition, like a spring which, bent in one direction, in returning to its state of rest bends as much the contrary way. The accidental image thus results from a particular modification of the organ of sight, in virtue of which it spontaneously gives us a new sensation after it has been excited by direct vision. If the prevailing impression be a very strong white light, its accidental image is not black, but a variety of colours in succession. According to M. Plateau, the retina offers a resistance to the action of light, which increases with the duration of this action; whence, after looking intently at an object for a long time, it appears to decrease in brilliancy. The imagination has a powerful influence on our optical impressions, and has been known to revive the images of highly luminous objects months, and even years, afterwards.