figure 30.
figure 30.
It is not unlikely that phenomena of a new and different description from any we have hitherto observed, may be produced from the same causes to which we have adverted. A certain optical writer remarks—‘If the variation of the refractive power of the air takes place in a horizontal line perpendicular to the line of vision, that is, from right to left, then we may have alateralMirage, that is, an image of a ship may be seen on the right or left hand of the real ship, or on both, if the variation of refractive power is the same on each side of the line of vision, and a fact of this kind was once observed on the Lake of Geneva. If there should happen at the same time, both a vertical and a lateral variation of refractive power in the air, and if the variation should be such as to expand or elongate the object in both directions, then the object would be magnified as if seen through a telescope, and might be seen and recognized at a distance at which it would not otherwise have been visible. If the refracting power, on the contrary, varied, so as to construct the object in both directions, the image of it would be diminished as if seen through a concave lens.’
Such, then, are some of the striking and interesting effects produced by the refraction and the reflection of the rays of light. As the formation of theimagesof objects by convex lenses, lays the foundation of the construction of refracting telescopes and microscopes, and of all the discoveries they have brought to light, so the property ofconcave specula, in forming similarimages, is that on which the construction ofReflectingtelescopes entirely depends. To this circumstance Herschel was indebted for the powerful telescopes he was enabled to construct—which were all formed on the principle of reflection—and for all the discoveries they enabled him to make in the planetary system, and in the sidereal heavens. The same principles which operate in optical instruments, under the agency of man, we have reason to believe, frequently act on a more expansive scale in various parts of the system of nature. The magnificentCrosswhich astonished the preacher and the immense congregation assembled at Migné, was, in all probability, formed by a vast atmospherical speculum formed by the hand of nature, and representing its objects on a scale far superior to that of human art; and probably, to the same cause is to be attributed the singular phenomenon of the coast of France having been made to appear within two or three miles of the town of Hastings, as formerly described, (see p. 53.) Many other phenomena which we have never witnessed, and of which we can form no conception, may be produced by the same cause operating in an infinity of modes.
The facts we have stated above, and the variety of modes by which light may be refracted and reflected by different substances in nature, lead us to form some conceptions of the magnificent and diversified scenes which light may produce in other systems and worlds, under the arrangements of the all-wise and Beneficent Creator. Light, in all its modifications and varieties of colour and reflection, may be considered as the beauty and glory of the universe, and the source of unnumbered enjoyments to all its inhabitants. It is a symbol of the Divinity himself; for “God isLight, and in Him is no darkness at all.” It is a representative of Him who is exhibited in the Sacred oracles, as “TheSunof Righteousness,” and “theLightof the world.” It is an emblem of the glories and felicities of that future world, where knowledge shall be perfected, and happiness complete; for its inhabitants are designated “the saints inlight;” and it is declared in Sacred history, to have been the first born of created beings. In our lower world, its effects on the objects which surround us, and its influences upon all sensitive beings, are multifarious and highly admirable. While passing from infinitude to infinitude, it reveals the depth and immensity of the heavens, the glory of the sun, the beauty of the stars, the arrangements of the planets, the rainbow encompassing the sky with its glorious circle, the embroidery of flowers, the rich clothing of the meadows, the valleys standing thick with corn, “the cattle on a thousand hills,” the rivers rolling through the plains, and the wide expanse of the ocean. But in other worlds the scenes it creates may be far more resplendent and magnificent. This may depend upon the refractive and reflective powers with which the Creator has endowed the atmospheres of other planets, and the peculiar constitution of the various objects with which they are connected. It is evident, from what we already know of the reflection of light, that very slight modifications of certain physical principles, and very slight additions to the arrangements of our terrestrial system, might produce scenes of beauty, magnificence and splendour of which, at present, we can form no conception. And, it is not unlikely that by such diversities of arrangement, in other worlds,an infinite varietyof natural scenery is produced throughout the universe.
In the arrangements connected with the planet Saturn, and the immense rings with which it is encompassed, and in the various positions which its satellites daily assume with regard to one another, to the planet itself, and to these rings—there is, in all probability, a combination of refractions, reflections, light, and shadows, which produce scenes wonderfully diversified, and surpassing in grandeur what we can now distinctly conceive. In the remote regions of the heavens, there are certain bodies composed of immense masses of luminous matter, not yet formed into any regular system, and which are known by the name ofNebulæ. What should hinder us from supposing that certain exterior portions of those masses form speculums of enormous size, as some parts of our atmosphere are sometimes found to do? Such specula may be conceived to be hundreds and even thousands of miles in diameter, and that they may form images of the most distant objects in the heavens, on a scale of immense magnitude and extent, and which may be reflected, in all their grandeur, to the eyes of intelligences at a vast distance. And, if the organs of vision of such beings, be far superior to ours in acuteness and penetrating power, they may thus be enabled to take a survey of an immense sphere of vision, and to descry magnificent objects at distances the most remote from the sphere they occupy. Whatever grounds there may be for such suppositions, it must be admitted, that all the knowledge we have hitherto acquired respecting the operation of light, and the splendid effects it is capable of producing, is small indeed, and limited to a narrow circle, compared with the immensity of its range, the infinite modifications it may undergo, and the wondrous scenes it may create in regions of creationto which human eyes have never yet penetrated,—and which may present to view objects of brilliancy and magnificence such as, “Eye hath not yet seen, nor ear heard, nor hath it entered into the heart of man to conceive.”
We have hitherto considered light chiefly as a simple homogeneous substance, as if all its rays were white, and as if they were all refracted in the same manner by the different lenses on which they fall. Investigations however, into the nature of this wonderful fluid, have demonstrated that this is not the case, and that it is possessed of certain additional properties, of the utmost importance in the system of nature. Had every ray of light been a pure white, and incapable of being separated into any other colours, the scene of the universe would have exhibited a very different aspect from what we now behold. One uniform hue would have appeared over the whole face of nature, and one object could scarcely have been distinguished from another. The different shades of verdure which now diversify every landscape, the brilliant colouring of the flowery fields, and almost all the beauties and sublimities which adorn this lower creation would have been withdrawn. But it is now ascertained that every ray of white light is composed of an assemblage of colours, whence proceed that infinite variety ofshade and colour with which the whole of our terrestrial habitation is arrayed. Those colours are found not to be in the objects themselves, but in the rays of light which fall upon them, without which they would either be invisible, or wear an uniform aspect. In reference to this point, Goldsmith has well observed: ‘The blushing beauties of the rose, the modest blue of the violet, are not in the flowers themselves, but in the light that adorns them. Odour, softness, and beauty of figure are their own; but it is light alone that dresses them up in those robes which shame the monarch’s glory.’
Many strange opinions and hypotheses were entertained respecting colours, by the ancients, and even by many modern writers, prior to the time of Sir Isaac Newton. The Pythagoreans called colour thesuperficiesof bodies; Plato said that it was a flame issuing from them. According to Zeno it is the first configuration of matter, and according to Aristotle, it is that which moves bodies actually transparent. Among the moderns, Des Cartes imagined that the difference of colour proceeds from the prevalence of the direct or rotatory motions of the particles of light. Grimaldi, Dechales, and others, thought the differences of colour depended upon the quick or slow vibrations of a certain elastic medium filling the whole universe. Rohault imagined that the different colours were made by the rays of light entering the eye at different angles with respect to the optic axis; and Dr. Hook conceived that colour is caused by the sensation of the oblique or uneven pulse of light; and this being capable of no more than two varieties, he concluded that there could be no more than two primary colours. Such were some of the crude opinions which prevailedbefore the era of the illustrious Newton, by whose enlightened investigations the true theory of colours was at last discovered. In the year 1666 this philosopher began to investigate the subject; and finding the coloured image of the sun, formed by a glass prism, to be of an oblong and not of a circular form, as according to the laws of refraction it ought to be, he was surprised at the great disproportion between its length and breadth, the former beingfivetimes the length of the latter; and he began to conjecture that light is nothomogeneal, but that it consists of rays some of which are much more refrangible than others. Prior to this period, philosophers supposed thatalllight, in passing out of one medium into another of different density wasequallyrefracted in the same or like circumstances; but Newton discovered that this is not the fact; but that there aredifferent speciesof light, and that each species is disposed both to suffer a different degree of refrangibility in passing out of one medium into another,—and to excite in us the idea of adifferent colourfrom the rest; and that bodies appear of that colour which arises from the peculiar rays they are disposed to reflect. It is now, therefore, universally acknowledged, that the light of the sun, which to us seems perfectly homogeneal and white, is composed of no fewer thansevendifferent colours, namelyRed,Orange,Yellow,Green,Blue,Indigo and Violet. A body which appears of a red colour has the property of reflecting the red rays more powerfully than any of the others; a body of a green colour reflects the green rays more copiously than rays of any other colour, and so of the orange, yellow, blue, purple and violet. A body which is of ablackcolour, instead of reflecting—absorbsall, or the greater part of therays that fall upon it; and, on the contrary, a body that appearswhitereflects the greater part of the rays indiscriminately without separating the one from the other.
Before proceeding to describe the experiments by which the above results were obtained, it may be proper to give some idea of the form and effects of thePrismby which such experiments are made. This instrument is triangular and straight, and generally about three or four inches long. It is commonly made of white glass, as free as possible from veins and bubbles, and other similar defects, and is solid throughout. Its lateral faces, or sides, should be perfectly plane and of a fine polish. The angle formed by the two faces, one receiving the ray of light that is refracted in the instrument, and the other affording it an issue on its returning into the air, is called therefracting angleof the prism, as ACB, (fig. 31.) The manner in which Newton performed his experiments, and established the discovery to which we have alluded, is as follows.
In the window-shutter EG, (fig. 31.) of a dark room, a hole F, was made, of about one third of an inch diameter, and behind it was placed a glass prism ACB, so that the beam of light, SF, proceeding directly from the sun was made to pass through the prism. Before the interposition of the prism, the beam proceeded in a straight line towards T, where it formed a round white spot; but being now bent out of its course by the prism, it formed an oblong image OP, upon the white pasteboard, or screen LM, containing the seven colours marked in the figure—theredbeing theleast, and thevioletthemostrefracted from the original direction of the solar beam, ST. This oblong image is called theprismatic spectrum. If the refracting angle of the prism ACB, be 64degrees, and the distance of the pasteboard from the prism about 18 feet, the length of the image OP will be about 10 inches, and the breadth 2 inches. The sides of the spectrum are right lines distinctly bounded, and the ends are semicircular. From this circumstance it is evident that it is still the image of the sun, but elongated by the refractive power of the prism. It is evident from the figure, that since some part of the beam, RO, is refracted much further out of its natural course WT, than some other part of the beam, as WP, the rays towards RO have a much greater disposition to be refracted than those toward WP; and that this disposition arises from the naturally different qualities of those rays, is evident from this consideration, that the refracting angle or power of the prism is the same in regard to the superior part of the beam as to the inferior.
figure 31.
figure 31.
By making a hole in the screen LM opposite any one of the colours of the spectrum, so as to allow that colour alone to pass—and by letting the colour thus separated fall upon a second prism—Newton found that the light of each of the colours was alike refrangible, because the second prism could not separate them into an oblong image, or into any other colour. Hence hecalled all the seven colourssimpleor homogeneous, in opposition towhitelight, which he calledcompoundor heterogeneous. With the prism which this philosopher used he found the lengths of the colours and spaces of the spectrum to be as follows: Red, 45; Orange, 27; Yellow, 40; Green, 60; Blue, 60; Indigo, 48; Violet, 80: or 360 in all. But these spaces vary a little with prisms formed of different substances, and as they are not separated by distinct limits, it is difficult to obtain any thing like an accurate measure of their relative extents. Newton examined the ratio between the sines of incidence and refraction of these decompounded rays (see p. 30,) and found that each of the seven primary colour-making rays, had certain limits within which they were confined. Thus let the sine of incidence in glass be divided into 50 equal parts, the sine of refraction into air of theleastrefrangible, and themostrefrangible rays will contain respectively 77 and 78 such parts. The sines of refraction of all the degrees ofredwill have the intermediate degrees of magnitude, from 77 to 77 one-eighth;Orangefrom 77 one-eighth to 77 one-fifth;Yellowfrom 77 one-fifth to 77 one-third;Greenfrom 77 one-third to 77 one-half;Bluefrom 77 one-half to 77 two-thirds;Indigofrom 77 two-thirds to 77 seven-ninths; andVioletfrom 77 seven-ninths to 78.
From what has been now stated, it is evident that, in proportion as any part of an optic glass bears a resemblance to the form of a prism, the component rays that pass through it must be necessarily separated, and will consequently paint or tinge the object with colours. The edges of every convex lens approach to this form, and it is on this account that the extremities of objects when viewed through them are found to be tingedwith the prismatic colours. In such a glass, therefore, those different coloured rays will havedifferent foci, and will form their respective images at different distances from the lens. Thus, suppose LN (fig. 32.) to represent a double convex-lens, and OB an object at some distance from it. If the object OB was of a pure red colour, the rays proceeding from it would form a red image atRr; if the object was of a violet colour, an image of that colour would be formed atVv,nearerthe lens; and if the object was white or any other combination of the colour-making rays, those rays would have their respective foci at different distances from the lens, and form a succession of images, in the order of the prismatic colours, between the spaceRrandVv.
figure 32.
figure 32.
figure 33.
figure 33.
This may be illustrated by experiment in the following manner. Take a card or slip of whitepasteboard, as ABEF, (fig. 33.) and paint one half ABCDred, the other half CF,violetor indigo; and tying black threads across it, set it near the flame of a candle G, then take a lens HI, and holding a sheet of white paper behind it, move it backwards and forwards upon the edge of a graduated ruler, till you see the black threads most distinctly in the image, and you will find the focus of the violetFE, much nearer than that of the redAC, which plainly shows that bodies of different colours can never be depicted by convex-lenses, without some degree of confusion.
The quantity of dispersion of the coloured rays in convex lenses depends upon the focal length of the glass; the space which the coloured images occupy being about the twenty-eighth part. Thus if the lens be twenty-eight inches focal distance, the space betweenRrandVv(fig 32) will be about one inch; if it be twenty-eight feet focus, the same space will be about one foot, and so on in proportion. Now, when such a succession of images formed by the different coloured rays, is viewed through an eye-glass, it will seem to form but one image, and consequently very indistinct, and tinged with various colours, and as the red figureRris largest, or seen under the greatest angle—the extreme parts of this confused image will be red, and a succession of the prismatic colours will be formed within this red fringe, as is generally found in common refracting-telescopes, constructed with a single object-glass. It is owing to this circumstance that the common refracting telescope cannot be much improved without having recourse to lenses of a very long focal distance; and hence, about 150 years ago, such telescopes were constructed of 80, and 100, and 120 feet in length. But still the image was not formed sodistinctly as was desired, and the aperture of the object-glass was obliged to be limited. This is a defect which was long regarded as without a remedy; and even Newton himself despaired of discovering any means by which the defects of refracting telescopes might be removed and their improvement effected. This, however, was accomplished by Dollond to an extent far surpassing what could have been expected, of which a particular account will be given in the sequel.
It was originally remarked by Newton, and the fact has since been confirmed by the experiments of Sir W. Herschel, thatthe different-coloured rays have not the same illuminating power. The violet rays appear to have the least illuminating effect; the indigo more, and the effect increases in the order of the colours,—thegreenbeing very great; between the green and the yellow the greatest of all; the yellow the same as the green; but the red less than the yellow. Herschel also endeavoured to determine whether the power of the differently-coloured rays toheatbodies, varied with their power to illuminate them. He introduced a beam of light into a dark room, which was decomposed by a prism, and then exposed a very sensible thermometer to all the rays in succession, and observed the heights to which it rose in a given time. He found that their heating power increased from the violet to the red. The mercury in the thermometer rose higher when its bulb was placed in the Indigo than when it was placed in the violet, still higher in blue, and highest of all at red. Upon placing the bulb of the thermometer below the red, quite out of the spectrum, he was surprised to find that the mercury rose highest of all; and concluded thatrays proceed from the sun, which have the power ofHEATING,but not of illuminating bodies. These rays have been calledinvisiblesolar rays. They were about half an inch from the commencement of the red rays; at a greater distance from this point the heat began to diminish, but was very perceptible even at the distance of 1½ inch. He determined that the heating power of theredto that of thegreenrays, was 2¾ to 1, and 3½ to 1, in red toviolet. He afterwards made experiments to collect those invisible calorific rays, and caused them to act independently of the light, from which he concluded that they are sufficient to account for all the effects produced by the solar rays in exciting heat; that they are capable of passing through glass, and of being refracted and reflected, after they have been finally detached from the solar beam.
M. Ritter of Jena, Wollaston, Beckman and others, have found that the rays of the spectrum are possessed of certainchemical properties—that beyond the least brilliant extremity, namely, a little beyond thevioletray, there areinvisiblerays which act chemically, while they have neither the power of heating nor illuminating bodies. Muriate of silver exposed to the action of the red rays becomes blackish; a greater effect is produced by the yellow: a still greater by the violet, and the greatest of all by theinvisibleraysbeyondthe violet. When phosphorus is exposed to the action of the invisible rays beyond the red, it emits white fumes; but the invisible rays beyond the violet extinguish them. The influence of these rays is daily seen in the change produced upon vegetable colours, which fade, when frequently exposed to the direct influence of the sum. What object they are destined to accomplish in the general economy of nature, is not yet distinctly known;we cannot however doubt that they are essentially requisite to various processes going forward in the material system. And we know that, not only the comfort of all the tribes of the living world, but the very existence of the animal and vegetable creation depends upon the unremitting agency of theCalorificrays.
It has likewise been lately discovered that certain rays of the spectrum, particularly theviolet, possesses the property of communicating the magnetic power. Dr.Morichini, of Rome, appears to have been the first who found that the violet rays of the spectrum had this property. The result of his experiments, however, was involved in doubt, till it was established by a series of experiments instituted by Mrs.Somerville, whose name is so well known in the scientific world. This lady having covered half of a sewing-needle, about an inch long, with paper, she exposed the other half for two hours, to the violet rays. The needle had then acquired North polarity. The indigo rays produced nearly the same effect; and the blue and green rays produced it in a still less degree. In the yellow, orange, red and invisible rays, no magnetic influence was exhibited, even though the experiment was continued for three successive days. The same effects were produced by enclosing the needle in blue or green glass, or wrapping it in blue and green ribbands one half of the needle being always covered with paper.
One of the most curious discoveries of modern times, in reference to the solar spectrum, is that of Fraunhofer of Munich—one of the most distinguished artists and opticians on the Continent.13He discovered that the spectrum is covered with dark and coloured lines, parallel to one another, and perpendicular to the length of the spectrum; and he counted no less than 590 of these lines. In order to observe these lines, it is necessary to use prisms of the most perfect construction, of very pure glass, free of veins—to exclude all extraneous light, and even to stop those rays which form the coloured spaces, which we are not examining. It is necessary also to use a magnifying instrument, and the light must enter and emerge from the prism at equal angles. One of the important practical results of this discovery is, that those lines are fixed points in the spectrum, or rather, that they have always the same position in the coloured spaces in which they are found. Fraunhofer likewise discovered in the spectrum produced by the light of Venus, the same streaks, as in the solar spectrum; in the spectrum of the light of Sirius, he perceived three large streaks which, according to appearance, had no resemblance to those of the light of the sun; one ofthem was in the green, two in the blue. The stars appear to differ from one another in their streaks. The electric light differs very much from the light of the sun and that of a lamp, in regard to the streaks of the spectrum—‘This experiment may also be made, though 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 the refracting angle parallel to the line of light. 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 position, breadth and intensities.’
From what has been stated in reference to the solar spectrum it will evidently appear, that white light is nothing else than a compound of all the prismatic colours; and this may be still farther illustrated by shewing, that the seven primary colours, when again put together, recompose white light. This may be rudely proved for the purpose of illustration, by mixing together seven different powders, having the colours and proportion of the spectrum; but the best mode, on the whole, is the following. Let two circles be drawn on a smooth round board, covered with white paper, as in fig. 34: Let the outermost be divided into 360 equal parts; then draw seven right lines as A,B,C, &c., from the center to the outermost circle, making the lines A and B include 80 degrees of that circle. The lines B and C, 40 degrees; C and D, 60; D and E, 60; E and F, 48; F and G, 27; G and A, 45. Then between these two circles paint the space AG red, incliningto orange near G; GF orange, inclining to yellow near F; FE yellow, inclining to green near E; ED green, inclining to blue near D; DC blue, inclining to indigo near C; CB indigo, inclining to violet near B; and BA violet, inclining to a soft red near A. This done, paint all that part of the board black which lies within the inner circle; and putting an axis through the centre of the board, let it be turned swiftly round that axis, so that the rays proceeding from the above colours, may be all blended and mixed together in coming to the eye. Then the whole coloured part will appear like a white ring a little grayish—not perfectly white, because no art can prepare or lay on perfect colours, in all their delicate shades, as found in the real spectrum.
figure 34.
figure 34.
That all the colours of light, when blended together in their proper proportions, produce a purewhiteis rendered certain by the following experiment. Take a large convex glass, and place itin the room of the paper or screen on which the solar spectrum was depicted (LM fig. 31), the glass will unite all the rays which come from the prism, if a paper is placed to receive them, and you will see a circular spot of a pure lively white. The rays will cross each other in the focus of the glass, and, if the paper be removed a little further from that point, you will see the prismatic colours again displayed, but in an inverted order, owing to the crossing of the rays.
From what has been stated above we may learn the true cause of those diversified hues exhibited by natural and artificial objects, and the variegated colouring which appears on the face of nature. It is owing to the surfaces of bodies being disposed to reflect one colour rather than another. When this disposition is such that the body reflects every kind of ray, in the mixed state in which it receives them, that body appearswhiteto us—which, properly speaking, is no colour, but rather the assemblage of all colours. If the body has a fitness to reflect one sort of rays more abundantly than others, by absorbing all the others, it will appear of the colour belonging to that species of rays. Thus, the grass isgreen, because it absorbs all the rays except the green. It is these green rays only which the grass, the trees, the shrubs, and all the other verdant parts of the landscape reflect to our sight, and which make them appear green. In the same manner the different flowers reflect their respective colours; the rose, the red rays; the violet, the blue; the jonquil, the yellow; the marigold, theorange, and every object, whether natural or artificial, appears of that colour which its peculiar texture is fitted to reflect. A great number of bodies are fitted to reflect at once several kinds of rays, and of consequence they appear under mixed colours. It may even happen, that of two bodies which should be green, for example, one may reflect the pure green of light, and the other the mixture of yellow and blue. This quality, which varies to infinity, occasions the different kinds of rays to unite in every possible manner, and every possible proportion; and hence the inexhaustible variety of shades and hues which nature has diffused over the landscape of the world. When a body absorbs nearly all the light which reaches it, that body appearsblack. It transmits to the eye so few reflected rays that it is scarcely perceptible in itself, and its presence and form make no impression upon us, unless as it interrupts the brightness of the surrounding space. Black is, therefore, the absence of all the coloured rays.
It is evident, then, that all the various assemblages of colours which we see in the objects around us,are not in the bodies themselves, but in the light which falls upon them. There is no colourinherentin the grass, the trees, the fruits, and the flowers, nor even in the most splendid and variegated dress that adorns a lady. All such objects are as destitute of colour, in themselves, as bodies which are placed in the centre of the earth, or as the chaotic materials out of which our globe was formed, before light was created. For where there is no light, there is no colour. Every object is black, or without colour, in the dark, and it only appears coloured as soon as light renders it visible. This is further evident from the following experiment. If we place a colouredbody in one of the colours of the spectrum which is formed by the prism, it appears of the colour of the rays in which it is placed. Take, for example, a red rose, and expose it first to the red rays, and it will appear of a more brilliant ruddy hue. Hold it in the blue rays, and it appears no longer red, but of a dingy blue colour, and in like manner its colour will appear different, when placed in all the other differently coloured rays. This is the reason why the colours of objects are essentially altered by the nature of the light in which they are seen. The colours of ribbons and various pieces of silk or woollen stuff are not the same when viewed by candle-light as in the day time. In the light of a candle or a lamp, blue often appears green, and yellow objects assume a whitish aspect. The reason is that the light of a candle is not so pure a white as that of the sun, but has a yellowish tinge, and therefore, when refracted by the prism, the yellowish rays are found to predominate, and the superabundance of yellow rays gives to blue objects a greenish hue.
The doctrine we are now illustrating is one which a great many persons, especially among the fair sex, find it difficult to admit. They cannot conceive it possible that there is no colour really inherent in their splendid attire, and no tints of beauty in their countenances. ‘What,’ said a certain lady, ‘are there no colours in my shawl, and in the ribbons that adorn my head-dress—and, are we all as black as negroes in the dark; I should almost shudder to think of it.’ Such persons, however, need be in no alarm at the idea; but may console themselves with the reflection, that, when they are stripped of all their coloured ornaments in the dark, they are certain thattheywill never be seen by any onein that state; and therefore, there is no reason to regret the temporary loss of those beauties which light creates—when they themselves and all surrounding objects areinvisible. But, to give a still more palpable proof of this position, the following popular experiments may be stated.
Take a pint of common spirit, and pour it into a soup dish, and then set it on fire; as it begins to blaze, throw a handful of salt into the burning spirit, and keep stirring it with a spoon. Several handfuls may thus be successively thrown in, and then the spectators, standing around the flame, will see each other frightfully changed, their colours being altered into a ghastly blackness, in consequence of the nature of the light which falls upon them—which produces colours very different from those of the solar light. The following experiment, as described by Sir D. Brewster, illustrates the same principle. ‘Having obtained the means of illuminating any apartment withyellowlight, let the exhibition be made in a room with furniture of various bright colours, and with oil or water coloured paintings on the wall. The party which is to witness the experiment should be dressed in a diversity of the gayest colours; and the brightest coloured flowers, and highly coloured drawings should be placed on the tables. The room being at first lighted with ordinary lights, the bright and gay colours of every thing that it contains will be finely displayed. If the white lights are now suddenly extinguished, and the yellow lamps lighted, the most appalling metamorphosis will be exhibited. The astonished individuals will no longer be able to recognise each other. All the furniture of the room, and all the objects it contains, will exhibit onlyonecolour. The flowerswill lose their hues; the paintings and drawings will appear as if they were executed in China ink, and the gayest dresses, the brightest scarlets, the purest lilacs, the richest blues and the most vivid greens, will all be converted into one monotonous yellow. The complexions of the parties, too, will suffer a corresponding change. One pallid deathlike yellow, will envelope the young and the old, and thesallowface will alone escape from the metamorphosis. Each individual derives merriment from the cadaverous appearance of his neighbour, without being sensible that he is one of the ghastly assemblage.’
——Like the unnatural hueWhich autumn paints upon the perished leaf,
——Like the unnatural hueWhich autumn paints upon the perished leaf,
——Like the unnatural hueWhich autumn paints upon the perished leaf,
From such experiments as these we might conclude, that were the solar rays of a very different description from what they are now found to be, the colours which embellish the face of nature, and the whole scene of our sublunary creation would assume a new aspect, and appear very different from what we now behold around us in every landscape. We find that the stars display great diversity of colour; which is doubtless owing to the different kinds of light which are emitted from those bodies; and hence we may conclude, that the colouring thrown upon the various objects of the universe is different in every different system, and that thus, along with other arrangements, an infinite variety of colouring and of scenery is distributed throughout the immensity of creation.
Theatmosphere, in consequence of its different refractive and reflective powers, is the source of a variety of colours which frequently embellish anddiversify the aspect of our sky. The airreflectsthe blue rays most plentifully, and must thereforetransmitthe red, orange, and yellow, more copiously than the other rays. When the sun and other heavenly bodies are at a high elevation, their light is transmitted without any perceptible change, but when they are near the horizon, their light must pass through a long and dense track of air, and must therefore be considerably modified before it reach the eye of the observer. The momentum of the red rays being greater than that of the violet, will force their way through the resisting medium, while the violet rays will be either reflected or absorbed. If the light of the setting sun, by thus passing through a long track of air, be divested of the green, blue, indigo, and violet rays, the remaining rays which are transmitted through the atmosphere, will illuminate the western clouds, first with an orange colour; and then, as the sun gradually sinks into the horizon, the track through which the rays must pass becoming longer, the yellow and orange are reflected, and the clouds grow more deeplyred, till at length the disappearance of the sun leaves them of a leaden hue by the reflection of the blue light through the air. Similar changes of colour are sometimes seen on the eastern and western fronts of white buildings. St. Paul’s Church, in London, is frequently seen at sun-set, tinged with a very considerable degree of redness; and the same cause occasions the moon to assume a ruddy colour, by the light transmitted through the atmosphere. From such atmospherical refractions and reflections are produced those rich and beautiful hues with which our sky is gilded by the setting sun, and the glowing red which tinges the morning and evening clouds, till their ruddy glare is temperedby the purple of twilight, and the reflected azure of the sky.
When a direct spectrum is thrown on colours darker than itself, it mixes with them: as the yellow spectrum of the setting sun, thrown on the green grass, becomes a greener yellow. But when a direct spectrum is thrown on colours brighter than itself, it becomes instantly changed into the reverse spectrum, which mixes with those brighter colours. Thus the yellow spectrum of the setting sun thrown on the luminous sky, becomes blue, and changes with the colour or brightness of the clouds on which it appears. The red part of light being capable of struggling through thick and resisting mediums which intercept all other colours—is likewise the cause why the sun appears red when seen through a fog,—why distant light, though transmitted through blue or green glass, appears red—why lamps at a distance, seen through the smoke of a long street, are red, while those that are near, are white. To the same cause it is owing that a diver at the bottom of the sea is surrounded with the red light which has pierced through the superincumbent fluid, and that the blue rays are reflected from thesurfaceof the ocean. Hence, Dr. Halley informs us that, when he was in a diving bell, at the bottom of the sea, his hand always appeared red in the water.
Thebluerays, as already noticed, being unable to resist the obstructions they meet with in their course through the atmosphere, are either reflected or absorbed in their passage. It is to this cause, that most philosophers ascribethe blue colour of the sky, the faintness and obscurity of distant objects, and the bright azure which tinges the mountains of a distant landscape.
Since the rays of light are found to be decomposed by refracting surfaces, and reflected in an infinite variety of modes and shades of colour, we need not be surprised at the changes produced in any scene or object by the intervention of another, and by the numerous modifications of which the primary colours of nature are susceptible. The vivid colours which gild the rising and the setting sun, must necessarily differ from those which adorn its noon-day splendour. Variety of atmospheric scenery will thus necessarily be produced, greater than the most lively fancy can well imagine. The clouds will sometimes assume the most fantastic forms, and at other times will be irradiated with beams of light, or, covered with the darkest hues, will assume a lowering aspect, prognostive of the thunder’s roar and the lightning’s flash—all in accordance with the different rays that are reflected to our eyes, or the quantity absorbed by the vapours which float in the atmosphere.
Light, which embellishes with so much magnificence a pure and serene sky, by means of innumerable bright starry orbs which are spread over it, sometimes, in a dark and cloudy sky, exhibits an ornament which, by its pomp, splendour and variety of colours, attracts the attention of every eye that has an opportunity of beholding it. At certain times, when there is a shower either around us, or at a distance from us in an opposite quarter to that of the sun, a species of arch or bow is seen in the sky, adorned with all the seven primary colours of light. This phenomenon,which is one of the most beautiful meteors in nature, has obtained the name of theRainbow. The rainbow was, for ages, considered as an inexplicable mystery, and by some nations it was adored as a deity. Even after the dawn of true philosophy, it was a considerable time before any discovery of importance was made, as to the true causes which operate in the production of this phenomenon. About the year 1571, M. Fletcher of Breslau, made a certain approximation to the discovery of the true cause, by endeavouring to account for the colours of the rainbow by means of a double refraction and one reflection. A nearer approximation was made by Antonio de Dominis, bishop of Spalatro, about 1601. He maintained that the double refraction of Fletcher,with an intervening reflection, was sufficient to produce the colours of the bow, and also to bring the rays that formed them to the eye of the spectator, without any subsequent reflection. To verify this hypothesis, he procured a small globe of solid glass, and viewing it when it was exposed to the rays of the sun—with his back to that luminary—in the same manner as he had supposed the drops of rain were situated with respect to them, he observed the same colours which he had seen in the rainbow, and in the same order. But he could give no good reasonwhythe bow should be coloured, and much less any satisfactory account of theorderin which the colours appear. It was not till Sir I. Newton discovered the different refrangibility of the rays of light, that a complete and satisfactory explanation could be given of all the circumstances connected with this phenomenon.
As the full elucidation of this subject involves a variety of optical and mathematical investigations,I shall do little more than explain the general principle on which the prominent phenomena of the rainbow may be accounted for, and some of the facts and results which theory and observation have deduced.
We have just now alluded to an experiment with a glass globe:—If, then, we take either a solid glass globe, or a hollow globe filled with water, and suspend it so high in the solar rays above the eye, that the spectator, with his back to the sun, can see the globered;—if it be lowered slowly, he will see it orange, then yellow, then green, then blue, then indigo, and then violet; so that the drop at different heights, shall present to the eye the seven primitive colours in succession. In this case, the globe, from its form, will act in some measure like a prism, and the ray will be separated into its component parts. The following figure will more particularly illustrate this point. Suppose A (fig. 35.) to represent a drop of rain—which may be considered as a globe of glass in miniature, and will produce the same effect on the rays of light—and letSdrepresent a ray from the sun falling upon the upper part of the drop atD. At the point of entering the drop, it will suffer a refraction, and instead of going forward toC, it will be bent toN. FromNa part of the light will be reflected toQ—some part of it will, of course, pass through the drop. By the obliquity with which it falls on the side of the drop atQ, that part becomes a kind of prism, and separates the ray into its primitive colours. It is found by computation that, after a ray has suffered two refractions and one reflection, as here represented, the least refrangible part of it, namely theredray, will make an angle with the incident solar ray of 42° 2´, asSfq; and theviolet, or greatest refrangible ray will make with the solar ray, an angle of 40° 17´, asScq; and thus all the particles of water within the difference of those two angles, namely 1° 45´—(supposing the ray to proceed merely from the centre of the sun)—will exhibit severally the colours of the prism, and constitute theinteriorbow of the cloud. This holds good at whatever height the sun may chance to be in a shower of rain. If he be at a high altitude, the rainbow will be low; if he be at a low elevation, the rainbow must be high; and if a shower happen in a vale, when the spectator is on a mountain, he will sometimes see thebow in the form of acomplete circlebelow him. We have at present described the phenomena only of a single drop; but it is to be considered that in a shower of rain there are drops at all heights and at all distances; and therefore the eye situated at G will see all the different colours. All those drops that are in a certain position with respect to the spectator will reflect the red rays, all those in the next station the orange, those in the next the green, and so on with regard to all the other colours.