the rainbow—decomposition of white light by the prism—formation of primary and secondary bows—rainbows in mountain regions—the rainbow a sacred emblem—lunar rainbow—light decomposed by clouds—their beautiful colours—examples.
By means of rain and rain clouds we get that beautiful appearance so well known as the rainbow. In order to form some idea of the manner in which the rainbow is produced, it is necessary to know something of the manner in which light is composed. Sir Isaac Newton was the first philosopher who clearly explained the composition of light, as derived from the sun. He admitted a ray of the sun into a darkened room through a small hole in the window shutters; in front of this hole he placed a glass prism, and at a considerable distance behind the prism he placed a white screen. If there had been no prism between the hole and the screen, the ray of light would have proceeded in the direction of the dotted lines, and a bright spot would have fallen upon the floor of the room, as shown in the figure. But the effect of the prism is to refract or bend the ray out of its ordinary course, and in doing so it does not produce a white spot upon the screen, but a long streak of beautiful colours, in the order marked in the figure, red being at the bottom, then orange, yellow, green, blue, indigo, and violet at the top.
Decomposition of white light
In order to account for the production of these colours from a ray of light, Newton supposed that such a ray is actually made up of seven distinct colours, which being mixed in proper proportions neutralize or destroy each other. In order to account for the decomposition of the ray of white light by the prism, and for the lengthened form of the spectrum, as it is called, he supposed that each of the seven coloured rays was capable of being bent by the prism in a different manner from the rest. Thus, in the figure, the red appears to be less bent out of the direction of the original ray than the orange—the orange less than the yellow, and so on until we arrive at the violet, which is bent most of all.
It is scarcely necessary to remark, that these views were found to be correct, except as regards the number of colours in the solar spectrum; for it is now ascertained, with tolerable certainty, that there are only three primitive or pure colours in nature, and these are red, yellow, and blue; and it is supposed that by mingling two or more of these colours in various proportions, all the colours in nature are produced.
Now, to apply this explanation to the production of the rainbow, which is usually seen under the following circumstances:—The observer is placed with his back to the sun, and at some distance before him rain is falling,—the air between the sun and the rain being tolerably clear. He then often sees two circular arcs or bows immediately in front of him. The colours of the inner bow are the more striking and vivid of the two. Each exhibits the same series of colours as in the spectrum formed by the prism; namely, red, orange, yellow, green, blue, indigo, and violet; but the arrangement of these colours is different in the two bows, for while in the inner bow the lower edge is violet and the upper red, in the outer bow the lower edge is red and the upper violet. The production of both bows is due to the refraction and reflexion of light, the drops of rain forming, in fact, the prism which decomposes the white light of the sun. The colours in the rainbow have the same proportional breadth as the spaces in the prismatic spectrum. “The bow is, therefore,” as Sir D. Brewster remarks, “only an infinite number of prismatic spectra, arranged in the circumference of a circle; and it would be easy, by a circular arrangement of prisms, or by covering up all the central part of a large lens, to produce a small arch of exactly the same colours. All we require, therefore, to form a rainbow, is a great number of transparent bodies capable of forming a great number of prismatic spectra from the light of the sun.”
The manner in which the drops of rain act as prisms, may, perhaps, be better understood with the assistance of the following diagram. Suppose the two lower circles to represent drops of rain which assist in forming the primary bow, and the two upper circles similar drops which help to produce the secondary bow; and let S represent rays of the sun falling upon them. The rays of the sun fall upon every part of the drop; but, as those which pass through or near the centre come out on the opposite side and form a focus, they need not be taken into account. Those rays, however, which fall on the upper side of the drops, will be bent or refracted, the red rays least, and the violet most; and will fall upon the back of the drop in such a manner as to be reflected to the under part of the drop; on quitting which they will be again refracted, so as to be seen at E, where there will appear to the observer a prismatic spectrum with the red uppermost, and the violet undermost. These remarks apply to those drops only which form the upper part of the bow, but it is obvious that a similar reasoning applied to the drops to the right and left of the observer, will complete the bow. The inclination of the red ray and the violet ray to the sun’s rays, is 42° 2' for the red, and 40° 17' for the violet, so that the breadth of the primary bow is 1° 45'.
Thus it will be seen, that the primary bow is produced by two refractions, and one intermediate reflection of the rays that fall on the upper sides of the drops of rain. It is different with the rays which enter the drops below. The red and violet rays will be bent or refracted in different directions; and, after being twice reflected, will be again bent towards the eye of the observer at E; but in this case the violet forms the upper part, and the red the under part of the spectrum. The inclination of these rays to the sun’s rays at S, is 50° 58' for the red ray, and 54° 10' for the violet ray; so that the breadth of the bow is 3° 10', and the distance between the primary and secondary bows is 8° 15'. Hence the secondary is formed in the outside of the primary bow, with its colours reversed, in consequence of their being produced by two reflexions and two refractions. The colours of the secondary bow are much fainter than those of the primary, because they undergo two reflexions instead of one.
There is something very wonderful in the rapidity and perfection with which these natural prisms, the falling drops of rain, produce these effects. In the inconceivably short space of time occupied by a drop falling through those parts of the sky which form the proper angles with the sun’s rays and the eye of the observer, the light enters the surface of the drop, undergoes within it one or two reflexions, two refractions and decompositions, and has reached the eye; and all this is done in a portion of time too small for the drop to have fallen through a space which we have the means of measuring.
It will be understood, that since the eyes of different observers cannot be in precisely the same place at the same time, no two observers can see the same rainbow; that is to say, the bow produced by one set of drops to the eye of one observer is produced by another set of drops to the eye of another observer.
A rainbow can never be greater than a semicircle, unless the spectator is on elevated ground; for if it were greater than a semicircle the centre of the bow would be above the horizon, while the sun, which must be in a line drawn through that centre and the eye of the observer, would be below the horizon: but in such a case, the sun could not shine on the drops of rain, and consequently there could be no rainbow.
When the rain cloud is of small extent only a portion of a bow is visible; when the cloud overspreads a large part of the sky a perfect bow appears. Sometimes the bow may be traced across a portion of blue sky, or it may appear to rest on the ground. In the former case, there are vapours in the air too thin to be seen, but sufficient to refract and reflect the rays of light; in the latter, the drops of rain, adhering to the grass and foliage, produce the same effect. A coloured bow, similar to that produced by rain, is sometimes seen in the spray of a fountain or of a water-fall, and also in mists that lie low upon the ground.
In mountainous and stormy regions rainbows are often seen to great advantage. In the islands off the Irish coast the author of “Letters from the Irish Islands,” describes the rainbow of winter “as gradually advancing before the lowering clouds, sweeping with majestic stride across the troubled ocean, then, as it gained the beach, and seemed almost within one’s grasp, vanishing amid the storm of which it had been the lovely but treacherous forerunner. It is, I suppose, a consequence of our situation, and the close connexion between sea and mountain, that the rainbows here are so frequent and so peculiarly beautiful. Of an amazing breadth, and of colours vivid beyond description, I know not whether most to admire this aËrial phenomenon, when suspended in the western sky, one end of the bow sinks behind the Island of Boffin, while at the distance of several leagues the other rests upon the misty hills of Ennis Turc; or when, at a later hour of the day, it has appeared stretched across the ample sides of Mulbrea, penetrating far into the deep blue waters that flow at its base. With feelings of grateful recollection, too, we may hail the repeated visits of this heavenly messenger, occasionally as often as five or six times in the course of the same day, in a country exposed to such astonishing, and, at times, almost incessant floods of rain.”
The beauty of the rainbow is not the only reason why we should regard it with interest. The rainbow was appointed by God himself as a sign of the covenant of mercy, made with Noah and with all mankind, after the flood. The words in which this declaration was made to mankind, are recorded in the Book of Genesis, chap. ix. ver. 11 to 16.
Burnet, in his “Sacred Theory of the Earth,” has some remarks on the first appearance of the rainbow to the inhabitants of the earth after the deluge. He says, “How proper and how apposite a sign would this be for Providence to pitch upon, to confirm the promise made to Noah and his posterity, that the world should be no more destroyed by water! It had a secret connexion with the effect itself, and was so far a natural sign; but, however, appearing first after the deluge, and in a watery cloud, there was, methinks, a great easiness and propriety of application for such a purpose. And if we suppose, that while God Almighty was declaring his promise to Noah, and the sign of it, there appeared at the same time in the clouds a fair rainbow, that marvellous and beautiful meteor which Noah had never seen before; it could not but make a most lively impression upon him, quickening his faith, and giving him comfort and assurance that God would be stedfast to his promise.”
A rainbow is sometimes formed by the rays of the moon falling upon drops of rain, in the same manner as the solar rays, and refracted and reflected by the drops; but the colours are faint in consequence of the feeble light of the moon compared with that of the sun. A lunar rainbow has been thus described by an observer:—“The moon was truly ‘walking in brightness,’ brilliant as she could be, not a cloud was to be seen near her; and over against her, toward the north-west, or perhaps rather more to the north, was a rainbow, a vast arch, perfect in all its parts, not interrupted or broken as rainbows frequently are, but unremittedly visible from one horizon to the other. In order to give some idea of its extent, it is necessary to say, that, as I stood toward the western extremity of the parish of Stoke Newington, it seemed to take its rise from the west of Hampstead, and to end perhaps in the river Lea, the eastern boundary of Tottenham. Its colour was white, cloudy, or greyish, but a part of its western limb seemed to exhibit tints of a faint sickly green. After some time the moon became darkened by clouds, and the rainbow of course vanished.”
Lunar Rainbow
The brilliant colours of the solar rainbow are frequently produced by the clouds without any prismatic arrangement. The light of the sun is decomposed by a process called absorption: for example, white light is composed of red, yellow, and blue rays, in certain proportions; now, if in passing through, or falling upon any substance whatever, the red rays are stifled or absorbed, while the yellow and blue are allowed to pass or to be reflected, it is obvious that such a substance cannot appear white, because one of the elements of white light, namely, the red, is wanting; it must therefore appear of such a colour as results from the combination of yellow and blue; the substance will therefore appear green. So, also, when white light falls upon what we call a red surface, the yellow and blue rays are stifled or absorbed, leaving the red only to be reflected. Now, when we consider the various ways in which this absorption may take place; one or two, or all of the coloured rays being absorbed in every possible proportion, it is easy to form some idea of the manner by which the innumerable tints of the sky are produced.
It has been calculated, that, of the horizontal sunbeams which pass through two hundred miles of air, scarcely a two thousandth part reaches the earth. A densely formed cloud must therefore detain a much larger share; and those dark and sombre forms, which sometimes make the sky so gloomy, can only result from the abundant absorption of the solar light. The brilliant whiteness which their edges occasionally exhibit, must result from the more copious transmission of light, so that the depths of shade in a cloud may be regarded as comparative measures of the varied thickness of its mass.
Sometimes the clouds absorb equally all the solar rays, in which case the sun and moon appear through them perfectly white. Instances are recorded in which the sun appeared of a pale blue. It has also been observed to be orange at its upper part, while the lower was of a brilliant red.
The position from which clouds are seen, has much to do with their colours; and it seems difficult sometimes to believe that the clouds, which in the evening are seen drenched with crimson and gold, are the same we beheld absolutely colourless in the middle of the day.
In the immediate neighbourhood of the sun the most brilliant colours may be disclosed; and their vividness and intensity diminish, and at last disappear at some distance from it. Parry noticed some white fleecy clouds, which, at the distance of fifteen or twenty degrees from the sun, reflected from their edges the most soft and tender tints of yellow, bluish green, and lake; and as the clouds advanced the colours increased gradually, until they reached a sort of limit two degrees below the solar orb. As the current continued to transport them, the vividness of colour became weakened by almost insensible degrees until the whole assemblage of tints vanished.
“Who can venture to imitate, by the pencil, the endless varieties of red and orange and yellow which the setting sun discloses, and the magical illusions which all the day diversify the vast and varied space the eye travels over in rising gradually from the horizon to the upper sky? Those who have paid any attention to colours, must be aware of the difficulty of describing the various tints and shades that appear, and which are known to amount to many thousands.”
The rapid changes of colour which the clouds undergo, seem to depend on something more than change of position either in the cloud or in the sun. Forster mentions an instance of some detached cirro-cumuli being of a fine golden yellow, but in a single minute becoming deep red. On another occasion he saw the exact counterpart in a cirro-stratus, by its instantly changing from a beautiful red to a bright golden yellow. “What, indeed, can be more interesting, than when by the breaking out of the sun in gleams, a cloud which a moment before seemed only an unshapened mass devoid of all interest and beauty, is suddenly pierced by cataracts of light, and imbued with the most splendid colours, varying every instant in intensity? Numerous examples occur of this beautiful play of colour, which cannot but remind us of the phenomena displayed by the pigeon’s neck and the peacock’s tail, by opal and pearl.
“After the sun is set, the mild glow of his rays is still diffused over every part; and it has been remarked, that the clouds assume their brightest and most splendid colours a few minutes after it is below the horizon. It is in the finest weather that the colouring of the sky presents the most perfect examples of harmony, in tempestuous weather it being almost always inharmonious. At the time of a warm sun-setting, the whole hemisphere is influenced by the prevailing colour of the light. The snowy summits of the Alps appear about sunset of a most beautiful violet colour, approaching to light crimson or pink. It is remarkable, also, as an example of that general harmony which prevails in the material world, that the most glowing and magnificent skies occur when terrestrial objects put on their deepest and most splendid hues. It has also been observed, that it is not the change of vegetation only, which gives to the decaying charms of autumn their finest and most golden hues, but also the atmosphere and the peculiar lights and shadows which then prevail; and there can be no doubt, on the other hand, that our perception of beauty in the sky is very much influenced by the surrounding scenery. In autumn all is matured; and the rich hues of the ripened fruits and the changing foliage are rendered still more lovely by the warm haze which a fine day at that season presents. So, also, the earlier hues of spring have a transparency, and a thousand quivering lights, which in their turn harmonize with the light and flitting clouds and uncertain shadows which then prevail.” [155]
Decorative picture of lady by river
Foot-print of a bird, and impression of rain-drops sand-stone
