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.
[Picture: 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 degrees 2' for the red,
and 40 degrees 17' for the violet, so that the breadth of the primary bow
is 1 degrees 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 degrees 58' for
the red ray, and 54 degrees 10' for the violet ray; so that the breadth
of the bow is 3 degrees 10', and the distance between the primary and
secondary bows is 8 degrees 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 aerial 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."
[Picture: 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}
[Picture: Decorative picture of lady by river]
[Picture: Foot-print of a bird, and impression of rain-drops sand-stone]