A MAN plants his garden in the spring, and out of the selfsame back-yard soil he gets the white of the lily, the blue of the violet, and the damask of the rose. The rains come down to water it, and when the storm is over the rainbow sets itself in the sky with all the seven sisters of light. The humming bird and the pigeon come to visit him, and as they move in the beams of noon they flash back metallic lustres to the sun. At evening the sun goes down and the heavens unlock their treasuries of color. They play upon the eye with the tints of the opal and the pearl; and then the moon comes up like a red balloon and turns to silver as it rises. And when the season is over, and the crops are gathered, and nature has felt the frosts of fall, there is a sudden, astonishing glory in the oak, the maple, and the birch.
All these things are the peculiar goal and property of Art.
But how comes it that this patch of garden clay can do such diverse things with a package of garden seed? How is it that the humming bird, the peacock, and the pigeon can get such rich hues out of the gross material of earth? And the rainbow — how does it maintain itself and stand with such unwavering steadiness in this deluge of falling drops?
All this is Science. And what is Science but the answer to questions?
Any man who knows that yellow paint mixed with blue will produce the color of grass knows by experience that yellow can lose itself in green. He probably has discovered also that red added to yellow will make orange, and that some of this may be concealed in the green, making it a deeper hue. And this will enable him to understand how the forest changes its colors in the fall.
Chlorophyll is the green pigment in leaves which enables them to derive energy from the sun. But while the coloring matter in leaves is predominantly green, there are four parts to it:a light green, a blue-green, a yellow, and an orange. When cold weather comes on, the tree hastens to call in the valuable substance from the leaves and store it away for another season. It is the green coloring matter that is called in, leaving the two yellows behind; and thus for a few short days the trees take on the colorful pageantry of autumn. While the green has chemical powers that constitute the very life of vegetation, the function of the two shades of yellow is but imperfectly known.
All summer the tree has been making sugar in the leaves and withdrawing the sugar for purposes of life and growth; but in the fall it takes sudden action with regard to the precious material of life itself, and saves all it can. It must act promptly before the leaves cast themselves loose and carry all this valuable substance away. The tree is a builder; it is a sentient creature with a regard for the future. As a builder it needs to have a certain wisdom and foresight of its own. Knowing it thus, we might say that the tree was the first Conservationist.
If color is a blessing, Americans need to be thankful for the humming bird, a creature peculiar to America. The rest of the world does not know it. And they need to be grateful for the spectacle of fall, a feast of color that the English have little conception of. And here I am reminded of an anecdote.
Our first notable landscape painter, Thomas Cole, went to England in 1829 with a selection of his paintings to be put on exhibition. The English came, and saw, and were far from conquered. The foliage of this man’s trees was all bright red and yellow! As such a thing was never seen in nature as they knew it, they felt that this was a school of art that needed to be nipped in the bud; and so the critics pooh-poohed, and the visitors had many a jovial nudge over the young artist and his new departure in painting.
Our autumns always create surprise. Dean Hole, of Rochester Cathedral, who came to see us in 1895, was an admirer of flowers. He was enamored of their fragrances, their colors, and their forms. He loved to see ‘a rose looking in at the window.’ Coming here with such an eye for beauty, he went to Central Park. Imagine his surprise when he saw its trees at the height of their autumn hues. He tried to write about it, using adjective after adjective. Then he gave it up and simply said, ‘Let my poor British brother try to imagine a poinsettia grown into a tree.’
The coloring of the hair and skin of the higher animals is accomplished by means of a colorless chemical acted upon by a ferment. This corresponds somewhat to modern practices in the art of dyeing. Wool, for instance, is dipped into a colorless derivative of naphthalene, and then it is oxidized to produce a beautiful brown. The chemist calls such a compound a chromogen. It is the antecedent of a pigment. The physiologist has the same name for the compound which Nature uses in coloring animals; but he is inclined to call it, more romantically, mother-of-pigment.
All the higher animals have this chromogen, whether human or not, and whether white or black. The pigment produced when the ferment acts upon it may be orange, yellow, red, brown, or chocolate deepening into black; and so, by a mixture of hues, there may be grays of any shade. The basic chemical is a protein — the lean-meat principle, which is nitrogenous—broken down into an amino acid; and there is a variety of ferments, some of which have been discovered and named, producing the variely of effects. The white man has this mother-of-pigment in his skin, but he has the ferment only in his hair; whereas the black man has it all over in a potent form. The white man may tan, but there is a limit to his coloring and he bleaches out again. These ferments evidently act to some extent by means of oxygen. The effects of peroxide of hydrogen when used as an article of the toilet show the power of oxygen over the animal pigments. By its use the dark-haired Rebecca of yesterday comes forth to-morrow as the fair Imogene. But Nature, not to be outdone, proceeds to push the hair forth again, black at the roots.
Gray hair occurs when the pigment is no longer supplied. When it is lacking, the natural color of the hair is gray, not white. In white hair we have something quite different. There are in nature what are known as structural color effects; and in this class belongs a white or other hue that is produced without the use of a pigment. That there can be white without any white matter in it may be proved by melting a snowball or by allowing a lily to wilt. In the case of the snowball the thousands of little crystals totally reflect the light, producing the sensation of white upon the eye; and in the lily a multitude of bubbles of air in the integument have the same effect. In the case of human hair, if air infiltrates and takes the place of the pigment when it leaves, all the colors of light are reflected, and that is white itself. Thus the crown of white hair is a special gift. It is a vestiture of light which one possesses in common with the lily, the swan, and the foam of the sea.
When we consider the coloring of plants we find that the most delicate and gorgeous effects are attained by pigments developed from sugar. These, the anthocyanins or so-called flower pigments, include blues, pinks, magentas, and even the most gorgeous of reds. They are of great scope, from the blue of the cornflower to the red of the apple and the most flamelike and deeper reds of the maple and the oak. Despite this astonishing range of color, there is but a slight change from the molecule of the one to the molecule of the other — how slight has only recently been discovered. These anthocyanins are blue or violet except when acted upon by acid, when they are red. They are anything but yellow.
As there are but three basic colors in nature, and any hue may be mixed out of three pigments, red, blue, and yellow, it will be seen that the garden soil can achieve its great variety of work with a very limited palette. The seven colors of the rainbow are but the three primary colors lapping over and blending at the edges, the violet at one end being the blue merging into darkness.
And now in taking up yellow, the other color in the trinity, we have got to go back for a moment to the coloring matter of the trees, the grass, and all verdant vegetation. We have already said that the two valuable chlorophylls — the one that is simply green and the one that is bluish green — are not to be considered merely as color, but as an indispensable substance which is responsible for the chemistry of plant life and therefore of all life on the globe. It uses sunlight to manufacture food out of carbon in the air. These leaf colors, whether the two greens or the two yellows, do not float about in the cell sap, as do the other colors we have been considering, but are produced and kept in separate little bodies in the cells called plastids. Like the animal colors, the greens have nitrogen in them, this being of the very substance of life itself. When the two greens are gone in autumn, and stored away in the tree, the two yellows are left in the leaf. These, the pure yellow and the one that is inclined to orange, are fatty pigments. They are widely distributed in nature. In the fatty pigments or lipochromes there are also reds, purples, and blues. Since animals, like plants, are made up of proteins, fats, and sugar, we might expect that some of the pigments of the one would occur in the other; and we find that this is true especially of the fatty pigments. They occur in the yellow spot of the eye, the purple of the retina, the yellow of the yolk of eggs, the orange of the carrot, and the red of the tomato.
In trees, while the lipochromes are responsible for the yellow such as we see in the willow, the maple, and the ash, the bright and flaming reds are made out of sugar, being due to an excess of sugar that remains in the leaf when cold weather comes on. This combines with other substances; and in the oak it combines with tannin, thus producing the red effect. To the yellow hidden in the chlorophyll of the leaf we owe the yellow and gold of butter.
There is yet another kingdom of color: the sunset cloud and the blue of the noonday sky. If we would know it we must learn that light is invisible. It is invisible in the sense that if you are in a dark place, and the sun shines in, its beam cannot be seen as you look across it.
When we consider how often we have seen light coming in at a keyhole or shining through a crack in the door, this must seem like a misstatement; but it is not. If all dust is taken out of the air of a dark room, the beam of light immediately disappears. It continues to come in, but cannot be seen. If you put an object in its path, the object will be in full light, but without any indication of the light’s pathway. You may even place a glass vessel of pure distilled water before the opening and there will be no sign of the light passing through the water. But if you blow some smoke into the air, or raise a dust, the air thus filled lights up at once. And if you drop a little milk or other dispersible substance into the water, the path of the beam can be seen. What we see is the reflected light from innumerable particles in the air or water.
We learn from this that wherever there is color there is substance. This being true, it is evident that the blue sky is substance. So far as we can see it, we are seeing something material. The blue color consists of reflection from matter dispersed in fine particles throughout the atmosphere. These suspended particles are not merely in the upper air, where the blue seems to be, but all through it, with the coarser particles below. Thus the sky is at our elbow, and the blue is the total effect of all the fine matter in the air as far as it extends. It is not the atmosphere that is colored; we are seeing substance that makes itself visible by reflection, as all substance does.
The wave lengths of the various colors in white light range from about 1/50,000 of an inch for the blue to 1/30,000 of an inch for the red. The reason the sky is blue is that such very fine particles of matter catch and reflect the short waves, the blue, more than they do the red. The color of the matter in space is blue because of its state of fine division, regardless of what color it might be in the mass.
It has generally been considered by physicists that the sky above our atmosphere would appear black. The reports of balloonists seem to lend proof to this view. Piccard saw the upper air as a deep blue verging into purple. In the past year it has been argued that there must be some star dust, however attenuated, in space, and that this, being illuminated by the stars, would show a light blue. It will receive attention as a theory. But still we may hold to the old teaching that if space is empty of dust it will be black. Though we might see the stars by their light impinging directly upon the eye, all else between would be dark. Without reflection there can be no illumination and no color.
When the sun is setting, and almost at the horizon, it has to travel a much greater distance through the atmosphere in reaching the eye. The short waves of light, the blue, are scattered; and only the long waves, the red, are transmitted through so great a thickness of the laden air.
The full moon, rising, always takes us aback by its redness and its swollen proportions. And here we are confronted by an astonishing fact. It has been found, by actual measurement, that the image made upon the seeing surface of the eye, the retina, is no larger when the moon is rising than when it is high up in the sky. We only think it is larger. We unconsciously compare it for size with the distant objects to which it seems so near; and we misinterpret. In short, it is not our eyes that are lying to us — it is our mind.
What sort of color does Nature use on a humming bird? How does she paint the metallic hues on a peacock’s tail? And how does she set the rainbow on a pigeon’s breast? The answer is that there are no such colors on these birds. We see them, but they are not there. It is all a magical effect, and in order to see how it is done we must take account of a few plain facts in physics.
It is natural for us to think that two lights are brighter than one light and that two noises are louder than one noise, but this is not necessarily true. The scientist can make two lights come together in such a way that darkness is produced; and he can make one sound, as of a tuning fork, impose itself upon another equal sound in such manner that the result is silence. In other words, he can make one wave length of sound or light kill off or blot out a part of the other sound or light; and this effect, produced not merely in the laboratory but in nature itself, is known as ‘interference.’ It plays a great part in modern scientific theory; and the fact that interference occurs in connection with light is the strongest proof that light is of a wavelike nature. The coloring of birds that we are now considering, opalescent, changing, and metallic, is caused by the fact that a part of the light, on the way to the eye, is interfered with and blotted out, leaving the rest of the light to strike the eye with the colors that are left.
The effect is produced by a very thin, transparent film or coating on the surface of the feathers; and the mechanism of it is interesting.
Light moves forward in lateral undulations comparable to the waves in water. Suppose that on a smooth sheet of water two circles of waves are propagated at the same moment. If, when they approach one another and come together, wave encounters wave and hollow coincides with hollow, these waves will combine their forces into still higher waves with corresponding deep hollows between. But if the waves of one meet the hollows of the other, they will nullify one another and produce smooth water. If the weaves are a foot from crest to crest, six inches of delay will cause wave and hollow to meet; and instead of the matching of wave and wave, on top of one another, we shall have the wave upon the hollow which cancels the motion and is known as interference. Hence the rule that interference is caused by the delay, so to speak, of half a wave length, or any odd number of half wave lengths. In waves of color, as of water, complete coincidence intensifies, while the opposite effect cancels or kills off.
In the case of the transparent film, the light that strikes the upper surface is partly reflected from that point to the eye, while the rest passes on through and is reflected to the eye from the under surface of the film. The ray of light that comes from the under surface, being delayed by that extra journey, falls somewhat behind the rays that were reflected from the upper surface. This delay, being just enough to cause the waves to strike in and join the other waves of light half a wave length behind some particular color, will produce the obliteration or killing off of that color; and in consequence the white light, being robbed of part of its color, will shine with the color that is left.
But how does it happen that this journey through the film, in and out again, is just enough to produce that half wave length of delay?
It must be remembered that the rainbow colors into which light can be split do not consist merely of the seven colors to which we have given names, but also of blended colors in between. From the short waves of the blue to the long waves of the red there is really an infinity of wave lengths or imperceptibly blending hues; and so, naturally, the delay of light in going through any thin film will fit in as a half wave length somewhere along the graduated scale of color. If it does not kill off one hue or wave length, it will kill off another.
The same point on the surface of a film does not always show the same color, all depending upon the angle from which you view it. If you are looking at it directly from the front, the distance through the film is small; if you look at it less directly, the distance is greater; these varying thicknesses affect the color that comes to the eye from different positions. Since light or color shines from any point in all directions, diffusely, it creates an infinitude of conditions for the eye to encounter.
To sum up: In the peacock and such iridescent birds there is a ground color of pigment, usually dark, upon which the iridescent effect is imposed. Thus two means of coloring are combined — the pigment method and the merely physical or mechanical. The brilliant hues that attract our attention are not due to the pigment, but purely to the interference of light waves.
This may seem a great deal of explanation to bring to bear upon a dove’s breast; but knowledge is knowledge. If one studies the Einstein theory, he will find that it took its origin in the Michelson-Morley experiment, in Cleveland, Ohio. This experiment, to determine whether there is really a stationary ether in space, was based upon this phenomenon of the interference of light. And so a little understanding of how light waves may interfere with one another may be of use to one when he puts into action that long-delayed intention of finding out just what the Einstein theory is all about.
If one wishes to contemplate the effect of ‘interference,’ let him look upon the play of color in a pearl, whose beauty is all due to the fact that it is built up in fine layers which have the effect of transparent film; or let him regard the delightful hues of the opal, a jewel that plays with light because it too is structurally made up of layers of substance and of air. And if one has not a pearl, an opal, a peacock, or a humming bird, let him spill a layer of colorless kerosene on a surface of water and take account of its iridescence in the sun. Or a soap bubble will do.
One may see nature from the point of view of the artist or that of the scientist. Certainly there can be no interference between the two. ‘Beauty is truth, truth beauty.’ And sometimes I have wondered whether Keats did not say even more than he was aware of when he produced that line.
One white-clouded day in summer the evening came on very quietly. The lake lost all motion and became a mirror. The landscape was like a picture. After watching it awhile my wife and I went down the bank and along the wooded shore to where our canoe was lying; and then, with a stroke of the paddles, we floated off into the midst of it.
Some loons and mud hens moved along in the distance with V-shaped wakes behind them; and we made a like mark with our canoe. The trees along the shore were reflected in every verdant detail. The hill across the lake stood upside down in the water. The woods and fields on the hill could all be seen plainly below; and when a girl made her appearance on a high slope in a white dress we first became aware of her presence in the water. The clouds, now turning pink, were reflected too; and the checkered loon was redoubled in the water.
As the sun verged toward the horizon the clouds became pinker and some deepened into red. My wife and I began to pay more attention to the scene below. The sky became more pink and fiery, and the pink reached out more toward the zenith. Gradually it wiped out all other reflections by the mere assertion of color; and thus it claimed the whole depth and surface of the lake.
As we looked into this nether world of clouds the scene became deeper and deeper, and yet more deep. We had started out on a good substantial surface of water, and now we were suspended above all this, a colorful heaven below us. Far down we could see roseate balconies hanging in space. There was a red tumult of billowy shapes for us to gaze at; and then, as the sun went lower, a spectacle of fiery crags and an abyss of fearful conflagrations. Instead of floating on a daylight surface, our smooth supporting medium seemed to have vanished. And, as we now took glances below, the dizzy scene seemed more and more fearful.
‘Shall we go back home?‘ I asked her.
‘Oh, yes. Let us get out of this,’ she answered.
When you turn your back on a spectre it does not help matters any, especially when you can still see the thing you are fleeing from.
After a quarter of a mile we reached the point of our peninsula and skimmed round on the shady side and thus to shore. And I, for my part, was glad to be there. ‘Long heath, brown furze, anything ’ — I was glad of a good opaque place whereon to stand.
That was many years ago, and the experience still keeps a place in our minds. Of all adventures by land or water, one of the tightest squeaks we ever had was our escape from that sunset.