THE SKY AND ITS COLOR.
In the chapters on light in Vol. II. it will be stated that we see all objects by a reflected light, except those that are self-luminous, such as the sun or any other source of light. We see the moon and many of the planets entirely by reflection. There are myriads of smaller objects, too small to be seen as such, even under a microscope, that still have a power to reflect light that is sensible to our vision. The air surrounding the globe is literally filled with these microscopic light reflectors. They serve to give us a diffused light which enables us to see clearly all visible objects. We have all noticed the effect of a single electric arc light, situated at a distance from any other source of light, and how it casts extremely dark shadows and very high lights; so much so that it is difficult to see an object perfectly in this light, because the part of an object that is under the direct rays of the lamp is so highly illuminated that the shadow, by comparison, has the effect of simply a dark blot without form or shape. Many of you have noticed in a country village, where the streets are lighted with electric arc lamps, what a difference there is in the illuminating effect between a clear and a foggy night. When there is a fog, or when the clouds hang low down, we get a reflection from these which tends to diffuse and soften the powerful light rays that are sent out by these lamps. This effect is especially noticeable when the night is only moderately foggy. Each globule of moisture floating in the air becomes a reflector of light, and by myriads of reflections and counter reflections the light (which on a clear night is concentrated) is diffused over a large area, producing an illumination which for practical purposes is far superior to that produced on a clear night. When the latter condition prevails the rays of light are so intense on objects immediately surrounding the lamps that one is blinded; so that the places which are in shadow seem darker than they would be if there were no light at all. The only way to prevent this effect is to have the lights so close together that there will be cross lights, which tend to break up the intensity of the shadows. This principle of light diffusion is taken advantage of to produce an even illumination in stores that are lighted only on one or two sides. This is effected by a series of prisms or reflecting surfaces that are cast upon the panes of glass.
If now there were no atmosphere—or, to state it differently—if there were no floating substances in the atmosphere, the sun would produce an effect upon the earth similar to that of a single electric light. The lights would be extremely high, and the shadows extremely dense. To one looking off into space, the sky, instead of having the blue appearance that we see, would have the effect of looking into a deep, dark abyss without illumination.
Tyndall has shown us by a beautiful experiment that if there be in a glass tube a mixture of gases related to each other in a certain way chemically, they will combine into small globules or particles similar to moisture in the air. If now a beam of light is thrown upon this tube and a dark screen put behind it, we shall, in the beginning of the experiment, simply see the dark screen. As soon, however, as the molecules of the gases have combined in sufficient numbers to produce particles of sensible size we begin to have a reflection of light from them, the color of which is constantly changing as the combining particles grow in size. At a certain stage in its progress the color which the mixture of gases assumes is a beautiful azure blue, rivaling in purity the finest skies of Greece or southern Italy.
The sun is the great lamp that illuminates the world, while the atmosphere, which is filled with particles of various substances, becomes the shade of the lamp which diffuses and softens the light and gives it its color tones, whether of warmth or coldness. We could not well do without the reflected light of the sky. The poetry of life would be sadly marred. The beautiful effects of color and purity of tone would be wanting. We need to bathe in light as much as in water, and the character of the light is almost as important as the character of the water. Imagine a world with an atmosphere devoid of all substances that would in any way reflect light or give to it softness or color tone. Imagine a sun or a moon without visible rays—for without a reflecting atmosphere there would be none. Imagine a sky that was no sky at all, but only a dark void, with no protecting vault. Think of the shadows, so dark that you could see nothing in them. These would be some of the effects that would come from an atmosphere that had no sky substance in it. Imagine the world lighted by one great arc light. The reflex action upon the race living in such a light would be anything but desirable. The world would develop into an arc-light civilization—if one can imagine what that would be like; certainly one of intensely violent contrasts. Look on this picture and let us be thankful for the blue sky and golden sunsets.
"But," you ask, "why is the sky blue?"
In one of the chapters on the subject of light in Vol. II. the properties of soap bubbles are discussed. It is shown that when a film is stretched across the mouth of a tumbler held in a position so that the film is perpendicular, by the action of gravity (the moisture constantly falling to the lower part of the film) it will continually grow thinner, and horizontal bands of color will appear upon it,—first red, then followed by the other colors of the solar spectrum, ending with violet.
It is also stated that every color of light has a definite wave length. Where a band of blue color appears upon the film we know that its thickness is right for the wave length of that particular color which is reflected from the back of the film to the eye. If we could conceive the blue vault of the heavens to be half a sphere of a soap bubble, the color that the sky would appear to us (if the light could be thrown upon it from beneath) would be determined by the thickness of this film. If the film was 1-156,000 of an inch the sky would be red instead of blue. To reflect the other colors the film would have to grow thinner for each color, in the progression from red to violet. The color of the sky is determined by a light-reflection from minute globules of moisture floating in the air. If the sky is blue, then the globules must be of the right diameter to reflect that color. The various tints and colorings of the sky are determined by what is found in the atmosphere, and this is the reason why skies differ in coloring and tone in different sections of the globe. The finest skies are probably found in semi-tropical regions like southern Italy, Greece, and California.
In 1892 I visited Greece in the early part of June. In crossing the Adriatic, from Brindisi to Patras in Greece, the route was through the Ionian Islands that are grouped along the southwestern shore of Albania. The sky was without a cloud, and its beautiful blue color was reflected in the waters of the Adriatic, and I never shall forget the impression made upon my senses when we first came in sight of the mountains on the west coast of Albania. At this point they rise abruptly from the water and are colored with that peculiar azure haze, mixed with a shading of warmth, which is an effect that distance gives in the classic atmosphere of old Greece. The effect upon the beholder is to intoxicate the senses and to fill him with that deliciously poetic feeling that always comes when standing in the presence of the sublime in nature. It was not the mountains themselves that produced the effect, for I had seen grander than these; but it was the sky on the mountains. When we look at a distant mountain it seems to be partly hidden by a peculiar haze that is the color of the sky at that time; we are really looking at the mountain through a portion of the sky. While in Athens I took a trip to the top of Mount Pentelicus, which separates the plains of Athens on the south from those of Marathon on the north. From the summit of this mountain we have a most wonderful view of the archipelago of the Ægean Sea—a beautiful map of blue water and brown islands that melt together in the distance. At our feet lay the historic plains of Marathon, and in the distance rose the snow-capped peaks of Mount Olympus. It is doubtful if the world furnishes a more beautiful combination of ocean, island, continent, and sky than can be seen from Mount Pentelicus. Myriads of brown islands set in the bluest of water—graceful in outline and multiform in shape—jutting headlands and land-locked harbors—strong in color and outline in the immediate foreground, but gradually melting together in the distance, the brown becoming bluer and the blue a softer blue till the whole is lost on the horizon in a sky that shades back to the zenith in an ever-changing azure that for purity of tone baffles all description.
What wonder that a people born under such skies and whose eyes have feasted on such beauties in nature should conceive and execute such a masterful work of art as the Parthenon! While the variation of landscape, the stretch of water filled with islands, and the mountains capped with eternal snow were a prominent part of the picture, it was the sky with its beautiful color-tones that after all gave it its wonderful charm.
The skies in a northern latitude are colder and grayer, due to the fact that nearly always there is a certain degree of condensation of moisture existing, which, while it does not take the form of a cloud, still gives a toning to the sky.
There is no doubt but that the color-tones of the sky have an influence upon the character and temperament of the people who live under them. Under semi-tropical skies the poetic nature is more strongly appealed to, and a man is more likely to be controlled by his dreamy imaginings than his cold calculations. We find this latter characteristic prevailing to a greater or less extent among the people who live under colder and sterner skies. If all these qualities or influences could be combined in the right way, the race would be stronger intellectually and in other ways. It is always dangerous to a race of people to be developed along certain lines only. The development should be symmetrical. The strongest men are not those who are simply coldly intellectual, neither those who are simply emotional and sentimental, but those in whom heart, mind, and soul are so related that each one of these elements re-enforces and strengthens the others.
At certain seasons of the year and in certain localities it is not uncommon to have wonderfully beautiful displays of coloring upon the skies and clouds at sunset. The question is often asked why we do not see these displays at other times in the day than at sunrise and at sunset—for the same effects are seen in the morning, but they are not noticed so often, because to do so would interfere with the habits of the average man and woman.
The reason for this change of coloring is the angle at which the sun's rays strike the clouds of an evening sky, which are reflected to our eyes. When the sun is high in the heavens it shines against the back of the clouds, from the point of view of a person standing on the surface of the earth. It also shines a shorter distance through the air at midday than at sunset. At sunset the rays are able to shine on the under side of a cloud, especially if it is high in the air. The moisture globules of which the cloud is made up are much larger than the transparent ones that are uncondensed and just as they were when released in the process of evaporation.
As we have already seen, the reflections from these minute globules give us the blue coloring of the sky and are very much smaller in diameter than a globule that is able to reflect the red ray. When these small globules are condensed into cloud a great number are combined into one globule, and they are of all sizes, from the globule of evaporation to that of the raindrop when precipitation takes place. We have, then, in the various stages of cloud formation all conditions present for reflecting the various colors and combinations of colors that are found in the solar spectrum. Hence it is that, under certain conditions of atmosphere and cloud formation, we see at sunset painted upon the sky those wonderful combinations of colors, more beautiful and delicate in shading, more various in combination and purer of tone, than any artist, however cunning his fingers or brilliant his pigments, has ever been able to truthfully reproduce. Even when the sky is cloudless it often assumes a brilliant hue, which is partly a reflection from invisible moisture globules and partly due to floating particles of dust that may have been driven up from the surface of the earth, or may be the ashes of meteorites disintegrated by contact with the air.
Some years ago, commencing in August, 1883, there was a wonderful exhibition of red skies at sunset that lasted for several hours after twilight ordinarily disappears. This phenomenon ran through a period of several weeks, gradually fading away. It was afterward determined that these displays were occasioned by small particles of ashes or dust floating high in the air, that were thrown off from the volcanic eruption of Krakatoa in the Island of Java. By the general circulation of the air the ashes were carried to all parts of the world, making a circuit of the earth in from twelve to thirteen days—which showed a velocity of over eighty miles an hour. This is an instance of the high velocity of the air currents in the upper regions of the atmosphere. The reason why the illumination extended so late in the night was because of the great height that these particles of dust attained. The higher the reflecting surfaces are in the air the longer they may be seen after sunset. Ordinary twilight is caused by a reflection of sunlight from the upper air; and from its duration as ordinarily observed it is estimated that the reflection does not proceed from a point more than thirty-six miles high. In the higher latitudes the twilight is long, from the fact that the sun does not go directly down, and if we go far enough north the whole night is twilight. In the tropical regions the twilight is shorter than at any other point on the globe for reasons that are obvious. The sun there goes directly down and is soon hidden behind the earth.
There are other optical effects to be seen sometimes on the horizon somewhat resembling twilight. The "aurora borealis" (northern lights), which we describe in Vol. III., is seen in the northern skies at certain times, and has very much the appearance of twilight in some of its phases. It is constantly changing, however, and is easily distinguished by anyone who has observed both. These appearances are undoubtedly electrical. There is another phenomenon seen in the arctic regions that causes a band of white light to appear on the horizon called "ice blink," and it is caused by the reflections from the great icebergs that abound in that region.
Curious optical effects are sometimes observed a little after sunset in the form of streamers or bands of light that shoot up into the sky, sometimes to a great height. These are undoubtedly due to cloud obstructions that partially shut off the sun's rays from a part of the sky, but allow it to shine with greater brilliancy in the path of these bands of light.
It will be seen from the foregoing that the sky in all of its phases is a product of sunlight and the substances that float in the air, including moisture, not only in the invisible state, but in all the stages of condensation, as well as particles of floating dust.