Harper's New Monthly Magazine, vol. 3, no. 18, November, 1851

by Various Harper

It is not our design just now to talk about the nature of the atmosphere; to attempt any analysis of light, or even to mention its recondite mysteries. But in a plain way we propose to look into the reason of those changes made by light in the appearance of the sky, those every-day sights with which we are the most familiar.

Blue sky itself, for example. Why is the sky blue? To explain that, we must state a few preliminary facts concerning light, and beg pardon of any one whose wisdom may be outraged by the elementary character of our information. There are some among our readers, no doubt, who may find it useful. In the first place, then, we will begin with the erection of a pole upon a play-ground, and, like boys and girls, we will go out to play about it with an india-rubber ball. The pole being planted upright, is said to be planted at right angles to the surface of the ground. Now, if we climb the pole, and throw our ball down in the same line with it, it will run down the pole and strike the ground, and then jump back again by the same road into our fingers. The bouncing back is called in scientific phrase, Reflection; and so we may declare about our ball, that if it strike a plane surface at right angles, it is reflected immediately back upon the line it went by, or, as scientific people say, "the line of incidence." Now, let us walk off, and mount a wall at a short distance from the pole. We throw our ball so that it strikes the ground quite close to the spot at which the pole is planted in the earth, and we observe that the said ball no longer returns into our hand, but flies up without deviating to the right or left (in the same plane, says Science) beyond the pole, with exactly the same inclination toward the pole on one side, and the surface of the ground on the other, as we gave it when we sent it down. So if there were a wall on the other side of our pole, exactly as distant and as high as our own, and somebody should sit thereon directly opposite to us, the ball would shoot down from our fingers to the root of the pole, and then up from the pole into his hand. Spread a string on each side along the course the ball has taken, from wall to pole, and from pole to wall. The string on each side will make with the pole an equal angle: the angle to the pole, by which the ball went, is called, we said, the angle of incidence; the angle from the pole, by which it bounced off, is called the angle of reflection. Now, it is true not only of balls, but of all things that are reflected; of light, for example, reflected from a looking-glass, or a sheet of water, that "the angle of reflection is equal to the angle of incidence."

The light that shines back to us from a sheet of water, has not penetrated through its substance, certainly. But now, let us be Tritons, or sea-nymphs, and let us live in a cool crystal grot under the waves. We don't live in the dark, unless we be unmitigated deep-sea Tritons. The deeper we go, the darker we find it. Why? Now, let us be absurd, and suppose that it is possible for light to be measured by the bushel. Ten bushels of light are poured down from the sun upon a certain bit of water; six of these, we will say, reflected from its surface, cause the glittering appearance, which is nothing to us Tritons down below. But light can pass through water; that is to say, water is a transparent substance; so the other four bushels soak down to illuminate the fishes. But this light, so soaking down, is by the water (and would be by any other transparent substance) absorbed, altered, partly converted into heat—when we understand exactly what Mr. Grove calls the Correlation of Physical Forces, we shall understand the why and how—we only know just now the fact, that all transparent bodies do absorb and use up light; so that the quantity of light which entered at the surface of our water suffers robbery, becoming less and less as if sinks lower down toward our coral caves.

Furthermore, beside reflection and absorption, there is one more thing that light suffers; and that we must understand before we can know properly why skies are blue, and stars are twinkling. That one thing more is called Refraction. A horse trots fairly over the stones, but slips the moment stones end, and he comes upon wood pavement. A ray of light travels straight as a dancing-master's back, so long as it is in air, or water, or glass, or any other "medium," as the books say, of a certain unvarying thinness or thickness, fineness or coarseness, or according to the school-word "density." But if a ray that has been traveling through warm and light air, suddenly plunges into air cold and heavy, it is put out of the way by such a circumstance, and in the moment of making such a change, it alters its direction. Still more, a ray of light that has been traveling in a straight line through air, is put out of its course on entering the denser medium of water; it is dislocated, refracted very much, alters its course, and then continues in a straight line on the new course, so long as the new medium continues. In the same way, a ray of light which travels through a medium that becomes denser and denser very gradually would be perpetually swerving from its straight path, and would travel on a curve. Our atmosphere is heaviest upon the surface of the earth, and becomes lighter and thinner as we rise; the ray, therefore, from a star comes to us after traveling in such a curve. But we see all objects in the direction of a perfectly straight line continued in the direction which the rays sent from them took at the moment of falling upon our sense of sight. Therefore we see all stars in a part of the heavens where they really are not; we see the sun before it really rises. Light entering a denser medium is refracted from, entering a lighter medium is refracted toward, a line drawn at right angles to its surface. Light entering a new medium at right angles—that is to say, not aslant—continues its own course unaltered.

There is but one more fact necessary to fill up the small measure of preliminary knowledge necessary for a general understanding of the phenomena produced by the mixing of light with air. Light in its perfect state is white, but the white light is a compound of other rays in due proportion, each ray being different in color and different in quality. So it takes place, because their qualities are different, that grass reflects the green ray and absorbs the rest, and therefore grass is green; while orange-peel reflects another ray, and swallows up the green and all the rest. These colors being in the light, not in the substance colored; in a dark room it is not merely a fact that we can not see red curtains and pictures; but the curtains really are not red, the paintings have no color in them, till the morning come, and artfully constructed surfaces once more in a fixed manner decompose the light. Beside the color of these rays, from which light is compounded, there are combined with them other subtle principles which act mysteriously upon matter. Upon the hard surface of a pebble there are changes that take place whenever a cloud floats before the sun. Never mind that now. The colored rays of which pure white light is compounded are usually said to be seven—Violet, Indigo, Blue, Green, Yellow, Orange, Red; and they may be technically remembered in their proper order by combining their initials into the barbarous word Vibgyor. These are called prismatic colors, because they were first separated by the passing of a ray of pure light through a prism. In that passage light is much refracted, and it happens that the contained rays all disagree with one another as to the extent to which they suffer themselves to be put out by a change of medium. Violet refracts most, and red least; the others stand between in the order in which they have just been named, the order in which you see them in the rainbow. So the rays after refraction come out in a state of dissension; all the rays—made refractory—having agreed to separate, because they are not of one mind, but of seven minds, about the degree to which they should be put out by the trouble they have gone through.

Now we have settled our preliminaries, we have got our principles; the next thing is to put them into practice. Let us first note what has been said of the absorption of light by transparent bodies. The air is one of the most transparent bodies known. On a clear day—when vapor (that is not air) does not mingle with our atmosphere—mechanical obstacles and the earth's figure form the only limits to our vision. You may see Cologne Cathedral from a mountain distant nearly sixty miles. Nevertheless, if the atmosphere had no absorbing power, only direct rays of the sun, or rays reflected from the substances about us, would be visible; the sky would be black, not blue; and sunset would abruptly pitch us into perfect night. The air, however, absorbs light, which becomes intermixed with its whole substance. Hold up your head, open your eyes widely, and stare at the noonday sun. You will soon shut your eyes and turn your head away; look at him in the evening or in the morning, and he will not blind you. Why? Remembering the earth to be a globe surrounded by an atmosphere, you will perceive that the sun's rays at noonday have to penetrate the simple thickness of the atmosphere, measured in a straight line upward from the earth; but in the evening or morning its beams fall aslant, and have to slip through a great deal of air before they reach us; suffering, therefore, a great deal of robbery; that is to say, having much light absorbed.

Now, why is the sky blue? Not only does the air absorb light; it reflects it also. The particles of air reflect, however, most especially the blue ray, while they let the red and his companions slip by. This constant reflection of the blue ray causes the whole air to appear blue; but what else does it cause? Let us consider. If air reflects or turns aside, or hustles out of its place the blue ray, suffering the rest to pass, it follows as a consequence that the more air a ray of light encounters, the more blue will it lose. The sun's rays in the morning and the evening falling aslant, as we have said, across a great breadth of our atmosphere, must lose their blue light to a terrible extent, and very likely reach us with the blue all gone, and red lord paramount. But so, in truth, the case is; and the same fact which explains the blueness of the atmosphere, explains the redness of the sunrise and the sunset. It will now easily be understood, also, why the blue color of the sky is deepest in the zenith, faintest when we look over the horizon; why the blue is at noon deeper than after mid-day; why it grows more intense as we ascend to higher elevations. From what we have already said, the reason of these things will come out with a very little thought. Again, in the example of our London fogs, &c., when in the upper portion of the dense mass the blue rays have been all refracted, there can penetrate only those other rays which make the lurid sky, with which we are familiar, or the genuine old yellow fog. Fog in moderation, the thin vapor on the open sea, and so forth, simply gives a lightness to the blue tint, or more plentiful, an absolute whiteness to the atmosphere.

Now let us see whether we are yet able to make out the philosophy of a fine autumn sunset. As the sun comes near the horizon, he and the air about him become red, because the light from that direction has been robbed of the blue rays in traversing horizontally so large a portion of the atmosphere. The sky in the zenith pales, for it has little but the absorbed or diffused light to exist upon. Presently, we see a redness in the east, quite opposite to the sun, and this redness increases till the sun sinks from our sight. In this case, the last rays of the sun that traverse the whole breadth of the atmosphere, reflected from the east, from vapors there, and more especially from clouds, come red to our eyes; no blue can be remaining in them. From the west, where the sun is setting, the rays come from the surrounding air, and from the clouds, variously colored; they lose their blue, but there remain the red, green, orange, yellow, and the purple rays; and some or all of these may make the tints that come to us, according to the state and nature of the clouds, the atmosphere, and other circumstances that may modify the process of refraction. The sun has set; it is immediately below the horizon, and its rays still dart through all our atmosphere, except that portion which is shielded from them by the intervening shadow of the earth. That shadow appears in the east, soon after sunset, in the shape of a calm blue arch, which rises gradually in the sky, immediately opposite to the part glorified by sunset colors. Over this arch the sky is red, with the rays not shut out by the round shadow of our ball. As the sun sinks, our shadow of course rises; and within it there can be only the diffused twilight, always blue. When this arch—this shadow of the earth—has risen almost to the zenith, and the sun is at some distance below the horizon, then the red color in the west becomes much more distinct and vivid; for the sun then shoots up thither its rays through a still larger quantity of intervening atmosphere; so that the redness grows as the sun sinks, until the shadow of the earth has covered all, and the stars—of which the brightest soon were visible—grow numerous upon the vault of heaven. When stars of the sixth magnitude are visible, then, astronomically speaking, twilight ends. The length of twilight will depend upon the number of rays of light that are reflected and dispersed, and that, again, will depend entirely on the atmosphere. Where there is much vapor, and the days are dull by reason of the quantity of kidnapped light, there compensation is made by the consequent increase of twilight. In the interior of Africa night follows immediately upon sunset. In summer the vapor rises to a great height, and pervades the atmosphere; the twilight then is longer than in winter, when the colder air contains less vapor, and the vapor it contains lies low.

Now, since the appearances at twilight depend on the condition of the sky, it follows that our weather-wisdom, drawn from such appearances, is based upon a philosophical foundation. When there is a blue sky, and after sunset a slight purple in the west, we have reason for expecting fine weather. After rain, detached clouds, colored red and tolerably bright, may rejoice those who anticipate a pic-nic party. If the twilight show a partiality for whitish yellow in its dress, we say that very likely there will be some rain next day; the more that whitish yellow spreads over the sky, the more the chance of water out of it. When the sun is brilliantly white, and sets in a white light, we think of storms; especially so when light high clouds that dull the whole sky become deeper near the horizon. When the color of the twilight is a grayish red, with portions of deep red passing into gray that hide the sun, then be prepared, we say, for wind and rain. The morning signs are different. When it is very red, we expect rain; a gray dawn means fine weather. The difference between a gray dawn and a gray twilight is this—in the morning, grayness depends usually upon low clouds, which melt before the rising sun; but in the evening grayness is caused by high clouds, which continue to grow denser through the night. But if in the morning there be so much vapor as to make a red dawn, it is most probable that thick clouds will be formed out of it in the course of the operations of the coming day.

Refraction of light has a good deal to do also with the twinkling of the stars; though there may go to the explanation of the phenomenon other principles which do not concern our present purpose. The air contains layers of different density, shifting over each other in currents. The fixed stars are, to our eyes, brilliant points of light; their rays broken in passing through these currents, exhibit an agitation which is not shown by the planets. The planets are not points to our sight, nor points to our telescopes; being much nearer, although really smaller, they are to our eyes of a decided, measurable size; so being in greater body, we at most could only see their edges scintillate; and this we can do sometimes through a telescope, but scarcely with the naked eye.