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The Sky

( Originally Published 1905 )

INVITED to write for the "Forum" an article that would have brought me face to face with "problems of life and mind," for which I was at the moment unprepared, and unwilling to decline a request so courteously made, I offered, if the editor cared to accept it, to send him a contribution on the subject here presented.

I mentioned this subject, thinking that, in addition to its interest as a fragment of "natural knowledge," it might permit of a glance at the workings of the scientific mind when engaged on the deeper problems which come before it. In the house of Science are many mansions, occupied by tenants of diverse kinds. Some of them execute with painstaking fidelity the useful work of observation, recording from day to day the aspects of Nature or the indications of instruments devised to reveal her ways. Others there are who add to this capacity for observation a power over the language of experiment, by means of which they put questions to Nature, and receive from her intelligible replies. There is, again, a third class of minds that can-not rest content with observation and experiment, whose love of causal unity tempts them perpetually to break through the limitations of the senses, and to seek beyond them the roots and reasons of the phenomena which the observer and experimenter record. To such spirits—adventurous and firm—we are indebted for our deeper knowledge of the methods by which the physical universe is ordered and ruled.

In his efforts to cross the common bourne of the known and the unknown, the effective force of the man of science must depend, to a great extent, upon his acquired knowledge. But knowledge alone will not do; a stored memory will not suffice; inspiration must lend its aid. Scientific inspiration, however, is usually, if not always, the fruit of long reflection—of patiently "intending the mind," as Newton phrased it; and as Copernicus, Newton, and Dar-win practiced it; until outer darkness yields a glimmer, which in due time opens out into perfect intellectual day. From some of his expressions it might be inferred that Newton scorned hypotheses; but he allowed them, never the less, an open avenue to his own mind. He propounded the famous corpuscular theory of light, illustrating it and defending it with a skill, power, and fascination which subsequently won for it ardent supporters among the best intellects of the world. This theory, moreover, was weighted with a supplementary hypothesis, which ascribed to the luminiferous molecules "fits of easy reflection and transmission," in virtue of which they were some-times repelled from the surfaces of bodies and sometimes permitted to pass through. Newton may have scorned the levity with which hypotheses are sometimes framed; but he lived in an atmosphere of theory, which he, like all profound scientific thinkers, found to be the very breath of his intellectual life.

The theorist takes his conceptions from the world of fact, and refines and alters them to suit his needs. The sensation of sound was known to be produced by aerial waves impinging on the auditory nerve. Air being a thing that could be felt, and its vibrations, by suitable treatment, made manifest to the eye, there was here, a physical basis for the "scientific imagination" to build upon. Both Hooke and Huyghens built upon it with effect. By the illustrious astronomer last named the conception of waves was definitely transplanted from its terrestrial birthplace to a universal medium whose undulations could only be intellectually discerned. Huyghens did not establish the undulatory theory, but he took the first firm step toward establishing it. Laying this theory at the root of the phenomena of light, he went a good way toward showing that these phenomena are the necessary outgrowth of the conception.

By analysis and synthesis Newton proved the white light of the sun to be a skein of many colors. The cause of color was a question which immediately occupied his thoughts; and here, as in other cases, he freely resorted to hypothesis. He saw, with his mind's eye, his luminiferous corpuscles crossing the bodily eye, and imparting successive shocks to the retina behind. To differences of "bigness" in the light-awakening molecules Newton ascribed the different color-sensations. In the undulatory theory we are also confronted with the question of color; and here again, to inform and guide us, we have the analogy of sound. Aerial waves of different lengths, or periods, produce notes of different pitch; and to differences of wave-length in that mysterious medium, the all-pervading ether, differences of color are ascribed. Hooke had already discoursed of "a very quick motion that causes light, as well as a more robust that causes heat. New-ton had ascribed the sensation of red to the shock of his grossest, and that of violet to the shock of his finest, luminiferous projectiles. Defining the one, and displacing the other of these notions, the wave-theory affirms red to be produced by the largest, and violet by the smallest waves of the visible spectrum. The theory of undulation had to encounter that fierce struggle for existence which all great changes of doctrine, scientific or otherwise, have had to endure. Mighty intellects, following the mightiest of them all, were arrayed against it. But the more it was discussed the more it grew in strength and favor, until it finally sup-planted its formidable rival. No competent scientific man at the present day accepts the theory of emission, or refuses to accept the theory of undulation.

Boyle and Hooke had been fruitful experimenters on those beautiful iridescences known as the "colors of thin plates." The rich hues of the thin-blown soap-bubble, of oil floating on water, and of the thin layer of oxide on molten lead, are familiar illustrations of these iris colors. Hooke showed that all transparent films, if only thin enough, displayed such colors; and he proved that the particular color displayed depended upon' the thickness of the film. Passing from solid and liquid films to films of air, he says: "Take two small pieces of ground and polished looking-glass plate, each about the bigness of a shilling; take these two dry, and with your forefingers and thumbs press them very hard and close together, and you shall find that, when they approach each other very near, there will appear several irises or colored lines." Newton, bent on knowing the exact relation between the thickness of the film and the color it produced, varied Hooke's experiment. Taking two pieces of glass, the one plane and the other very slightly curved, and pressing both together, he obtained a film of air of gradually in-creasing thickness from the place of contact outward. As he expected, he found the place of contact surrounded by a series of colored circles, still known all over the world as "Newton's rings." The colors of his first circle, which immediately surrounded a black central spot, Newton called "colors of the first order" ; the colors of the second circle, "colors of the second order," and so on. With unrivalled penetration and apparent success, he applied his theory of "fits" to the explanation of the "rings." Here, however, the only immortal parts of his labors are his facts and measurements; his theory has disappeared. It was reserved for the illustrious Thomas Young, a man of intellectual calibre resembling that of Newton himself, to prove that the rings were produced by the mutual action—in technical phrase, "interference" —of the light-waves reflected at the two surfaces of the film of air enclosed between the plane and convex glasses. The colors of thin plates were "residual colors"—survivals of the white light after the ravages of interference. Young soon translated the theory of "fits" into that of "waves"; the measurements pertaining to the former being so accurate as to render them immediately available for the purposes of the latter.

It is here that Newton's researches and opinions touch the subject of this article. The color nearest to the black spot, in the experiment above described, was a faint blue —"blue of the first order corresponding to the film of air when thinnest. If a solid or liquid film, of the thickness requisite to produce this color, were broken into bits and scattered in the air, Newton inferred that the tiny fragments would display the blue color. Tantamount to this, he considered, was the action of minute water-particles in the incipient stage of their condensation from aqueous vapor. Such particles suspended in our atmosphere ought, he supposed, to generate the serenest skies. Newton does not appear to have bestowed much thought upon this subject; for to produce the particular blue which he regarded as sky-blue, thin plates with parallel surfaces would be required. The notion that cloud-particles are hollow spheres, or vesicles, is prevalent on the Continent, but it never made any way among the scientific men of England. De Saussure thought that he had actually seen the cloud-vesicles, and Faraday, as I learned from himself, believed that he had once confirmed the observation of the illustrious Alpine traveller. During my long acquaintance with the atmosphere of the Alps I have often sought for these aqueous bladders, but have never been able to find them. Clausius once published a profound essay on the colors of the sky. The assumption of small water drops, he proved, would lead to optical consequences entirely at variance with facts. For a time, therefore, he closed with the idea of vesicles, and endeavored to deduce from them the blue of the firmament and the morning and evening red.

It is not, however, necessary to invoke the blue of the first order to explain the color of the sky; nor is it necessary to impose upon condensing vapor the difficult, if not impossible, task of forming bladders, when it passes into the liquid condition. Let us examine the subject. Eaude-Cologne is prepared by dissolving aromatic gums or resins in alcohol. Dropped into water, the scented liquid immediately produces a white cloudiness, due to the precipitation of the substances previously held in solution. The solid particles are, however, comparatively gross; but, by diminishing the quantity of the dissolved gum, the precipitate may be made to consist of extremely minute particles. Brucke, - for example, dissolved gum-mastic, in certain proportions, in alcohol, and carefully dropping his solution into a beaker of water, kept briskly stirred, he was able to reduce the precipitate to an extremely fine state of division. The particles of mastic can by no means be imagined as forming bladders. Still, against a dark ground—black velvet, for example—the water that contains them shows a distinctly blue color. The bluish color of many liquids is produced in a similar manner. Thin milk is an example. Blue eyes are also said to be simply turbid media. The rocks over which glaciers pass are finely ground and pulverized by the ice, or the stony emery imbedded in it; and the river which issues from the snout of every glacier is laden with suspended matter. When such glacier water is placed in a tall glass jar, and the heavier particles are permitted to subside, the liquid column, when viewed against a dark background, has a decidedly bluish tinge. The exceptional blueness of the Lake of Geneva, which is fed with glacier water, may be due, in part, to particles small enough to remain suspended long after their larger and heavier companions have sunk to the bottom of the lake.

We need not, however, resort to water for the production of the color. We can liberate, in air, particles of a size capable of producing a blue as deep and pure as the azure of the firmament. In fact, artificial skies may be thus generated, which prove their brotherhood with the natural sky by exhibiting all its phenomena. There are certain chemical compounds—aggregates of molecules-the constituent atoms of which are readily shaken asunder by the impact of special waves of light. Probably, if not certainly, the atoms and the waves are so related to each other, as regards vibrating period, that the wave-motion can accumulate until it becomes disruptive. A great number of substances might be mentioned whose vapors, when mixed with air and subjected to the action of a solar or an electric beam, are thus decomposed, the products of decomposition hanging as liquid or solid particles in the beam which generates them. And here I must appeal to the inner vision already spoken of. Remembering the different sizes of the waves of light, it is not difficult to see that our minute particles are larger with respect to some waves than to others. In the case of water, for example, a pebble will intercept and reflect a larger fractional part of a ripple than of a larger wave. We have now to imagine light-undulations of different dimensions, but all exceedingly minute, passing through air laden with extremely small particles. It is plain that such particles, though scattering portions of all the waves, will exert their most conspicuous action upon the smallest ones; and that the color-sensation answering to the smallest waves—in other words, the color line—will be predominant in the scattered light. This harmonizes perfectly with what' we observe in the firmament. The sky is blue, but the blue is not pure. On looking at the sky through a spectroscope, we observe all the colors of the spectrum; blue is merely the predominant color. By means of our artificial skies we can take, as it were, the firmament in our hands and examine it at our leisure. Like the natural sky, the artificial one shows all the colors of the spectrum, but blue in excess. Mixing very small quantities of vapor with air, and bringing the decomposing luminous beam into action, we produce particles too small to shed any sensible light, but which may, and doubtless do, exert an action on the ultra-violet waves of the spectrum. We can watch these particles, or rather the space they occupy, till they grow to a size able to yield the firmamental azure. As the particles grow larger under the continued action of the light, the azure becomes less deep; while later on a milkiness, such as we often observe in nature, takes the place of the purer blue. Finally the particles become large enough to reflect all the light-waves, and then the suspended "actinic cloud" diffuses white light.

It must occur to the reader that even in the absence of definite clouds there are considerable variations in the hue of the firmament. Everybody knows, moreover, that as the sky bends toward the horizon, the purer blue is impaired. To measure the intensity of the color De Saussure invented a cyanometer, and Humboldt has given us a mathematical formula to express the diminution of the blue, in arcs drawn east and west from the zenith down-ward. This diminution is a natural consequence of the predominance of coarser particles in the lower regions of the atmosphere. Were the particles which produce the purer celestial vault all swept away, we should, unless helped by what has been called "cosmic dust," look into the blackness of celestial space. And were the whole atmosphere abolished along with its suspended matter, we should have the "blackness" spangled with steady stars; for the twinkling of the stars is caused by our atmosphere. Now, the higher we ascend, the more do we leave behind us the particles which scatter the light; the nearer, in fact, do we approach to that vision of celestial space mentioned a moment ago. Viewed, therefore, from the loftiest Alpine summits, the firmamental blue is darker than it is ever observed to be from the plains.

It is thus shown that by the scattering action of minute particles the blue of the sky can be produced; but there is yet more to be said upon the. subject. Let the natural sky be looked at on a fine day through a piece of transparent Iceland spar cut into the form known as a Nicol prism, It may be well to begin by. looking through the prism at a snow slope, or a white wall. Turning the prism round its axis, the light coming from these objects does not undergo any sensible change. But when the prism is :directed toward the sky the great probability is that, on turning it, variations in the amount of light reaching the eye will be observed. Testing various portions of the sky with due diligence, we at length discover one particular direction where the difference of illumination becomes a maximum. Here the Nicol, in one position, seems to offer no impediment to the passage of the skylight; while, when turned through an arc of ninety degrees from this position, the light is almost entirely quenched. We soon discern that the particular line of vision in which this maximum difference is observed is perpendicular to the direction of the solar rays. The Nicol acts thus upon skylight because that light is polarized, while the light from the white wall or the white snow, being unpolarized, is not affected by the rotation of the prism.

In the case of our manufactured sky not only is the azure of the firmament reproduced, but these phenomena of polarization are observed even more perfectly than in the natural sky. When the air-space from which our best artificial azure is emitted is examined with the Nicol prism, the blue light is found to be completely polarized at right angles to the illuminating beam. The artificial sky may, in fact, be employed as a second Nicol, between which and a prism held in the hand many of the beautiful chromatic phenomena observed in an ordinary polariscope may be reproduced.

Let us now complete our thesis by following the larger light-waves, which have been able to pass among the aerial particles with comparatively little fractional loss. Without going beyond inferential considerations, we can state what must occur. The action of the particles upon the solar light increases with the atmospheric distances traversed by the sun's rays. The lower the sun, therefore, the greater the action. The shorter waves of the spectrum being more and more withdrawn, the tendency is to give the longer waves an enhanced predominance in the transmitted light. The tendency, in other words, of this light, as the rays traverse ever-increasing distances, is more and more toward red. This, I say, might be stated as an inference, but it is borne out in the most impressive manner by facts. When the Alpine sun is setting, or, better still, some time after he has set, leaving the limbs and shoulders of the mountains in shadow, while their snowy crests are bathed by the retreating light, the snow glows with a beauty and solemnity hardly equalled by any other natural phenomenon. So, also, when first illumined by the rays of the unrisen sun, the mountain heads, under favorable atmospheric conditions, shine like rubies. And all this splendor is evoked by the simple mechanism of minute particles, themselves without color suspended in the air. Those who referred the extraordinary succession of atmospheric glows, witnessed some years ago, to a vast and violent discharge of volcanic ashes, were dealing with "a true cause." The fine floating residue of such ashes would, undoubtedly, be able to produce the effects ascribed to it Still, the mechanism necessary to produce the morning and the evening red, though of variable efficiency, is always present in the atmosphere. I have seen displays, equal in magnificence to the finest of those above referred to, when there was no special volcanic outburst to which they could be referred. It was the long-continued repetition of the glows which rendered the volcanic theory highly probable.

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