The Blue Color of the Sky

THE blue color of the sky on a clear day is familiar to all. And yet how many have considered the source of this delicate mantle of azure which Nature spreads over the dome of the heavens? The beautiful tints of the sky are universally admired,and every one has welcomed with mental relief the break in the clouds which gives a glimpse of the firmament when gloom and darkness have long hovered over the Earth. The color of this blue naturally appears the more striking when seen in immediate contact with the clouds.

Probably our very familiarity with the every-day appearance of the sky diminishes our wonder at one of the most exquisite colors in the physical world, and for this reason we seldom inquire into its origin. It certainly is a remarkable circumstance in the history of the human mind that some of the most obvious of natural phenomena, which every one notices and no one especially dwells upon, should have escaped the attention of philosophers to such an extent that even now their causes are not fully understood, while other phenomena much more remote, and having little connection with daily life, excite such wonder that they have long since been duly explained and appreciated. These latter phenomena obviously are cases where “distance lends enchantment to the view,” and therefore after all are not so unnatural as they at first appear.

It is undeniable that a singular charm often attaches to objects remote from us either in time or space, and a similar mental attitude is frequently illustrated in the history of the Physical Sciences. This subtle psychological tendency arises from a natural disposition to endow those things which we see in the distance, or learn of only by report, with all the perfections of descriptive language so framed as to convey the salient qualities of interest, without the imperfections usually revealed by personal contact and close observation. The creations of the imagination are more ideal than the works of Nature, and we always see these remote objects under the fascination of the imagination.

The blue color of the sky on a bright clear day has been constantly noticed by the individual from childhood. To the primitive lay mind the azure tint of the firmament is simply its natural color. But our daily experience shows that the visible dome of the heavens is only an appearance, and Science teaches us to inquire critically into the nature of things. The cause of this color viewed from a scientific standpoint has been almost as elusive as the fabled philosopher’s stone, which during the Middle Ages was for centuries an object of profound research. The same may be said of the familiar color of the deepblue sea, which has elicited the admiration of dwellers on the ocean shores from the earliest ages of mankind; and yet probably no great number of individuals have inquired into the cause of this color.

Viewed from an artistic standpoint, the ancient Greeks, who were so much favored by auspicious influences both human and divine, were especially fortunate in their location in a region of the world where the color phenomena of sea, sky, and mountains assume a beauty not only unsurpassed but probably unapproached at any other point of the terrestrial globe. These vivid impressions of the Physical Universe, working upon the free minds of the most gifted race of antiquity, turned their idealizing tendency to Art, Poetry, and Science, whence has come the most beautiful language and literature in history. The sea-faring Greeks beheld daily the bluest of skies reflected in dark blue seas beneath their feet; and at the distant horizon snow-capped mountains of bluish purple appeared to prop the firmament above the Earth like the fabled Atlas of old. Admiration for these wonders of nature finds expression in the gorgeous colors which they bestowed on their temples in imitation of the divine spirit pervading the world, and which they worshiped in majestic edifices of noble simplicity.

It was natural for the Greeks to inquire into physical phenomena, so far as the knowledge of the times permitted, and nothing excited their wonder and admiration more than the blue canopy of the heavens, from which the gods of Homer descended to their ministrations in the affairs of men. Indeed, Zeus or Jupiter means the Father of the Skies, the deity who presides over the orderly and beautiful Cosmos. This spirit is admirably conveyed by Kaulbach’s justly celebrated painting in the National Gallery at Berlin, where the Greeks of the Homeric age are seen on the seashore near an imposing temple, mingling with the nymphs of the blue sea, while the gods are ascending to Heaven over the arches of a brilliant rainbow which illuminates the sky, after the manner of the token which God set in the clouds as a sign of the everlasting covenant made with Noah and all living creatures after the Flood.

If the Physical Sciences had been developed in antiquity, it is safe to say that the Greek spirit of devotion to all that is artistic and beautiful in the Cosmos would have led them to inquire as minutely into the colors of the sea and sky as they did into those sublime relations of Art, Philosophy, and Mathematical Science, which have filled subsequent generations with admiration and despair. Nothing could surpass the artistic and æsthetic spirit of the age of Æschylus and Sophocles, Phidias and Praxiteles, Aristotle and Plato.

Yet astonishing as were the intellectual creations of the Greeks, there is no record of the scientific study of the familiar color of the firmament. Nor indeed could such study be expected, when we consider the infancy of the sciences at that early epoch, and the amazing difficulties of the problem as made known by the scientific methods of our own age. We look therefore in vain for a correct understanding of the cause of the color of the sea and sky among the ancients, not because artistic appreciation or scientific ability was lacking, but because the state of research was then much too primitive to fathom the depths of a problem at once familiar and profound.

The color of the sky has to be studied in connection with the theory of light, and as this was not well understood by the ancients, we find scientific theories of the colors of natural objects only in modern times, chiefly since the epoch of the great Newton.

The simple propagation of light in right lines was well known to the ancients. Archimedes understood the conic sections and the elementary theories of optics so well that by means of reflecting mirrors of his own construction he was enabled to burn the ships of the besieging Romans in the harbor of Syracuse. The astronomer Ptolemy clearly understood the reflection of light from mirrors, and even recognized the effects of atmospheric refraction upon the light of the stars and planets. But all the ancients thought the velocity of light was infinite, or that it passed instantaneously from one part of the earth to another; and even in modern times similar views continued to prevail until the year 1675, when Roemer discovered from irregularities in the eclipses of Jupiter’s satellites that light is propagated across the Earth’s orbit in measurable time. This discovery is one of the most fortunate in the annals of history; and yet when first announced Roemer’s theory seemed so extraordinary that for a time it was scarcely believed. The realization of Roemer’s observations of the satellites of Jupiter depended upon the astronomical telescope which Galileo had invented sixtyfive years before, and applied with such revolutionary effect to the study of the heavens. These discoveries opened up new views of the nature of light, and it subsequently came to be the subject of profound philosophical research and experimentation, especially by the illustrious Newton, who analyzed the spectrum in 1666, and during the next ten years was much occupied with developing a theory of the colors of natural bodies. These were the first strictly scientific attempts to explain the color of objects by principles deduced from experimental research, in which the ancients had been singularly deficient. Unfortunately, the novelty of the new theory of colors gave rise to professional jealousies which involved Sir Isaac Newton in disputes so bitter that he afterwards regretted publishing his work. He blamed his imprudence in parting with so substantial a blessing as his peace of mind to run after the shadow of fame, and said if he got rid of certain controversies with Linus he would bid adieu to such experiments forever except such as he did for his own satisfaction, or left to come out after him. He declared that “a man must either resolve to put out nothing new, or make himself a slave to defend it.”

Before the memorable work of Newton some of the great Continental painters of the Renaissance had formed theories of light and color based upon the mixture of pigments; and a few of them naturally attempted to account for the blue color of the sky. Leonardo da Vinci, who had devoted much attention to the composition of colors in his extensive artistic designs, conjectured that the blue color of the sky was the result of the mixing of the white sunlight reflected from the upper layers of the atmosphere with the intense blackness of space. Historically this is the first explanation of the color of the sky worthy of mention, and its simplicity reminds one of the early speculations of the Ionian philosophers that the world is composed of the elements water, fire, air, and earth. Though resembling the natural science of the primitive Greeks, this explanation after all comes nearer the modern theories than might be expected, for these declared that the blue color of the sky is due to reflections from very minute particles of oxygen and nitrogen in the upper layers of the atmosphere.

Before touching upon these recent investigations it seems advisable to elucidate the historical steps by which such views were established. Newton’s study of the color of the sky was a part of the brilliant optical experiments which he finished about the year 1675. While absorbed in these labors during the year 1666, the young philosopher admitted a beam of sunlight into his chamber through a small aperture in the window shutter. On passing it through a triangular prism of glass he produced the famous experiment of colors, leading at once to the solar spectrum; and when this spectrum was again passed through a reversed prism he produced white light. To a keen youth of twentyfour these experiments opened a very wide field of optical investigation, and for the next ten years he was largely occupied with researches into the nature of light, and especially with investigating the colors of thin films of transparent bodies. He used soap bubbles as the most practicable means of getting films of water of the requisite thinness, and studied the colors which they exhibit.

It is well known that under the action of gravity the water composing such a thin shell tends to run down on all sides, so that the walls of the bubble grow thin at the top and thicken toward the bottom. After a time the bubble becomes so thin at the top that further flow of water from this point can hardly take place, and finally the bubble bursts. But before this last stage is reached a degree of thinness in the walls of the bubble is attained, which causes it to glow with brilliant iridescent colors. Newton noticed that on top of the thin bubble illuminated by white sky light a black spot is formed; with increase of thickness downward from this point on all sides, a red band next appears, then a blue one; then, again, red and blue, red and blue, and so on; the colors showing more extremes of red and purple in the higher orders. This blue band, which first expands outward from the black spot at the top, and descends slowly with the subsidence of the water, Newton called the “blue of the first order; ” and although somewhat dingy, he judged it to be of the same tint as the blue of the sky.

Newton’s theory of the colors of bodies rests upon the iridescent effects produced by white light falling upon thin plates of the given substances; and he says the color will be the same when the plates are cut up into infinitely thin strips, and again cut crosswise into particles ; so that he explains the color of powdered paint by referring it to the color of plates of the same thickness as the grains of powder.

Reasoning from analogy, he inferred that the transparent globules in the air were small particles of water, such as a thin soap bubble would yield when cut up into small particles. The following passages from Newton’s famous Treatise on Optics, published in 1704, are of interest: —

“If we consider the various phenomena of the Atmosphere, we may observe that when Vapors are first raised, they hinder not the transparency of the Air, being divided into parts too small to cause any reflexion in their superficies. But when in order to compose drops of rain they begin to coalesce and constitute globules of all intermediate sizes, those globules, when they become of a convenient size, reflect some colors and transmit others, may constitute clouds of various colours according to their sizes. And I see not what can be rationally conceived in so transparent a substance as water for the production of these colours, besides the various sizes of its fluid and globular parcels. . . .

“The blue of the first order, though very faint and little, may possibly be the color of some substances; and particularly the azure of the skys seems to be of this order. For all vapors, when they begin to condense and coalesce into small parcels, become first of that bigness whereby such an azure must be reflected, before they can constitute clouds of other colours. And so, this being the colour which vapours begin to reflect, it ought to be the colour of the finest and most transparent skys in which vapours are not arrived to that grossness requisite to reflect other colours, as we find it by experience. ”

Newton’s explanation seemed so plausible that for a long time it was generally accepted as correct. But since the discovery of the blue clouds which Tyndall artificially produced in the laboratory about a third of a century ago, and Lord Rayleigh’s subsequent mathematical investigations of the reflection of light from small particles, it has been replaced by the theory of Tyndall as verified by Rayleigh, an account of which will be given below.

Before taking up this recent work it may be remarked that the French physicist Mariotte about 1675 adopted the naturalistic view that it is an inherent quality of the sky to reflect blue light. Under the influence of this opinion the great Euler in 1762 wrote: “It is more probable that all the particles of the air should have a faintly bluish cast, but so very faint as to be imperceptible, until presented in a prodigious mass, such as the whole extent of the atmosphere, than that this color is to be ascribed to vapors floating in the air, which do not pertain to it. In fact the purer the air is, and the more purged from exhalation, the brighter is the lustre of heaven’s azure, which is sufficient proof that we must look for the cause of it in the nature of the particles of the air. ”

Sir John Herschel about 1830 still adhered to Newton’s original view that the color of the sky is a blue of the first order, and he made extensive use of this theory. When Clausius in 1847 attempted to test Newton’s theory mathematically, he reached the conclusion that if the heavenly bodies are to appear sharply defined through such a medium the particles of water in the air must have the form of thin shells or hollow spheres, whose parallel surface would not greatly refract the waves of light, but, when the bubbles are sufficiently thin, would yet reflect the blue of the first order. This singular doctrine of vesicular vapor did not originate with Clausius, but had come down from the speculative age of Leibnitz and Descartes; in recent years it has been entirely abandoned as having no foundation in nature.

It was discovered by Arago in 1810, and more fully established by the observations of Sir David Brewster about 1840, that blue sky light is always polarized in a plane passing through the Sun, the point of the sky observed, and the observer. According to the laws of polarization of light by reflection, this proved that the light of the sky is sunlight reflected from solid particles in the air. Moreover, the maximum polarization occurs in a great circle of the heavens ninety degrees from the Sun. In 1853 the German physicist Brücke showed that the light scattered by fine particles in a turbid medium is blue, and that the blue of the sky is in reality much deeper than Newton had supposed, being of at least the second or third order.

In 1869 Tyndall showed by some very beautiful experiments which have since become famous that when the particles causing the turbidity are so exceedingly fine as to be invisible with a powerful microscope, the scattered light is not only a magnificent blue, but is polarized in the plane of scattering, the amount of the polarization being a maximum at an angle of ninety degrees with the incident light. The definition of objects seen through this fine-grained medium was found to be unimpaired by the turbidity. Here for the first time the physicist at work in the laboratory had produced all the essential qualities of blue sky light. Tyndall’s experiment was recognized as giving the key to the problem which had wellnigh proved the riddle of the ages.

Using a glass tube about a yard in length and some three inches in diameter containing air of one tenth the ordinary density mixed with nitrite of butyl vapor, which is extremely volatile, and then exposing the mixture to the action of a concentrated beam of electric light which would pass almost unhindered through the transparent ends of the tube, Tyndall was enabled to precipitate the attenuated vapors in the form of a blue cloud. This cloud is not visible in ordinary daylight, and to be seen must be surrounded with darkness, the vapor alone being illuminated. The blue cloud differs in many ways from the finest ordinary clouds, and, in fact, occupies an intermediate position between these clouds and true cloudless vapor. By graduating the quality of vapor admitted into the tube, Tyndall found that the precipitation may be obtained of any desired degree of fineness, so that particles could be produced sufficiently coarse to be visible to the naked eye, or so fine as to be hopelessly beyond the reach of the most powerful microscope. The light emitted by the blue cloud in a direction perpendicular to the beam of incident light was found to be completely polarized, and the polarization was the more perfect the deeper the blue of the cloud. Tyndall demonstrated that the blue cloud would result from particles of any kind provided they are sufficiently fine, and the analogy of the blue sky was so evident that he concluded that the phenomenon of the firmamental blue found definite explanation in these experiments. He assumed the existence of fine particles of water in the higher regions of the air, and his studies on the heat-retaining power of aqueous vapor, which does not extend very high above the Earth, led him to think that these particles are in a solid state, owing to the intense cold to which they are exposed in the rare medium of oxygen and nitrogen composing the upper layers of the atmosphere.

In these experiments Tyndall felt confident that “particles might be precipitated whose diameters constitute but a very small fraction of the wave length of violet light.¹ ... In all cases, and with all substances, the cloud formed at the commencement, when the precipitated particles are sufficiently fine, is blue, and it can be made to display a color rivaling that of the purest Italian sky.” On account of certain difficulties incident to the use of aqueous vapor at the pressure and temperature desirable in these experiments, he made no actual use of water in any form; yet he says: “That water-particles, if they could be obtained in this exceedingly fine state of division, would produce the same effects, does not admit of reasonable doubt. . . . Any particles, if small enough, will produce both the color and polarization of the sky. But is the existence of small water-particles, on a hot summer’s day, in the higher regions of our atmosphere,inconceivable ? It is to be remembered that the oxygen and nitrogen of the air behave as a vacuum to radiant heat, the exceedingly attenuated vapors of the higher atmosphere being therefore in practical contact with the cold of space.”

Tyndall concludes his theory of the color of the sky thus: “Suppose the atmosphere surrounded by an envelope impervious to light, but with an aperture on the sunward side, through which a parallel beam of solar light could enter and traverse the atmosphere. Surrounded on all sides by air not directly illuminated, the track of such a beam would resemble that of a parallel beam of the electric light through an incipient cloud. The sunbeam would be blue, and it would discharge light laterally in the same condition as that discharged by the incipient cloud. The azure revealed by such a beam would be to all intents and purposes a ‘ blue cloud.'”

Lord Rayleigh’s profound mathematical investigations prove that when white light is transmitted through a cloud of particles small in comparison with the cube of the shortest wave length, the light scattered laterally is polarized in the plane of scattering, the maximum of polarization is ninety degrees from the incident light, and the intensity of the scattered light varies inversely as the fourth power of the wave length. This result takes no account of light which has undergone more than a single scattering. All the facts brought out by Lord Rayleigh have been shown to agree with phenomena observed in the laboratory when light is passed through turbid media; and very recently this illustrious physicist has shown that about one third of the total intensity of the blue light of the sky may be accounted for by the scattering due to the molecules of oxygen and nitrogen in the air, entirely independent of the dust and aqueous vapor which assume great importance in the lower layers of the atmosphere. Solid particles of water, ozone, and very fine aggregations of oxygen and nitrogen condensed under the intense cold prevailing in the upper regions of the atmosphere enable us to account for the rest of the sky light in accordance with Rayleigh’s mathematical theory.

It is worthy of remark that but for the brightness of the sky the stars could be seen in daylight. Even as matters stand, some of the brighter of them have been seen after sunrise by explorers in high mountains, where the air is very clear and the sky dark blue. If we could go above the atmosphere the sky would appear perfectly black, and stars would be visible right close up to the Sun. Astronomers observe bright stars in daytime by using long focus telescopes, the dark tubes of which cut off the side light; and persons in the bottoms of deep wells have noticed stars passing overhead, the side light being reduced by the great depths of the wells.

The sky is bluer in the zenith than elsewhere, because the path traversed by scattered light is here the shortest, so that it appears with less admixture of white light reflected from haze and water vapor, and less absorption of blue light in the same watery envelope. Near the horizon, where the path traversed by the light reflected from the Sun is very long, there should be a great increase in the whiteness of the background, and this is fully verified by experience. The sky is generally more or less milky near the horizon, and if it assumes a perfectly blue color it is usually just after a heavy rain. At this time all the dust is washed out of the air and the watery haze has been precipitated. Even then the blue remains deepest in the zenith, for the reasons above mentioned.

In the average condition of the sky the haze is usually sufficiently prevalent to render our sunsets and sunrises yellowish or reddish. This is due mainly to selective absorption of the blue rays by water vapor, smoke, and dust in the air. The existence of this selective absorption is a fortunate circumstance for painters, poets, and writers, who have used these beautiful and familiar adornments of Nature to fascinate the minds and charm the imaginations of mankind in all ages.

The study of the polarization and color of the sky viewed scientifically is very useful to meteorologists, as indicating the size and kind of condensation taking place in the atmosphere. Considerable observational data on these points have been collected in the past by Sir David Brewster and Professor James D. Forbes, and by the Swedish physicist Rubenson,but a vastly greater work is being done now by the scientists of the United States Weather Bureau in supplying valuable observations for the future study of the atmosphere.

The great aerial ocean over our heads is made up of an infinite multitude of moving currents and streams of varying density and temperature, all in process of continued change and adjustment due to the heating of the atmosphere by the Sun during the day and cooling by radiation at night. The atmosphere is full of little waves or streaming masses of air somewhat resembling the ripples in a shallow stream of water flowing over gravel. And if the astronomer will point his telescope on a bright star and remove the eye-piece, so as to look directly upon the object-glass illuminated by the light of the star, he may see these streaming currents dancing in all their complexity. It is these little waves in the air which cause the twinkling of the fixed stars. As the waves are passing before our eyes they act like prisms, deflecting the light first this way and then that, producing flashes of the spectral colors and sometimes almost extinguishing the stars, so that momentarily they appear to go out. In high dry countries where the atmosphere is quiescent these waves are greatly diminished in importance; and astronomers have noticed that in such localities the scintillation of the stars almost ceases. There the air is quite free from agitating currents, and the astronomers can make good observations. At present such regions are known chiefly in Peru, and in the high dry plateaus of the southwestern part of the United States.

Having thus penetrated the cause of the blue color of the sky, it is not a very great leap to infer that a similar explanation holds for the color of the ocean, which next to the sky offers to our senses the most attractive tints of the great objects in nature. The saline and other mineral substances dissolved in the waters of the sea may be looked upon as infinitely small particles in a turbid medium ; and these should reflect the sunlight and give a bluish green appearance to the ocean, just such as we observe. For the salts are not in chemical combination with the water, but merely dissolved in the medium, and thus constitute an infinitely fine mixture of molecules and particles suspended in a colorless fluid. The light of the Sun penetrates the ocean to a considerable depth before all the reflections are produced, and the depth of this layer is such that some of the shorter waves of blue are absorbed, while the slightly longer waves of green are transmitted. This accounts for the appearance of the well-known greenish tinge in the color of the ocean.

If the sea water is full of air bubbles, as in the neighborhood of breakers, or when turning violently before a moving ship, the light reflected from the surface of these bubbles suffers a double absorption by the water before it reaches the eye, thus producing some of the exquisite colors of the sea. Near the shore, or in shoal water, another cause sometimes comes into play, namely, fine solid particles suspended in the water. Such particles, whether in air or in water, if sufficiently small, may produce colors due to their minuteness alone, as we have seen in the experiments of Tyndall. If the particles are somewhat coarser, like fine grains of soil washed down in the erosion of rivers, they may give the water a muddy appearance, as in the China Sea; while again, if excessively minute, they may produce the deep blue seen in the West Indies and the equatorial Pacific. Extremely minute animalculæ, both living and dead, are said to affect the color of the sea water in many places. Owing to the suspension of such mineral matter in the waters of the ocean, they are not penetrable by the Sun’s rays to any very great depth. After a depth of a few hundred fathoms has been attained, the darkness becomes so great that attempts at submarine photography have to be made by artificial electric light sent down for the purpose. And sea animals of all kinds living in the bottom of the ocean are wrapt in perpetual night of such blackness that Nature has beneficently provided them with phosphorescent powers for illuminating their surroundings, not unlike the common bull’s-eye lamp so frequently used for exploring dark corners. The phosphorescent lamps of the denizens of the deep sea serve for the explorations needed in their daily life, and also for gratifying the sense of color, which is preserved and even highly developed among animals dwelling in the total darkness of the uttermost abysses of the ocean.

The beauty of pictorial works of Art dealing with ocean scenery depends very largely upon the magnificent coloring of the background; and here, as in the case of the aerial ocean over our heads, the color is due to reflection of light by small particles suspended in the fluid medium. According to Helmholtz, the blueness of the eyes is also due to the action of suspended particles. The “dark blue sea ” of Homer, and the endless variety of allusions to the color of the ocean in the literature of all ages, thus find a curious and instructive explanation in the light of modern Science.

Let us now consider how the theory of Tyndall and Rayleigh works when the lower strata of the atmosphere are filled with dust and water vapor in its various forms. It is well known that but little water vapor ascends to a very great height above the Earth’s surface. The temperature decreases so rapidly as we ascend, that at a height of 29,000 feet the thermometer falls to sixteen degrees below zero centigrade, as was observed by the English aeronauts Glaisher and Coxwell in 1862. At this height the color of the sky was noticed to be “an exceedingly deep Prussian blue, ” and the air was “almost deprived of moisture. ” In an ascent to the height of 23,000 feet made at Paris in 1804 Gay-Lussac found the temperature nine degrees below zero centigrade, and the dryness of the air so extreme that hygrometric substances such as paper and parchment became dried and crumpled, as if they had been near a fire. At this great height he noticed that the sky had a dark blue tint, and that the absolute silence prevailing was impressive. Most of the moisture in the atmosphere had been left behind before the balloon entered the rare abode of the cirrus clouds, which surround the tops of our highest mountains.

In high altitudes in the Rocky Mountains, the Andes, and the Alps, travelers notice the striking blueness of the sky, and the rarity and dryness of the atmosphere. The writer recalls very vividly the blue aspect of the sky as seen from the top of the San Francisco Mountains in Arizona, which have an altitude of 13,000 feet above the sea; and in an ascent of Popocatepetl to a height of 16,000 feet the sky also appeared deep blue. The same color was noticed at other points of the Rocky Mountains and in the Alps of Switzerland, where the contrast between the blue of the sky and white snow on the mountain peaks appeared so striking as to attract the instant notice of the thoughtful observer. Similar phenomena have been noticed by travelers who have explored high mountains in all parts of the globe, and theory and observation agree in indicating that water vapor is confined mainly to the lower part of the atmosphere, though in the form of cirrus clouds the height has been shown occasionally to exceed ten miles. At this height the water of course is frozen, and the clouds are made up of crystals of ice and snow.

One of the simplest means of verifying these views, that the water vapor and dust of the air are confined to the layers within a few miles of the sea level, is to notice the shadows cast by heavy clouds on mountains at the setting or rising of the Sun. The great beams which spread out fanlike from the setting Sun teach us a great deal about the atmosphere. We always see a blue streak where the clouds or mountains cast a shadow; while the surrounding region of the sunset sky is whitish, golden, purple, or even reddish, and sometimes the colors are amazingly brilliant. Thunder clouds seldom exceed the height of five miles, and yet the shadows cast by them at the time of sunset are conspicuously blue. The blue color of the shadow indicates that the predominant part of the blue light of the sky originates at great height, while the whitish, yellow, and reddish colors are confined to the lower strata of the air. The persistence of the blue color for more than an hour after sunset, when the sky light is reflected from illuminated particles in the rare medium more than one hundred miles above the Earth’s surface, also strengthens this view. In the spaces intervening between the blue beams the lower layers of the atmosphere are directly illuminated by the Sun, and reproduce Homer’s “rosy-fingered dawn. ” This color is due to the absorption of blue light in the denser and more turbid medium of the lower air, through which only the longer waves, as the yellow, orange, and red, can be freely transmitted.

It was this gorgeous aspect of the rising Sun, casting shadows from the clouds and mountains of Greece against a sky naturally rich in color, which gave the Greek poets their elegant conceptions of the dawn. The sun-god Apollo, worshiped at Delphi, without doubt owes much of his mystery and impressiveness to the towering mountains which surround the seat of the ancient oracle. Nothing could be more majestic than mountains like Parnassus, to the east of Delphi, from which the morning sun looks down into the precipitous gorges in front of that famous temple. The Sun emerges suddenly from his hiding behind overhanging peaks, and is seen radiating with all brilliancy in a sky of the deepest blue. The natural color of the Greek landscape combined with the gorgeous phenomena of the rising Sun bursting upon a scene where shadows from mountains and clouds fill the air with luminous beams of purple and azure, without doubt accounts for much of the glory of Apollo at the Temple of Delphi. As seen by the art-loving Greeks of the primitive ages nothing could be more beautiful or more impressive than this grand natural spectacle, which we now explain by the reflection of light from myriads of minute particles suspended in the atmosphere. Most of the deep sky blue comes from excessively minute particles at a great height; while the “ rosy-fingered dawn ” arises from aqueous vapor, and haze, and innumerable particles of smoke and dust floating near the earth.

Those who have visited Egypt, where the atmosphere is usually clear, and so free from clouds that the annual rainfall is only an inch and a half, have been impressed by the absence of a pure deep blue sky. The vault of the firmament appears rather whitish, or muddy, due of course to the absorption of the blue by dust diffused from the dry regions of Sahara. While the Egyptian sky is very bright, the white light is so pronounced that the blue does not appear particularly attractive. The skies of Italy and the Alps, on the other hand, frequently are clear blue. Of all the places which the writer has visited Greece has the purest and deepest blue sky. The color frequently is so striking that one does not wonder at even the most vivid descriptions in Greek literature. While traveling in Greece during the spring of 1891 the writer took particular occasion to notice the color of the sky, sea, and mountains. The atmospheric colors are much the most brilliant known in any part of the world. The mountains of Greece seen at a distance of more than ten miles appear of deep indigo blue tinged with a delicate purple of inexpressible beauty.

The admirable paintings in the National Museum at Berlin, representing restorations of various places of classic celebrity, as Athens, Olympia, and Syracuse, convey this rich coloring of bluish purple in great vividness, but are not in the least degree overdrawn. They are among the most beautiful paintings in the world, and eminent scholars have regretted that they are not extensively reproduced.

It is probable that the climate of Greece, from a combination of several natural causes, is such that the atmospheric reflection and absorption become especially pronounced. And as this sky was evidently the same in classic antiquity as it is to-day, this color phenomenon affords an interesting proof of the unchanging climatic conditions of that part of our globe during the last two thousand years.

In most parts of the United States our skies are whitened by water vapor, haze, and dust; and we usually see the deepest blue just after rainy days, when the haze and moisture have been precipitated, and the particles of dust washed out of the atmosphere by the falling rains.

It is perhaps fortunate, from an æsthetic point of view, that the appearance of the sky varies so much as it does. The infinite varieties of color which it affords when so delicately frescoed with clouds of all forms and of all shades of color and intensity, combined with vegetable and mineral hues upon the land, whether in the green of spring, the smoky blue of Indian Summer, the purple of autumn, or the whiteness of winter, yield in due succession a constant mental relief, and have inspired most of the exquisite delineations of Nature in pictorial Art as well as in Literature. The soft hues with which the land is clothed give to the whole aspect of the world a lifelike appearance, and the light of the Sun reflected from the blue sky and luminous clouds fills the whole scene with such vivid radiation, that the Universe becomes to a modern student as truly an inspiration as the orderly and beautiful Cosmos was to the primitive Greeks. As Goethe says: —

“ Angels are strengthen’d by the sight, Though fathom thee no angel may ; Thy works still shine with splendour bright, As on Creation’s primal day.”

Now that Science has at length added her share to these pleasurable contemplations by showing the causes from which the inspirations of the mind have sprung, the result of explaining the color of the sea and sky, phenomena often considered almost obvious and yet for long ages wholly obscure, may be ranked among the most gratifying triumphs of the human mind.

T. J. J. See.