imum 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 as

sumed 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.

1

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. . . . 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 exist ence 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

1 Which is about one sixty-thousandth of an inch.

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 pre

sent 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 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