speeches, but five in number, were delivered under circumstances of the utmost physical difficulty. But their success, first with the audiences that had to be conquered, and then with a half-hostile public, was one of the notable triumphs of our heroic period; and Dr. Abbott is to be thanked for putting it so effectively on record. For the rest of Beecher's career—it is no easy task to write of the most conspicuous member of the family which inspired the remark that mankind is divided into "the good, the bad, and the Beechers." It would be harder for most biographers than it has been for Dr. Abbott, for, except in such a chapter as the discreet and restrained "Under Accusation," into which the whole miserable Tilton business is com

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pressed, the author has permitted himself the fluency of one whose constant practice has made it easy to expatiate on any theme. Some condensation might therefore have been well. Yet the book leaves a clear impression of an extraordinary personality: the preacher who, using his text, as he said himself, as a gate not to swing upon, but to push open and go in, made his pulpit a living power; the editor, who observed no rules or office hours, yet profoundly affected the type of journalism with which he had to do; the writer and public speaker, of persuasive wit and eloquence. The figure of Beecher could not be spared from an American gallery of the last century, and Dr. Abbott's picture bids fair to stand as the authoritative portrait.

M. A. De Wolfe Howe.

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 es

caped 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 per

sonal 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 imagina

tion.

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 Eschylus 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 aneients. Archimedes understood the conie 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 max