E. L. TROUVELOT, (continued 5)

The Crab Nebula of Lord Rosse, near ζ Tauri, No. 1,157, is one of the interesting objects of this class. It has curious appendages streaming off from an oval, luminous mass, which give it a distant resemblance to the animal from which it derives its name. The Bifid nebula in Cygnus, Nos. 4,400 and 4,616, is another object of this class. It consists of a long, narrow, crooked streak, forking out at several places, and passing through χ Cygni. Observers, having failed to recognize the connection existing between its different centres of brightness, have made distinct nebulæ of this extended object.

The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious object, with a general resemblance to the instrument from which it derives its name. Lord Rosse's telescope has shown many stars in it, projected on a nebulous background, and Prof. Bond seems to have thought that it showed traces of resolvability, although in the study which I made of this nebula with the same instrument used by the latter observer, I failed to perceive any such traces. Dr. Huggins finds its spectrum gaseous.

The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by Sir J. Herschel as a loose cluster of at least 100 stars, I have found to be involved in an extensive, although not very bright, nebula, which would seem to have escaped his scrutiny. In a study and drawing of this nebula made in 1876, its general form is that of an open fan, with the exception that the handle is wanting, with deeply indented branches on the preceding side, where the brightest stars of the cluster are grouped. From its peculiar form, this object might appropriately be called the Fan nebula.

The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I have made a study and two drawings, one with a refractor 6⅓ inches in aperture, and the other conjointly with Prof. Holden, with the great telescope of the Naval Observatory, is a bright and very complicated object. Its general appearance in small instruments, with low power, is that of a long, narrow pisciform mass of light, from which proceeds on the preceding side, the great double loop from which it derives its name. But in the great Washington refractor its structure becomes very complicated, forming various bright nebulous masses and wisps of great extension. Prof. Holden, who has made a careful, comparative study of the published drawings of this object, thinks there are reasons to believe that its western branch has moved relatively to the stars found within its loop. The spectrum of this nebula is gaseous.

The Trifid nebula, No. 4,355, in the same constellation, is also a very remarkable object, although it is not so bright as the last. This nebula, which I have studied with the refractor of the Cambridge Observatory, consists of four principal masses of light, separated by a wide and irregular gap branching out in several places. These masses, which are brighter along the dark gap, gradually fade away externally. A group of stars, two of which are quite bright, is found near the centre of the nebula, on the inner edge of the following mass, and close to the principal branch of the dark channel. A little to the north, and apparently forming a part of this nebula, is a globular-looking nebula, having a pale yellow star at its centre. Prof. Holden's studies on this nebula show that the triple star, which was centrally situated in the dark gap from 1784 to 1833, was found involved in the border of the nebulous mass following it, from 1839 to 1877; the change, he thinks, is attributable either to the proper motion of the group of stars or to that of the nebula itself.

In the same vicinity is found the splendid and very extensive nebula No. 4,361, in which is involved a loose, but very brilliant star-cluster. This nebula and cluster, which I have studied and drawn with a 6⅓ inch telescope, is very complicated in structure, and divided by a dark irregular gap into three principal masses of light, condensing at one point around a star, and at others forming long, bright, gently-curved branches, which give to this object a strong resemblance to the wings of a bird when extended upwards in the action of flying. From this peculiarity this object might appropriately be called the Winged nebula. Its spectrum is that of a gas.

The variable star η Argus is completely surrounded by the great nebula of the same name, No. 2,197, first delineated by Sir J. Herschel, during his residence at the Cape of Good Hope, in 1838. This object, which covers more than ⁴⁄₇ of a square degree, is divided into three unequal masses, separated by dark oval spots, comparatively free from nebulosity, and is suspected to have undergone changes since Herschel's time.

In the same field with the double star, ζ Orionis, the most easterly of the three bright stars in the belt of Orion, is found another irregular nebula of the Trifid type. From the drawings which I have made of this object, it appears to be composed of three principal unequal masses, separated by a wide, irregular, dark channel, two of the masses being quite complicated in structure, and forming curved, nebulous streams of considerable length and breadth. This nebula, like the next to be described, seems to be connected with the Galaxy by the great galactic loop described in another section.

By far the most conspicuous irregular nebula visible from our northern States, is the great nebula in Orion, No. 1,179, represented on Plate XV. This object, visible to the naked eye, is the brightest and the most wonderful nebula in the heavens. It is situated a little to the south of the three bright stars in the belt of Orion, and may be readily detected surrounding the star θ, situated between and in a line with two faint stars, the three being in a straight line which points directly towards ε, the middle star of the three in Orion's belt. The area occupied by this nebula is about equal to that occupied by the Moon.

In its brightest parts the nebula in Orion appears as a luminous cloud of a pentagonal form, from which issue many luminous appendages of various shapes and lengths. This principal mass is divided into secondary masses, separated by darkish, irregular intervals. These secondary masses in their turn appear mottled and fleecy. Towards the lower part of the pentagonal mass is found a roundish dark space, comparatively devoid of nebulosity, in which are involved four bright stars forming a trapezium, and several fainter ones. The four bright stars of the trapezium constitute the quadruple star θ Orionis, from which the nebula has received its name. The cloud-like pentagonal form is brightest on the north-west of the trapezium, and is surrounded on three sides by long, soft, curved wisps, fading insensibly into the outer nebulous mass in which they are involved. On the east a broad, wavy wing spreads out, and sends an important branch southward. South-east of the trapezium are found several curious dark spaces, comparatively devoid of nebulosity, especially those on the east, which give to this nebula a singular character. Close to the north-eastern part of the nebula, or rather in contact with it, is found a small, curiously-shaped nebula, condensing around a bright star into a blazing nucleus. From this centre it continues northward in a narrow diffused stream, which spreads out in passing over the stars c¹ and c²; and after having sent short branches northward, it curves back to the south and joins the main nebula on the west of its starting point, having thus formed a great loop which is not shown on the Plate. The nebula also forms a loop towards the south, which is partly shown on Plate XV., a small branch of which, passing through τ Orionis, the nebulous star shown at the top of the Plate, and extending southward, is not here represented.

On ordinary nights the nebula in Orion is a splendid object, and inspires the observer with amazement; but this is as nothing compared with the grand and magnificent sight which it presents during the very rare moments when our atmosphere is perfectly clear and steady. I have seen this nebula but once under these favorable circumstances, and I was surprised by the grandeur of the scene. Then could be detected features to be seen at no other time, and its fleecy, floculent, cloud-like masses glittered with such intensity that it seemed as if thousands of stars were going to blaze out the next moment. Although I observed the nebula under such favorable conditions, and with the fifteen-inch refractor of the Cambridge Observatory, yet I was disappointed in my expectations, and distinguished no new stars or points of light, and nothing more than a very bright mass, finely divided into minute blazing cloudlets. Although I failed to resolve this nebula into stars, yet Lord Rosse, Bond and Secchi thought they had caught glimpses of star dust. Its spectrum, however, proves to be mainly that of incandescent gases, probably hydrogen and nitrogen. In the curved wisps found in this nebula, Lord Rosse and others saw indications of a spiral structure.

Several bright stars are found scattered over this nebula, and besides those forming the trapezium, there are three in a row, a little to the south-east of that group, which are quite bright and remarkable. Among the stars involved in this nebula, few show signs of having a physical connection with it, although it seems probable that the group of the trapezium is so connected. Some of these stars are variable. The small stars represented on this Plate, as on others of the series, are somewhat exaggerated in size, as was unavoidable with any process of reproduction which could be adopted.

In 1811, W. Herschel was led to suspect that some changes had occurred in this nebula, but changes in such complicated and delicate objects are not easily ascertained, since, for the most part, we have for comparison with our later observations only coarse drawings made by hands unskilled in delineation.

Although comparatively rare, double and multiple nebulæ may be found in the sky. When this occurs, their constituents most commonly belong to the class of spherical nebulæ. Sometimes the components are separated and distinct, at other times one of them is projected upon the other, either really or by the effect of perspective. Sometimes one is round and the other elongated. It is probable that while some of these nebulæ are physically associated and form a system, others appear to be so only because they happen to be almost in a line with the observer. A double nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair type of those which are separated. The first is a globular nebula, and the last an oval one, with a star at its centre. The double nebula, Nos. 858 and 859, in Taurus, which I have also studied, is a type of the cases in which one nebula is partly projected on another. In this instance both the nebulæ are globular.

The nebulæ in general show very little color in their light, which is ordinarily whitish and pale. Some, however, present a decided bluish or greenish tint. The great nebula in Orion has a greenish cast, and we have seen that some planetary nebulæ are bluish.

It has been a question whether nebulæ are changing. It has already been stated that Prof. Holden believes there is ground to suspect that the Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near ε Tauri has been lost and found again several times. Two other nebulæ in the same constellation have presented curious variations. One, near a star of the tenth magnitude, exhibited variations of brightness like those of the star itself, and for a time disappeared. The other, near ζ Tauri, increased in brightness for three months, after which it disappeared. In 1859 Tempel discovered a nebula in the Pleiades, which has shown some fluctuations. In 1875 I made a long study of this object, and drew it carefully a dozen times, but I was not able to see any changes in it within the two or three months during which my observations were continued. But on Nov. 24, 1876, it was found of a different color, being purplish and very faint. On Dec. 23, 1880, it was found just as bright and visible as when I drew it in 1875, and on Oct. 20, 1881, it appeared faint and purplish again, as in 1876. On this last night, and on those which followed it, it was impossible for me to trace the nebulosity as far as in 1875. I consider this as due to a variation in the light of this object, which in 1875 was bright enough to be well seen while the Moon after her First Quarter was within ten or fifteen degrees from the Pleiades.

From the observations of M. Laugier, it appears that some nebulæ have a proper motion, comparable to that of stars. From the displacement of the lines of their spectra by their motion in the line of sight, Dr. Huggins found that no nebula observed by him has a proper motion surpassing 25 miles per second. The Ring nebula in Lyra appears to move from us at the rate of 3 miles per second, and that in Orion recedes about 17 miles per second.

The important question arises, are all the irresolvable nebulæ in the heavens to be considered as so many star-clusters, differing only from them by the minuteness of their components, or their immense distance from us; or are they cosmical clouds, composed of luminous vapors, similar to the matter composing the heads and tails of comets? Originally, W. Herschel, with many astronomers, thought that all these objects were stellar aggregations, too distant to be resolved into stars; but he subsequently modified his opinion, and accepted the idea that some of them are of a gaseous nature.

No direct proof that the nebulæ are gaseous could be obtained, however, before the spectroscope was known. The attempt to analyze the light of the nebulæ with this instrument was made in 1864, by Dr. Huggins, who directed his spectroscope to the planetary nebula, No. 4,373, in Draco. Its spectrum was found to consist of three bright, distinct lines, the brightest of which corresponded with the strongest nitrogen line, and the feeblest with the hydrogen C line. Besides these lines, it gave also a very faint, continuous spectrum, apparently due to a central point of condensation. By this observation, the gaseous nature of a nebula was for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ, of which one-third gave a gaseous spectrum, consisting of several bright lines, the brightest of which invariably corresponded with the lines of nitrogen. The others gave a continuous spectrum, with the red end usually deficient. These results indicate that if some of the so-called nebulæ are due to an aggregation of stars, either too minute or too remote in space to be individually resolved, others are in a gaseous state. Yet the faint, continuous spectrum, given by some nebulæ, in addition to their gaseous spectra, seems to show that these nebulæ have some stars or matter in a different state, either involved in them or projected on their surface.

The idea of diffused matter distributed here and there in space, and gradually condensing into stars, is by no means new. As early as 1572, Tycho Brahé proposed such an hypothesis, to explain the sudden apparition of a new star in Cassiopeia, which he considered as formed by the recent agglomeration of the "celestial matter" diffused in space. Kepler adopted the same idea to explain the new star which appeared in Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the same opinion. The hypothesis of a self-luminous, nebulous matter diffused in space, and forming here and there immense masses, has been proposed from the origin of the telescope, and was adopted by Sir William Herschel, who in his grand speculations on the universe considered the nebulæ as immense masses of phosphorescent vapors, gradually condensing around one or several centres into stars or clusters of stars. The evidence afforded by the spectroscope seems to be in favor of such an hypothesis, and shows us that gaseous agglomerations exist in space.

According to our modern conception, the visible universe is but an infinitely small portion of the infinite universe perceived by our mind. The great blazing centre around which our little, non-luminous globe pursues its endless journey, is only an humble member of a cluster comprising four hundred equally powerful suns, as they are believed to be, although they appear to us as little twinkling stars. The nearest of these stars is 221,000 times as far from the Sun as the Sun is from the Earth, and yet this entire cluster is only one among the several hundred Star-clusters composing the great galactic nebula in which we are involved, comprising thirty or fifty millions of such suns. Among the 4,000 irresolvable nebulæ in the sky, perhaps over one-half are supposed to be galaxies, like our own galaxy, composed of star-clusters, and millions of stars. Besides these remote galaxies, vast agglomerations of yet uncondensed, nebulous matter exist in space, and form the nebulæ proper, in which the genesis of suns is slowly elaborated. Although the visible universe is limited by the penetrating power of our instruments, yet we see in imagination the infinite universe stretching farther and farther; but we know not whether this invisible universe is totally devoid of matter, or whether it also is filled with millions and millions of suns and galaxies.

APPENDIX

KEY TO THE PLATES

PLATE I.—GROUP OF SUN-SPOTS AND VEILED SPOTS. Observed June 17, 1875, at 7h. 30m. A. M.

The background shows the sun's visible surface, or photosphere, as seen with a telescope of high power at the most favorable moments, composed of innumerable light markings, or granules, separated by a network of darker gray. The granules, each some hundreds of miles in width, are thought to be the flame-like summits of the radial filaments or columns of gas and vapor which compose the photospheric shell. The two principal sun-spots of the group here represented show the characteristic dark umbra in the centre, overhung by the thatch-like penumbra, composed of whitish gray filaments. The penumbral filaments are not supposed to differ in their nature from those constituting the ordinary photosphere, save that they are seen here elongated and violently disturbed by the force of gaseous currents. Both spots are traversed partly or wholly by bright overlying faculæ, or so-called luminous bridges, depressed portions of which, in the left-hand spot, form the gray and rosy veils commonly attendant upon this class of spots. In each of these spots, also, the inner ends of projecting penumbral filaments have fallen so far within the umbra as to appear much darker than the rest. At the right of the upper portion of the left-hand spot, is a mass of white facular clouds, honey-combed by dark spaces, through which are seen traces of the undeveloped third spot of the triple group first observed. If seen upon the sun's limb, this would have presented the appearance of a lateral spot. Above the right-hand spot is a small black "dot," or incipient spot, without distinct penumbra. The irregular dark rift below the two large spots and connecting them is a spot of the crevasse type, with very slight umbra, a still better example of which is seen in a westward prolongation of the penumbra of the left-hand spot. In the upper left-hand corner of the Plate are seen several small faculæ, appearing as irregular whitish streaks amongst the granules. In the pear-shaped darkening of the solar surface below and at their left, is seen a veiled spot, two of which attended this group.

Approximate scale, 2500 miles—1 inch.

PLATE II.—SOLAR PROTUBERANCES. Observed May 5, 1873, at 9h. 40m. A. M.

A view of an upheaval of the chromosphere, or third outlying envelope of the sun, as observed with the tele-spectroscope, or telescope with spectroscope attached.

The method of the observation requires a word of explanation. Save on the rare occasions of a total solar eclipse, no direct telescopic view of the solar prominences or flames is possible, owing to the fact that the intense white light from the sun's main disk entirely obscures the feeble pink light of the chromosphere. A few years ago Messrs. Jannsen and Lockyer found that a spectroscope of high dispersive power so weakens the spectrum of ordinary sun-light as to show the spectrum of bright lines given by the chromosphere, on any clear day. The telescope is adjusted so that a portion of the sun's limb, usually near a group of active sun-spots, shall be presented before the opened slit of the spectroscope. The light of the chromosphere thus admitted along with some diffused sun-light from the earth's atmosphere, produces a spectrum of intensely bright lines, widely separated, on the fainter background of the strongly dispersed spectrum of sun-light. The most prominent of these bright lines are those known as the C line (scarlet), F line (blue), which with several others are due to the hydrogen present in the chromosphere, the D₃ line (orange) ascribed to a little known substance called "helium", and occasionally the sodium lines D₁, D₃, (yellow). By adjusting the slit upon the scarlet C line, the appearances represented in Plate II. were observed as through an atmosphere of scarlet light: in the D or F lines identical appearances may be seen, but somewhat less clearly defined, as through yellow or blue light respectively. Hence the solar times, as here observed with the spectroscope in the hydrogen C line, are seen through a portion only (the scarlet rays) of the light coming from but one substance (hydrogen) of the companion incandescent substances present in the chromosphere. The color of the collective chromospheric light is seen directly with the telescope during an eclipse (See Plate III.) to be a delicate rosy pink.

Description of the Plate.—The black background represents the general darkness of the eye-piece to the spectroscope. The broad red stripe stretching from top to bottom of the Plate is a portion of the red band of the spectrum, magnified about 100 times as compared with the actual spectroscopic view. The upper and lower edges of the cross-section of dusky red correspond with the edges of the slit, opened widely enough to admit a view of the chromospheric crest and of the whole height of the protuberances at once. With a narrower opening of the slit this background would have been nearly black, its reddish cast increasing with the amount of opening and consequent admission of diffused sun-light. Rising above the lower edge of the opening is seen a small outer segment of the chromosphere, which, as a portion of the sun's eastern limb, should be imagined as moving directly towards the beholder. The seams and rifts by which its surface is broken, as well as the distorted forms of the huge protuberances show the chromosphere to be in violent agitation. Some of the most characteristic shapes of the eruptive protuberances are presented, as also cloud-like forms overtopping the rest. In the immediate foreground the bases of two towering columns appear deeply depressed below the general horizon of the segment observed, showing an extraordinary velocity of motion of the whole uplifted mass toward the observer. The highest of these protuberances was 126,000 miles in height at the moment of observation. The triple protuberance at the left with two drooping wings and a tall swaying spire tipped with a very bright flame, shows by its more brilliant color the higher temperature (and possibly compression) to which its gases have been subjected. The irregular black bands behind this protuberance indicate the presence there of less condensed and cooler clouds of the same gases. The dimmer jets of dame rising from the chromosphere are either vanishing protuberances, or, as in the case of the smallest jet shown at the extreme right of the horizon, are the tops of protuberances just coming into view.

Approximate scale, 6000 miles—1 inch.

PLATE III.—TOTAL ECLIPSE OF THE SUN. Observed July 29, 1878, at Creston, Wyoming Territory.

A telescopic view of the sun's corona or extreme outer atmosphere and of the solar flames or prominences during a total eclipse. At the moment of observation the dark disk of the moon, while still hiding the sun's main body, had passed far enough eastward to allow the rosy pink chromospheric prominences to be seen on its western border. On all sides of the sun's hidden disk, the corona shows its pale greenish light extending in halo-like rays and streamers, and two very remarkable wings stretch eastward and westward very nearly in the plane of the ecliptic and in the direction of the positions of Mercury and Venus respectively at the time of observation. The full extent of these wings could not be shown in the Plate without reducing its scale materially, since the westerly wing extended no less than twelve times the sun's diameter, and the easterly wing nearly as far, or over ten million miles. A circlet of bright light immediately bordering the moon's disk is the so-called inner corona, next to which the wings and streamers arc brightest, thence shading off imperceptibly into the twilight sky of the eclipse. Other noteworthy peculiarities of the corona, as observed during this eclipse, are the varying angles at which the radiating streamers are seen to project, the comparatively dark intervals between them, and the curved, wisp-like projections seen upon the wings. An especially noticeable gap appears where the most westerly of the upward streamers abruptly cuts off the view of the long wing. The largest and brightest of the curving streamers on the westerly wing coincides with the highest flame-like protuberance. To some observers of this eclipse the upward and downward streamers seemed pointed at their outer extremities and less regular in form.

Approximate scale, 135,000 miles—1 inch.

PLATE IV.—AURORA BOREALIS. As observed March 1, 1872, at 9h. 25m. P. M.

The view presents the rare spectacle of an aurora spanning the sky from east to west in concentric arches. The Polar Star is nearly central in the background, the constellation of the Great Bear on the right and Cassiopeia's chair on the left. The large star at some distance above the horizon on the right is Arcturus. The almost black inner segment of the aurora resting upon the horizon, has its summit in the magnetic meridian, which was in this case a little west of north, its arc being indented by the bases of the ascending streamers. Both streamers and arches were, when observed, tremulous with upward pulsations and there was also a wave-like movement of the streamers from west to east. The prevailing color of this aurora is a pale whitish green and the complementary red appears especially at the west end of the auroral arch. The summits of the streamers are from four hundred to live hundred miles above the earth and the aurora is therefore a phenomenon of the terrestrial atmosphere rather than of astronomical observation proper.

PLATE V.—THE ZODIACAL LIGHT. Observed February 20, 1876.

An observation of the cone of light whose axis lies along the Zodiac, whence it derives its name. It is drawn as seen in the west, with its base in the constellation Pisces, and its apex near the familiar group of the Pleiades in the constellation Taurus. The first bright star above the horizon in the base of the cone is the planet Venus and at some distance above is the reddish disk of Mars, the two being in rare companionship as evening stars. Above the constellation Pisces, two bright stars of Aries lie just outside the cone at the right. The nearest bright star above these at the right is Beta, the leading star of the constellation Triangula. Further at the right the three prominent stars nearly in a line are, in ascending order, Delta, Beta and Gamma of the constellation Andromeda. Above these at the left, the brightest star of a quadrangular group of four is the remarkable variable star Algol (Beta) of the constellation Perseus, which changes from the second to the fourth magnitude in a period of less than three days. At the left and a little above the Pleiades is the ruddy star Aldebaran, one of the Hyades and chief star in the constellation Taurus. These are the principal stars visible in this portion of the sky at the time of the observation. Their relative positions are represented as seen in the sky and not by the common method of star-atlases, which allows for the change from a spherical to a plane surface. Their magnitude in the order of brightness is indicated only approximately.

PLATE VI.—MARE HUMORUM. From a study made in 1875.

A view of one of the lunar plains, or so-called seas (Maria), with an encircling mountainous wall consisting of volcano-like craters in various stages of subsidence and dislocation. The sun-light coming from the west casts strong shadows from all the elevations eastward, and is just rising on the terminator, where the rugged structure of the Moon's surface is best seen. The lighter portions are the more elevated mountainous tracts and crater summits. The detailed description of this Plate given in the body of the Manual is repeated here for convenience of reference: The Mare Humorum, or sea of moisture, as it is called, is one of the smaller gray lunar plains. Its diameter, which is very nearly the same in all directions, is about 270 miles, the total area of this plain being about 50,000 square miles. It is one of the most distinct plains of the Moon, and is easily seen with the naked eye on the left-hand side of the disk. The floor of the plain is, like that of the other gray plains, traversed by several systems of very extended but low hills and ridges, while small craters are disseminated upon its surface. The color of this formation is of a dusky greenish gray along the border, while in the interior it is of a lighter shade, and is of brownish olivaceous tint. This plain, which is surrounded by high clefts and rifts, well illustrates the phenomena of dislocation and subsidence. The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000 feet in height, is seen in the upper left-hand corner of the gray plain. Close to Vitello at the east is the large broken ring-plain Lee, and farther east, and a little below, is a similarly broken crater called Doppelmayer. Both of these open craters have mountainous masses and peaks on their door, which is on a level with that of the Mare Humorum. A little below, and to the left of these objects, is dimly seen a deeply imbedded oval crater, whose walls barely rise above the level of the plain. On the right-hand side of the great plain is a long fault, with a system of fracture running along its border. On this right-hand side may be seen a part of the line of the terminator, which separates the light from the darkness. Towards the lower right-hand corner is the great ring-plain Gassendi, 55 miles in diameter, with its system of fractures and its central mountains, which rise from 3,000 to 4,000 feet above its floor. This crater slopes towards the plain, showing the subsidence to which it has been submitted. While the northern portion of the wall of this crater rises to 10,000 feet, that on the plain is only 500 feet high, and is even wholly demolished at one place where the floor of the crater is in direct communication with the plain. In the lower part of the sea, and a little to the west of the middle line, is found the crater Agatharchides, which shows below its north wall the marks of rills impressed by a flood of lava, which once issued from the side of the crater. On the left-hand side of the plain is seen the half-demolished crater Hippalus resembling a large bay, which has its interior strewn with peaks and mountains. On this same side can be seen one of the most important systems of clefts and fractures visible on the Moon, these clefts varying in length from 150 to 200 miles.

Approximate scale, 15 miles—1 inch.

PLATE VII.—PARTIAL ECLIPSE OF THE MOON. Observed October 24, 1874.

A view of the Moon partially obscured by the Earth's shadow, whose outline gives ocular proof of the earth's rotundity of form. The shadowed part of the Moon's surface is rendered visible by the diffused sun-light refracted upon it from the earth's atmosphere. Its reddish brown color is due to the absorption, by vapors present near the earth's surface, of a considerable part of this dim light. On both the obscured and illuminated tracts the configurations of the Moon's surface are seen as with the naked eye. The craters appear as distinct patches of lighter color, and the noticeably darker areas are the depressed plains or Maria. The large crater Tycho, at the lower part of the disk, is the most prominent of these objects, with its extensive system of radiating streaks. The largest crater above is Copernicus, at the left of which is Kepler and still above are Aristarchus and Herodotus appearing as if blended in one. Above and at the left of the great crater Tycho, the first dark tract is the Mare Humorum of Plate VI., seen in its natural position, with the crater Gassendi at its northern (upper) extremity and Vitello on its southern (lower) border. The advancing border of the shadow appears, as always, noticeably darker than the remainder, an effect probably of contrast. The illuminated segment of the Moon's disk has its usual appearance, the lighter portions being the more elevated mountainous surfaces, and the dark spaces the floors of extensive plains.

Approximate scale, 140 miles—1 inch.

PLATE VIII.—THE PLANET MARS. Observed September 3, 1877, at 11h. 55m. P. M.

A view of the southern hemisphere of Mars, when in the most favorable position for observation, and when exceptionally free from the clouds, which very frequently hide its surface configurations. Since, of all the planets, Mars is most like the earth, Plate VIII. may give a fair idea of the appearance of our globe to a supposed observer on Mars. The dark gray and black markings, are regarded as tracts of water, or of some liquid with similar powers of absorbing light; and, for the same reason, the lighter portions, of a prevailing reddish tint, are supposed to be bodies of land, while the bright white portions are variously due to clouds, to polar snow or ice, and the bright rim of white along the limb, to the depth of the atmosphere through which the limb is seen. The chief permanent features of the planet's surface have been named in honor of various astronomers.

The large dark tract on the left is De La Rue Ocean, the isolated oval spot near the centre is Terby Sea, and on the right is the western end of Maraldi Sea, with strongly indented border. Directly north of (below) De La Rue Ocean, is Maedler Continent; above it stretches Jacob Land; and surrounding Terby Sea is Secchi Continent. Extending into the centre of De La Rue Ocean is a curious double peninsula, called, in consequence of the dimness of former observations, Hall Island. The sharply defined, white-crested northern borders of De La Rue Ocean and Maraldi Sea may indicate the existence there of lofty coast ranges, more or less constantly covered with opaque clouds strongly reflecting light. The white spot in the centre of Maedler Continent, of a temporary nature, has a similar explanation. The intervals of olivaceous gray on Secchi Continent and elsewhere may perhaps be ascribed to the flooding and drying up of marshes and lowlands, as these markings have been observed to vary somewhat in connection with the change of seasons on Mars. The greenish tints observed along the planet's limb, alike on the darker and lighter surfaces, are probably due to an optical effect, the green being complementary to the prevailing red of the disk. The brilliant oval white spot near the southern (upper) pole of the planet is a so-called polar spot, in all probability consisting of a material similar to snow or ice and here observed in the midst of a dark open sea.

Approximate scale, 300 miles—1 inch.

PLATE IX.—THE PLANET JUPITER. Observed November 1, 1880, at 9h. 30m. P. M.

This planet is perpetually wrapped in dense clouds which hide its inner globe from view. The drawing shows Jupiter's outer clouded surface with its usual series of alternate light and dark belts, the disk as a whole appearing brighter in the centre than near the limb. The darker gray and black markings indicate in general the lower cloud-levels; that is, partial breaks or rifts in the cloudy envelope, whose prevailing depth apparently exceeds four thousand miles. While the deepest depression in the cloudy envelope is within the limits of the Great Red Spot, the vision may not even here penetrate very deeply. Two of Jupiter's four moons present bright disks near the planet's western limb, and cast their shadows far eastward on the disk, that of the "second satellite" falling upon the Red Spot. On the Red Spot are seen in addition two small black spots, no explanation of which can yet be offered. The broad white ring of clouds bordering the Red Spot appeared in constant motion. The central, or equatorial belt, shows brilliant cloudy masses of both the cumulus and stratus types, and the underlying gray and black cloudy surfaces are pervaded with the pinkish color characteristic of this belt. The dark circular spots on the wide white belt next north showed in their mode of formation striking resemblances to sun-spots. They afterward coalesced into a continuous pink belt. The diffusion of pinkish color over the three northernmost dark bands, as here observed, is unusual. About either pole is seen the uniform gray segment or polar cap. The equatorial diameter is noticeably longer than the polar diameter, a consequence of the planet's extraordinary swiftness of rotation. To the same cause may also be due chiefly the distribution of the cloudy belts parallel to the planet's equator, though the analogy of the terrestrial trade-winds fails to explain all the observed phenomena.

Approximate scale, 5,500 miles—1 inch.

PLATE X.—THE PLANET SATURN. Observed November 30, 1874, at 5h. 50m. P. M.

Saturn is unique amongst the planets in that its globe is encircled by a series of concentric rings, which lie in the plane of its equator, and consist, according to present theories, of vast throngs of minute bodies revolving about the planet, like so many satellites, in closely parallel orbits. The globe of Saturn, like that of Jupiter, is surrounded by cloudy belts parallel to its equator. The broad equatorial belt, of a delicate pinkish tint, is both brighter and more mottled than the narrower yellowish white belts, which alternate with darker belts of ashy gray on both the north and south sides, but are seen here only on the northern side. The disk has an oval shape, owing to the extreme polar compression of the globe.

The outer, middle and inner rings, with their various subdivisions, are clearly shown in Plate X., and are best seen on the so-called ansæ, or handles, projecting on either side. The gray outer ring is separated by the dusky pencil line into two divisions, both of which appear slightly mottled on the ansæ, as if with clouds. The middle ring has three sub-divisions which are clearly distinguishable, although separated by no dark interval, viz., a brilliant white outer zone, distinctly mottled, as seen on the extremities of the ansæ, and two interior zones of gradually diminishing brightness. The gauze or dusky ring is seen at its full width on the ansæ, but on the background of the strongly illuminated globe only its outer and presumably denser border is visible. The shadow of the globe on the rings is seen on the lower portion of the eastern ansæ. The shadow on the dusky ring is with difficulty perceptible; the shadow on the middle ring is slightly concave toward the planet, which concavity is abruptly increased on the outer zone of this ring; while the shadow on the outer ring slants away from the globe. These appearances are fully accounted for by supposing a general increase of level from the inner edge of the dusky ring to the outer margin of the middle ring, and a uniform lower level on the outer ring. Other observers have regarded the deflection of the shadow as an effect of irradiation. The inner margin of the double outer ring presents on both ansæ a number of slight indentations, which, if not actual irregularities in the contour of this ring, may be explained as shadows caused by elevations on the outer border of the middle ring, or possibly by over-hanging clouds.

Approximate scale, 6,500 miles—1 inch.

PLATE XI.—THE GREAT COMET OF 1881. Observed on the night of June 25-26, at 1h. 30m. A. M.

A view of the comet 1881, III., drawn as if seen with the naked eye, the minute details, however, being reproduced as seen with the telescope. The star-like nucleus is attended by four conical wings which cause it to appear diamond-shaped. The coma appears double, the brilliant inner coma, immediately enveloping the nucleus, being surrounded by a fainter exterior coma, which has a noticeable depression corresponding to that of the inner edge of the principal coma. The tail is divided lengthwise by a dark rift and is brightest on its convex or forward side. An inner portion of the tail, brighter than the rest, is more strongly curved, as if by solar repulsion. Stars are seen through the brighter parts of the tail, as they may be seen even through the coma and nucleus, with little diminution of their light.

PLATE XII.—THE NOVEMBER METEORS. As observed on the night of November 13-14, 1868.

A partially ideal view of the November Meteors, combining forms observed at different times during the night of Nov. 13th, 1868. It is not, however, a fanciful view, since a much larger number of meteors were observed falling at once during the shower of November, 1833, and at other times. The locality of the observation is shown by the Polar Star seen near the centre of the Plate, and Cassiopeia's Chair at the left. The general direction of the paths of the meteors is from the north-east, the radiant point of the shower having been in the constellation Leo, beyond and above Ursa Major. While the orbits of the meteors are, in general, curved regularly and slightly, several are shown with very eccentric paths, among them one which changed its course at a sharp angle. In the upper left-hand corner appear two vanishing trails of the "ring-form," and several others still further transformed into faint luminous patches of cloud. Red, yellow, green, blue and purple tints were observed in the meteors and their trails, as represented in the Plate.

PLATE XIII.—PART OF THE MILKY WAY. From a study made during the years 1874, 1875 and 1876.

The course of the portion of the Galaxy represented in Plate XIII. is as follows: From Cassiopeia's chair, three bright stars of which appear at the upper edge of the Plate, the Galaxy, forming two streams, descends south, passing partly through Lacerta on the left, and Cepheus on the right; at this last point it approaches nearest to the Polar Star, which is itself outside of the field of view. Then it enters Cygnus, where it becomes very complicated and bright, and where several large cloudy masses are seen terminating its left branch, which passes to the right, near the bright star Deneb, the leader of this constellation. Below Deneb, the Galaxy is apparently disconnected and separated from the northern part by a narrow, irregular dark gap. From this rupture, the Milky-way divides into two great streams, separated by an irregular dark rift. An immense branch extends to the right, which, after having formed an important luminous mass between the stars Gamma and Beta, continues its southward progress through parts of Lyra, Vulpecula, Hercules, Aquila and Ophiuchus, where it gradually terminates a few degrees south of the equator. The main stream on the left, after having formed a bright mass around Epsilon Cygni, passes through Vulpecula and then Aquila, where it crosses the equinoctial just below the star Eta, after having involved in its nebulosity the bright star Altair, the leader of Aquila. In the southern hemisphere the Galaxy becomes very complicated and forms a succession of very bright, irregular masses, the upper one being in Scutum Sobieskii, while the others are respectively situated in Sagittarius and in Scorpio; the last, just a little above our horizon, being always considerably dimmed by vapors. From Scutum Sobieskii, the Galaxy expands considerably on the right, and sends a branch into Scorpio, in which the fiery red star Antares is somewhat involved. In the upper left-hand corner of the Plate, at some distance from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes so magnificent an object to telescopic view.

PLATE XIV.—STAR-CLUSTER IN HERCULES. From a study made in June, 1877.

In the constellation Hercules, a small nebulous mass is faintly visible to the eye, a telescopic view of which is presented in Plate XIV. It is one of the most beautiful of the easily resolvable globular clusters. The brilliancy of the centre gives the cluster a distinctly globular appearance, while the several wings curving in various directions, have suggested to some observers an irregularly spiral structure. The large stars of the cluster are arranged in several groups which correspond, in a general way, with the faintly luminous wings.

PLATE XV.—THE GREAT NEBULA IN ORION. From a study made in the years 1875-76.

This nebula, which is one of the most brilliant and wonderful of telescopic objects, readily visible to the naked eve as a patch of nebulous light immediately surrounding the middle star of the three which form the sword of Orion, and a little south of the three well-known stars forming the belt. The small stars in this, as in other Plates of the series, are somewhat exaggerated in size, as was unavoidable with any mode of reproduction that could be employed. The bright pentagonal centre of the nebula is traversed by less luminous rifts, the several subdivisions thus outlined being irregularly mottled as if by bright fleecy clouds. Toward the lower part of this bright pentagonal centre is a comparatively dark space containing four bright stars which form a trapezium and together constitute the quadruple star Theta Orionis, which, to the naked eye, appears as the single star in the centre of the sword. On three sides of the central mass extend long bright wisps, whose curves fail, however, to reveal the spiral structure often attributed to this nebula. On the east a broad wing, with wave-shaped inner border, stretches southward. East of the trapezium are two especially noticeable dark spaces. Close to the main nebula on the north-east, a small faint nebula surrounds a bright star, and a branch from another faint stream of nebulous matter forming a loop to the southward, encloses the nebulous star (Iota Orionis) shown at the top of the Plate.