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.