CATALOGUE of Positive Copies of the ORIGINAL NEGATIVES OF JUPITER TAKEN AT THE LICK OBSERVATORY IN 1890, AND NOW IN THE LIBRARY OF THE R. A. S.

The copies were made by A. STANLEY WILLIAMS, Esq., F. R. A. S., and are to be deposited in the collection at the Lick Observatory.

DEAR PROF. HOLDEN: I send this to let you know that clouds so obscured the moon on the evening of the 15th that no observations could be taken of the eclipse.

THE ROYAL ASTROPHYSICAL OBSERVATORY OF POTSDAM,* BY ARMIN O. LEUSCHNER.

From time to time there have appeared in these publications pictures of many of the larger observatories of the world with descriptions from the pen of Prof. HOLDEN. Upon my return from Germany this fall, where I visited a number of observatories, Prof. HOLDEN invited me to write an account of the Potsdam Observatory, which I take pleasure in doing. Much of the short account that follows is condensed from Lieut. WINTERHALTER'S Report on European Observatories (see Publ. A. S. P., vol. III, page 40). The accompanying wood-cut is copied from the same report with the kind permission of the superintendent of the U. S. Naval Observatory.

During the early decades of the present century most of the observatories of Europe were built or rebuilt. It was natural on every account that they should be placed in the midst of towns and cities near the administrative offices of the Universities or of the governments to which they belonged. The vast industrial progress of civilized countries in the last half of the XIXth Century has brought with it the electric light, heavy traffic in the streets, railways and tramways, and clouds of dust and smoke overhanging all great cities. Thus it is that nearly all the observatories founded or rebuilt in the years 1830-50, as well as many of the older institutions, found themselves in 1870-1880 in very undesirable situations. The transparency of the air was ruined by the dust and smoke, the darkness of the night-sky was lost in the presence of the electric light, and observations of precision, which require a perfectly steady foundation, could not be made on sites which were subject to the tremors caused by the passage of wagons, trams and railway cars.

The Royal Observatory of Berlin (founded in 1705, rebuilt in 1835) suffered from the causes just mentioned. It was desired to add to this observatory new departments devoted to astronomical physics, spectroscopy, photography, etc., etc., and it was obvious that such new departments ought not to be located near the parent institution in the midst of a great city. Little progress, however, was made in obtaining the consent and the support of the government until the year 1871, when the late emperor,

* Professor H. C. VOGEL, Director.-See the plate, page 24.

FREDERICK WILLIAM, then crown-prince, became deeply interested in the question of founding an observatory such as was desired. Accordingly about 1874, chiefly through his influence, it was determined to found a Sonnenwarte (solar-observatory) as distinguished from a Sternwarte (star-observatory) near Berlin, and the new establishment was located in the vicinity of Potsdam, the summer residence of German emperors, which can be reached by local train from Berlin in about thirty minutes. The grounds reserved for the observatory comprise thirty-two acres, and these again are bordered by the Royal Parks for which Potsdam is famous and by the grounds reserved for a large geodetic observatory, now in process of construction. The situation is a model one on every account.

The grounds themselves, especially the portions immediately surrounding the observatory buildings and the residences of the director and of the astronomers, are ornamented with flower-beds, lawns, decorative shrubs, trees, etc., and in all respects form a most attractive park. The observatory reservation is easily. reached on foot from the Potsdam railway-station in about fifteen minutes. An excellent walk and drive, gently ascending from the city and leading through a wonderful grove of old birch trees, brings us up to a huge gate, the entrance to the grounds. There the wanderer halts bewildered by these high enclosures and that huge gate, through the iron bars of which he discerns at a distance the walls of a grand building shining through dense trees. And had he not already been informed that these walls belonged to the new Astrophysikalisches Observatorium, he might well take them for one of the old castles which remind us of by-gone generations -so completely is this modern structure hidden from our view when we first reach the gate. Yet only for a moment might we thus be misled, for upon having been admitted to the grounds, we at once encounter a modest building of recent origin, -the home of the machinist, who attends to the machinery which furnishes heat, light and water to the observatory, buildings and grounds. On we go and presently find ourselves face to face with a small round building, unmistakably the upper portion of a well. And a wonderful well this is too, for it has a depth of forty-six metres and in it is a little geophysical observatory. At a depth of about twenty-four metres a small observing-room is built into one side of the well, and at various other depths meteorological instruments. are suspended. Connected with this well is a moderate-sized

engine-house through which the water from the well is forced up to the highest level on the grounds, namely, to a reservoir located in the tower above the main entrance of the observatory. The engine-house also contains gas-works, which supply a large number of street-lanterns placed in profusion throughout the grounds.

Leaving the well and engine-house and passing the residences of the astronomers, of which the directors' dwelling-house is nearest to the observatory, we reach, after a little walk through shady paths, the main building, the center of attraction in this wonderful home of science. The scope of the observatory was enlarged about 1882 (or at least the enlarged scope was recognized by a change of name) and it is now known as the Astrophysikalisches Observatorium—an observatory devoted rather to researches into the physical conditions of all the stars, than to the astronomy which has to do with merely fixing their positions. The building, consisting of a central body and two wings, is surmounted by three domes, which cover a refractor (objective by SCHROEDER, mounting by REPSOLD) of about twelve inches aperture, an 8-inch refractor by GRUBB (used for observations of solar spots) and a 5-inch refractor by STEINHEIL (used for spectroscopic observation of the sun and for photometric work, etc.). The ground floor of the building contains the laboratories and offices; the basement contains the steam-engine, dynamo, instrument-makers' shop, etc.

In order to secure an even temperature, portions of the roof are covered with sods in which grass is grown, which is a decided novelty in observatory construction, but one which should be effective. Upon entering the building under a square tower, which as was said contains the large reservoir, we find ourselves in a spacious hall running in the direction of the meridian. A marble plate with inscription in gold set in the wall to our left reminds us of the untiring interest taken by the late emperor FREDERICK in giving the world this grand observatory. To the left also are the offices of the director, Prof. VOGEL, and of Dr. SCHEINER, the astronomer. Here it is where these wonderful photographs of star-spectra are scrutinized under the microscope. Here, too, have been written by these eminent astronomers the many astronomical papers dealing with new and important discoveries. Passing through a sort of ante-chamber which is used for photographic purposes, we enter a large apartment containing one of the most beautiful instruments ever constructed for spectrographic investi

gations, a spectrometer by BAMBERG admitting of a very high dispersion and especially valuable on account of its excellently constructed micrometer and because of the accuracy of its circle, whose divisions, as Prof. VOGEL assured me, are almost perfect. This instrument is chiefly used on the solar spectrum, the rays of the sun being thrown upon the objective of the collimator in a horizontal direction by means of a heliostat.

There are a great many other instruments in the glass-cases of this room, the most interesting of which is a spectrometer (by SCHROEDER in Hamburg) with nineteen prisms. This instrument has been extensively used in studying the spectrum of the sun. To the left under the large telescope is the clock-room, and in a projection of the building to the south is a heliograph with which photographs of the sun are regularly taken. The heliograph is a fixed telescope of 6.3 inches in aperture and 157 inches focus, into which the sun's image is reflected by a heliostat. (The axis of the Potsdam heliograph is inclined parallel to the axis of the earth; in other respects it is similar in general principle to the 40-foot horizontal photoheliograph of the Lick Observatory.)

To the right of the main hall are an instrument-room and a study for assistants, where on a large table the latest numbers of astronomical bulletins of all countries are exhibited, and in a conspicuous place among these we are pleased to see the Publications of the Astronomical Society of the Pacific.

From the main body of the building arched corridors lead to the east and west wings whose towers contain the two smaller telescopes already mentioned. The east tower harbors also a rapidly-growing library. To the north, in front of these wings, various meteorological instruments are exposed. From the domes we step on the roof from which a beautiful view of the surrounding country and cities may be obtained and from here into the big dome with the large equatorial. This instrument has been used for very many important researches and is at present engaged in the determination of the motions of the brighter stars in the line of sight by means of photography. An ingenious addition to the eye-end of the telescope carries the very powerful spectroscope. The spectrum of the star and the comparison spectrum are photographed by exposures of from fifteen to sixty minutes, and subsequently enlarged by another ingenious device so as to be suitable for measurement.

The results already obtained for the motion of stars in the line