PUBLICATIONS

OF THE

Astronomical Society of the Pacific.

VOL. III. SAN FRANCISCO, CALIFORNIA, NOVEMBER 28, 1891.

No. 19.

PHOTO

ENLARGED DRAWINGS FROM LUNAR GRAPHS TAKEN AT THE LICK OBSERVATORY.

BY PROF. DR. L. WEINEK OF PRAGUE.*

In November, 1889, Professor EDWARD S. HOLDEN kindly made the proposition to me that he would send to Prague a series of the best negatives of the moon, taken with the 36-inch refractor of the Lick Observatory, in order that they might serve as a basis for my drawings of lunar craters and lunar landscapes made at the telescope, as well as to furnish subjects for special studies of the moon's surface. I was all the more eager to entertain such a proposition, because the Prague Observatory is possessed of only a six-inch telescope on a very inconvenient mounting, and because the opportunity was offered to take part in the magnificent results of the at present-largest instrument in the world.

Up to this time I had made fifty-four lunar drawings at the telescope, and I was justified, by reason of considerable practice of eye and hand, in taking up this new labor.

Since the beginning of the year 1890, Professor HOLDEN has sent to Prague continuously a large number of plates for the different days of a lunation, and I am greatly indebted to him for creating an incentive to do this work, as well as for the valuable donation of these beautiful plates to the Prague Observatory from the Lick Observatory.

For the purpose in view I had built at Dresden a suitable apparatus before beginning the task. In this the photographic plate is viewed by transmitted light by means of two eye-pieces of 41.15 and 25.20 millimetres (equivalent) focus. (Linear enlargement. 7.8 and 12.1 for my distinct vision of 28 centim.) The holder or

* Translated for the Society by OTTO VON GELDERN, Esq.

stand of the eye-pieces possessed a double motion in two co-ordinate directions normal to each other; it admitted of a ready and rapid adjustment of the portion of the moon viewed, and kept the chosen lens at a constant distance from the plate. The illumination of the latter may be obtained by daylight or by means of a lamp; in the case of the former a parabolic reflector may be so set or turned as to furnish the desired degree of light. For drawing, diffused daylight is used, by inserting a dull ground glass between the plate and the reflecting mirror. The window at the left hand of the draughtsman throws light at the same time on the small drawingboard which holds the paper. A handy little drawing-table

attached to the lower part of the apparatus, completes the outfit. That the person engaged in viewing or drawing may not become too easily fatigued, the plate with the mirror may be turned backward and set at will to any desired angle. The apparatus is arranged for large plates as well as for very small ones.

Since every direct photographic enlargement presents numerous defects, and will be found inferior to the original, particularly in strength and intensity, it was not a superfluous undertaking to prepare enlarged drawings or tintings from the original platesdrawings that were to be made in this case with the greatest exactness and absolute truth as to strength and clearness-for the reason, that work done in this manner may be continued at any time, and can be constantly controlled and improved upon.

In this particular two methods were available; the first, by having very faint photographic enlargements of certain particularly interesting portions of the lunar surface thrown upon suitable paper and by retouching them subsequently to the fullest intensity of the original; and the second, by discarding the photographic aid altogether and projecting a mathematically accurate enlargement to any desired scale, by means of suitable instruments, directly upon the best white drawing-paper, upon which the representation is then newly developed and entirely built up.

In the first or photographic method, which I used in making a four-fold enlargement of the Mare Crisium, from Apollonius in the south to Geminus in the north, taken from the beautiful Lickexposure of August 23d, 1888, a work requiring 3434 hours, I found it very unfavorable that the so-called salt paper (salz-papier), which is commonly in use in photographing coloring, is of so hygroscopic a nature that it requires the most careful and painstaking drying at a considerable expense of time; and again I

have found that every photographic paper will lose in whiteness by reason of the chemical process it has to undergo, and in consequence will not admit of showing the lighter portions of the moon with sufficient brightness. It was principally for the latter reason that I soon adopted the second method. Since an exact tracing of contours was unavoidable, and having found that even the most transparent papers with millimetre graduations were unsatisfactory, I had glass scales made containing precise divisions into square millimetres. These as well as the above described drawing apparatus were made by the mechanician HEYDE of Dresden, who after a few trials succeeded in etching these scales. Such a scale is now laid, with its etched side down, upon the plate and held there by means of two springs at opposite ends; that part of the lunar surface which it is desired to enlarge is followed in the smaller squares on the glass, and reproduced in similar squares previously drawn on the paper to any required degree of enlargement. In this manner the originals of the two lithographic copies accompanying this paper, of the magnificent ring-plain of Archimedes were drawn, magnified ten times.

The picture on the left belongs to the Lick-exposure of August 15th, 1888 (age of the moon eight days), and required for its final and finished completion in tints 4434 working hours, while the one on the right is from an exposure taken at the Lick Observatory on the 27th of August, 1888 (age of the moon twenty days), and required a working-time of 43 hours. Each is of a size of five by seven centimetres.

Let us now proceed to the scientific discussion of these drawings. In what follows the one on the left will be designated I, that on the right II.

At first glance it is recognized, that Archimedes appears somewhat larger on I than on II, although both drawings were magnified from the originals exactly ten times. From this it must be concluded that the lunar diameter of the photograph taken on the 15th of August, 1888, must have been larger than on the one taken August 27th, 1888. And indeed we find that, measuring in the line of the zero meridian (direction from the eastern wall of Autolycus to the west wall of Ptolemy) the diameter is 130.0 millimetres for the first named plate, while for the second it is only 119.7 millimetres. This is further explained by the fact that I was taken near the time of the moon's perigee (which took place on August 14th at o" Greenwich mean time) whereas II was ob

tained near the apogee of the moon (occurring on August 28th at 1 Greenwich mean time). If for an approximation the corrections for refraction, parallax and libration be left unconsidered, the arithmetical mean of the dimensions of I and II may be looked upon as that corresponding to the mean distance of the moon from the earth, which, as well known, was used in the large lunar charts of one metre by LOHRMAN and MÄDLER, and the 2-metre chart made by SCHMIDT. Since the latter is based upon observations extending through a period of over thirty years, and is the most detailed, let us adopt it for the purpose of making our comparisons. Archimedes is shown upon section IV of the same. As far as the effects of libration are specially concerned, a simple reflection will show that on account of the juxtaposition of Archimedes to the central meridian, it is principally the libration in latitude that need be considered. Since the centre of the moon was 4° above the ecliptic on the 15th of August, and on the 27th 5° below it, the first exposure would necessarily show more of the southerly portions, and the second more of the northerly parts of the lunar surface. But since Archimedes is situated in about 30° selenographic latitude in the northern hemisphere, it follows that this walled-plain must lie nearer to the visible edge of the moon on I than upon II, which is confirmed by comparing the two photographs. For this reason the meridional dimensions of I must appear perspectively shortened and those of II perspectively lengthened, when compared with those that occur under a mean geocentric libration in latitude (the moon's centre being in the ecliptic); and in our case this must be so in nearly equal amounts, so that, roughly approximated, we may consider the mean of I and II as free from all libratory effects.

In order to make this subject intelligible to the reader without the aid of a contour drawing from I or II, and without adopting a nomenclature of figures or letters for the numerous objects of each picture; and in order to permit a most careful comparison with SCHMIDT's chart, it will be well to characterize the single objects by means of polar-co-ordinates referred to some easily determined initial point. As such the centre of the ring-plain might be taken, provided that its periphery would prove sufficiently regular for the purpose. But this is not the case. I have therefore joined the centres of the two craters, located southeast and northwest from Archimedes respectively, crater A (shown in the picture in the upper right hand corner) and C (at the lower