EXAMINATION OF URANUS FOR THE DETECTION OF NEW SATELLITES. As the opposition of Uranus comes at a time of the year when our weather is most unfavorable, there have been very few occasions when the planet has been really well seen. Magnifying powers of 520 and 700 have generally been used, and very rarely 1000. In our best seeing, it would be advantageous to employ 1500 and even 2000. Under fairly good conditions, we have usually seen Umbriel and Ariel in all parts of their orbits. We are satisfied that no new satellite half as bright as Ariel at elongation exists within the orbit of Umbriel. It is not likely that any such object exists within the orbit of Titania. When we come to the space beyond Oberon it is not possible to speak with certainty from our past observations. With an aperture as large as 36 inches, many small stars are visible in the field of view. If the atmosphere is steady and the circumstances are good, their places can be quickly determined. If, however, the conditions are unfavorable, as they ordinarily have been, it is often necessary to employ all the available time in making sure of the existence of some of the objects seen, and therefore it is possible that others have escaped notice. It is not practicable to be more definite in statement at present. E. S. H., J. M. S. DISCOVERY OF a New CRATER ON THE MOON Negatives OF THE LICK OBSERVATORY, BY PROFESSOR WEINEK OF PRAGUE. [It is known that Professor WEINEK is making a careful study of the Moon negatives of the Lick Observatory, and is recording his results in drawings from the negatives which he enlarges 10fold. See Publ. A. S. P. vol. III, page 253. He has already spent more than 200 hours of actual work on such drawings on 101 different days and has completed several drawings which it is hoped to publish shortly. Some 40-50 hours of work are required for a single crater. He has lately discovered a new crater on one of our negatives and at his request I give a brief description of its position, etc. I may add that a letter lately received from Professor WEINEK says that he has found that our best negatives (as for example that of August 31, 1890) require to be enlarged in the drawing twenty-fold in order to show all the features. This corresponds to a magnifying power of more than 1000 diameters in the telescope; or to put it in another way, the diameter of the full Moon on such a drawing would be more than nine feet. The results already obtained by Professor WEINEK completely justify the opinion of our Moon negatives which I expressed in the Publications, vol. II, p. 15. E. S. H.] Extract from a letter of Professor Weinek. K. K. STERNWARTE IN PRAG, 1891, May 23. I send you to-day a rapid sketch of a part of the Moon southwest of Pallas and Bode (southeast of Triesnecker) copied from the negative of the Moon made at the Lick Observatory August 15, 1888. This shows east of A and B [SCHMIDT'S Map, section I] a crateriform object which is not laid down by either SCHMIDT, MAEDLER, LOHRMANN, or NEISON. * * * To explain my description of the position of the new object, I take as the origin of co-ordinates (O) the middle point of A B. The meridian through O lies, according to SCHMIDT, in the picture 7° to the left of B O, above, and 7° to the right of A O, below [north]. I call the position-angle (Þ) the angle from the northend of the meridian counted towards the east. This angle together with the distance (d) determines the place of the new crater. [On my drawing, ten-fold enlarged, I find] AB=33.7 m m.; d=8.5 m m.; p=75°; diameter= 1.8 mm. "The question now is whether this is due to some imperfection of the plate. If it is a real object, the discovery is interesting, for SCHMIDT has drawn everything in this neighborhood as level. * * * If the object is real it has a different character from that of the two craters towards the south, because its shadow shows a greater extent. It cannot be funnel-shaped, but it must have a flat base and its west wall must be higher than the east one, and the west wall must decline gently towards the exterior. It may be called a crater-pit or round valley * L. WEINEK. [This discovery of Professor WEINEK'S was verified by me on the L. O. negatives of Aug. 24 and Nov. 3, 1890. The negative of Sept. 22, 1890, shows the region very well, and it is especially interesting as exhibiting an extensive system of rills, all of which are new, and some of which are formed of confluent craters. Professor WEINER'S new crater lies at the intersection of two of these rills. A sketch of the neighborhood, showing the new crater and some of the rills was made from the L. O. negative of Sept. 22 and sent to Professor WEINEK on June 10. On June 15 I examined the region with the large telescope and sent Professor WEINEK a copy of my drawing. It shows the new crater as circular with an interior shadow. It seems to be situated in the exact centre of a raised mound (of its own lava?) On the edge of this mound is a companion crater also new. Other new features were seen and mapped, and they will be published in due time. E. S. H.] APPOINTMENT OF DR. HENRY CREW AS ASTRONOMER IN THE LICK OBSERVATORY. The Regents of the University of California elected Dr. HENRY CREW, Instructor of Physics in Haverford College, as Astronomer in the Lick Observatory at their regular meeting of July 14, 1891. E. S. H. PERMANENT IMPROVEMENTS AT MOUNT HAMILTON. Observatory for the Crocker Photographic Telescope: A new observatory has been built to cover the WILLARD photographic lens (a 5.9 inches, f= 31 inches) and its mounting by BRASHEAR, which have been presented to the Lick Observatory by Hon. C. F. CROCKER. The dome is ten feet in diameter, and attached to it is a photographic dark room, etc., about ten feet by eleven feet. The building has been placed on the slope of the hill a few feet south of HUYGHENS reservoir. This equatorial mounting is stout enough to carry an eight-inch refractor. New Store House and Wood Shed on the Summit: The proposed improvements to the surroundings of the tomb of JAMES LICK make it necessary to provide a store house to contain various tools and pieces of apparatus (the electric lighting engine presented to the Lick Observatory by the EDISON Manufacturing Company, among others), and accordingly the Regents of the University have set apart the sum of $1500 to provide for such a building, and for a wood shed, which is much needed. These buildings will be placed on the summit-plateau almost directly east of the 75-foot dome. E. S. H. ERRATUM IN NEISON'S "MOON." In the Map facing page 130 interchange the names Autolychus and Aristillus. E. S. H. ON THE DETERMINATION OF STELLAR MAGNITUDES BY MEANS OF PHOTOGRAPHY, BY DR. C. V. L. CHARLIER. [The following extracts from a private letter to Professor HOLDEN are printed by permission of Dr. CHARLIER; and Professor SCHAEBERLE has added a brief commentary.] You will allow me to express my opinion as to your discussion of the determination of photographic magnitudes in No. 5 of your Publications (Vol. I, page 112), and to add a few words regarding the memoir of Professor SCHAEBERLE on the same subject (Vol. I, page 51). Against the fine method of Mr. SCHAEBERLE for determining the magnitudes of stars by photography it seems to me that the following remarks must be made: 1) . . . . The formula (d = a + Blog D+y. D. log f) that he employs to express the relation between the dimensions of the star discs, the time of exposure and the aperture of the objective, is not suitable for general application. In fact it must be remembered that for the most of the instruments hitherto examined the diameters of the star-discs seem to increase proportionally to a certain power of the time of exposure. Now in all these cases I think the formula of Mr. SCHAEBERLE cannot be applied. Take for instance the instrument which I have employed in Stockholm in my photometric researches, for which I found the law (2) d = dt. If we seek to determine the constants ƒ and g in the formula of Mr. SCHAEBERLE d=f+g log t so as to satisfy the observed values of the diameters of Polaris (page 7 in my memoir), we obtain f 21.6 g 43.3 and the different computed values of d as compared with the observed ones are given in the following table, where I have written down also the values, which follow according to the formula (2). SCHAEBERLE: 22, 35, 48, 61, 74, 87, 100, 112, 125, 139 OBSERVED: CHARLIER: 34, 41, 45, 56, 60, 78, 89, 108, 131, 156 92, 110, 131, 156 You find from this comparison that (2) almost completely reproduces the observations while the formula of Mr. SCHAEBERLE shows a distinct and systematic difference. It is very interesting that photographic instruments seem to belong, in general, to two different classes. For one class the formula of Mr. SCHAEBERLE is the most suitable—for instance for the photographic telescope employed in the Lick Observatory or in Potsdam-for the other, and it seems the greater number, the increase of the star-discs proceeds proportionally to a certain power of the time of exposure. Is the cause of this anomaly to be sought in the different qualities of the plates employed? Or does it depend upon the objective? It seems to me that the theory of the development of the image is closely connected with this phenomenon. 2.) The other remark that I would make on Mr. SCHAEBERLE'S method is as follows: The application of (1) for the determination of stellar magnitudes requires us to assume that the brightness of the image of a star in the focus of an objective is proportional to the free aperture of the objective. Now this assumption is somewhat uncertain. Almost all observers, who have made researches in stellar photometry, have shown, that this proportionality only exists within narrow limits. For the ZÖLLNER photometer of the Observatory in Upsala, for instance, I have obtained (Astrophotometrische Studien, page 11) a difference greater than half a magnitude in applying the law in question by diminishing the aperture to the fifth of its original value. The application of the method of Mr. SCHAEBERLE to this instrument would accordingly be allowable only when the difference in stellar magnitude is very small (in maximum 1.5 magnitudes). Now this may be peculiar to the instrument, in question, and the circumstances may be different for the instrument of Mr. SCHAEBERLE and perhaps more favorable. But the main point is that the proportionality of the aperture of the objective and the brightness of the image is somewhat doubtful. The difficulty can only be removed by a special research on the relation between these two factors for the special instrument employed in such investigations. Finally, I venture to say a few words regarding your own proposal to establish a system for determining photographic magnitudes. It now seems to me that the method proposed by you is in principle the best possible. In the research which I made on the brightness of the Pleiades, the circumstances were more favorable than in the general problem. Most of the stars being of the same spectral type it was a priori probable that the ratio between the photographic brightness of the different stars * See Publ. A. S. P. Vol. I. page 119. |