ASTRONOMY, PROGRESS OF, IN 1891. 



rapid rotation of Jupiter and Saturn on their 

 axes prevents prolonged exposures. So great is 

 this latter hindrance that as yet no photo- 

 graphs of these two great planets have been se- 

 cured that equal the delineations made by skilled 

 draughtsmen. In photographing the nebulae, 

 however, the exposures may last for hours if de- 

 sired, and, in extent and depth of structural de- 

 tail, the photographs far eclipse all visual revela- 

 tions even when assisted by mammoth telescopes. 



All attempts to photograph the solar corona 

 without an eclipse have thus far been futile, as 

 the sky luminosity so fogs the negative plates 

 as to obliterate every coronal imprint. Com- 

 menting on this matter, Mr. Burnham, in hi? 

 " Cayenne Eclipse Report," says : " Those who 

 have given long exposures, or advocated doing so, 

 with the object of getting the greatest possible 

 extension of corona on the plate, must have over- 

 looked the fact that the background of the sky 

 is luminous, and would soon fog a sensitive plate 

 if all other light were excluded. The problem, 

 photographically considered, has nothing to do 

 with the matter of photographing a nebula on 

 the black ground of the sky at night. There the 

 only light which reaches the plate comes from 

 the object to be photographed, and the exposure 

 can be indefinitely prolonged, with the result of 

 constantly increasing the impression made by 

 the fainter nebulous light. Undoubtedly the 

 coronal light would act on the plate in the same 

 manner if all other light could be excluded, and 

 we should have a picture in extent and detail far 

 beyond the most wonderful display ever seen 

 with the naked eye." 



It is the opinion of physicists that, above our 

 atmosphere, the sky by day, even in the neigh- 

 borhood of the sun, would be intensely black, 

 under which conditions the corona could be 

 photographed as well as the brightest nebula by 

 night. Were it possible to attain to this, it is 

 highly probable that during a total solar eclipse 

 a single photographic expert would give more 

 instruction than the combined results of a large 

 and expensive expedition. But no such condi- 

 tion can ever be realized at ordinary heights, 

 though an appreciable step toward it may be 

 reached by working from elevations of 15,000 or 

 20,000 feet above the sea. In pursuance of this 

 idea, M. Janssen has proposed the building of 

 an observatory on Mont Blanc, providing rock 

 could be reached at a reasonable depth ; but as 

 borings have been made through the ice cap to a 

 depth of thirty feet, and, horizontally, to a dis- 

 tance of ninety feet without finding the desired 

 rocky foundation, the project has been postponed 

 until another season, at least, and, perhaps, per- 

 manently abandoned. 



Photographic Chart of the Sky. The last 

 reunion of the International Committee on the 

 photographic chart of the entire sky was re- 

 cently held. Of the eighteen observatories as- 

 signed to the task, all are ready, and some have 

 already secured a numbei of satisfactory trial 

 plates. Many delicate and difficult questions 

 arose, which can not be discussed here, most of 

 which, it is hoped, have been solved, though 

 doubtless others will present themselves as the 

 work progresses. About three years have been 

 occupied in constructing the ph'otographic tele- 

 scopes, in organizing, and in settling many pre- 



liminaries made necessary by want of experience. 

 The conference demands a great photographic 

 chart of the heavens with exposures of forty 

 minutes, with which it is expected to reach stars 

 down to the fourteenth magnitude ; and, as each 

 plate is to be limited to four square degrees, and, 

 as each star, to avoid -errors, is to appear on two 

 plates, over 22,055 photographs will be required. 

 Besides the plates requisite for the great chart, a 

 second set to form a catalogue is to be made, with 

 shorter exposures, and to include stars only to the 

 eleventh magnitude. The plates are to be pre- 

 pared by photographing upon them very faint, 

 delicate lines, called by the French reseaux, five 

 millimetres apart, which will greatly facilitate 

 the tedious process of measuring and cataloguing 

 several million stars. 



The following are the observatories taking 

 part in this work, with the number of plates ne- 

 cessary to complete each zone : 



Observatories. No. of plates. 



Greenwich 1,149 



Rome ] ,040 



Catane 1,008 



Hclsingfors l.OOS 



Potsdam 1,232 



Oxford 1,180 



Paris 1,260 



Bordeaux 1,260 



Toulouse 1,080 



Observatories. No. of plates. 



Algiers 1,560 



San Fernando 1,2CO 



Tacubaya 1,260 



Santiago 1.260 



La Plata 1,360 



Uio de Janeiro 1 ,073 



Cape of Good Hope 1,512 



Sydney 1,400 



Melbourne 1,149 



In giving a sufficiently long exposure to secure 

 very faint stars, the brighter are overexposed 

 and their images enlarged, though to just what 

 extent is difficult to determine. Again, stars of 

 different colors require longer or shorter time to 

 impress themselves on the gelatin film, and the 

 ever-changing conditions of the atmosphere at 

 different times and stations impose great diffi- 

 culties, which it seems impossible to obviate. 



Variable Stars. Prof. Seth C. Chandler, an 

 authority on variables, makes the, statement that 

 two thousand variable stars are visible with an 

 opera-glass, while hundreds of thousands are re- 

 vealed by the largest telescopes. Their periods 

 range from 7 h 46 m 48", the shortest known, to 

 several years in length. The shortest is known 

 as S. Antliae, No. 3,407 of Chandler's Cata- 

 logue, right ascension A. D. 1900-0, 9 h 27 m 54', 

 declination south 28 12'. A photograph of its 

 spectrum, made at Harvard College Observatory 

 in April, 1890, indicated by the widening of 

 some of the lines and the narrowing of others 

 that it belongs to an intermediate class between 

 variable stars of first and second type. 



Spectroscopic Binaries. The number of 

 double stars now known, many of which are bi- 

 naries, amounts to several thousand, and the list 

 is constantly being augmented by further dis- 

 coveries. These are visually double by the aid 

 of the telescope, though some are so close as to 

 suffer only a slight elongation under the highest 

 powers our greatest telescopes will bear. It is 

 not these we wish to consider, but a new class of 

 binaries, which, while known to be such, are not 

 thus seen even in the most powerful telescopes. 

 They are called spectroscopic binaries, and num- 

 ber only about a dozen, including a few suspects ; 

 'but in the near future, because of the increase 

 i ) the number and efficiency of spectroscopes in 

 tift hands of zealous astronomers, this roll must 

 be greatly extended. 



Dr. Vogel has examined early photographs 



