Astronomy. — ''Investigation of a galactic cloud in A(///ila." Hy 

 Dr. A. Pannkkop^k. (Coramnnieated by Piof. W. de Sittkr). 



(Communicated in the meeting of March 29, 1919). 



A coinmunication to the Meeting of this Academy on 

 Dec. 8, 1911, described how, by means of some photographs, 

 it is possible to obtain data about the increase of star-density with 

 decreasing limiting magnitude. There it was stated already that 

 Prof. Hektzsprung of Potsdam, by means of the Zeisstriplet of the 

 Astrophysical Observatory, had made some photographs (of the 

 galactic cloud N.W. of y Aquilae), in order to test the method. 

 Various circumstances, however, prevented a final discussion of these 

 plates until quite recently. 



The plates are 20 X 20 cm., the centre lies near x Aquilae, and 

 the region that was photographed is 6° square. The plates immediately 

 used for this purpose are : 



Nr. 328 Sept. 2 1910 Expos. 600, 600, 1 90, 60, 19, 6, 2 sec. (plate A) 

 Nr. 329 Sept. 2 1910 Expos. 1900, 1900 sec. (plate /^) 



Nr. 1260 Aug. 241911 Expos. 40'", Halbgitter North (plate C'J 



Nr. 1261 Aug. 241911 Expos. 45™, Halbgitter South (plated/,) 



As for the counting of plates A and B no reseau was printed 

 on the plates themselves, a reseau of öVa ram. interval was photo- 

 graphed on a separate glassplate instead, which reseau-plate, during 

 the counting, was firmly clasped to the counting-plates. 



1. The countings. On plate A were counted in each square firstly 

 the numbers of stars with only 2 equal images, secondly those with 

 moreover a 3''^ image, (190^ exp.) thirdly those with 4 images, (a 

 still visible image therefore for 60-), those with 5, and with 6 images. 

 The respective limiting magnitudes differ about 1 magnitude; accord- 

 ing to some provisional comparisons with a photograph of the North 



polar region they amounted to 13,0 9,0. The uncertainty and 



the subjective differences of conception so common in star-counting, 

 the faintest star-images not being discernible from casual spots in 

 the plate, are practically done away with here, as every star must 

 present two equal images at a known distance, or as a faint image 

 must present itself at a given spot near the brighter images. Yet 

 this does not do away entirely with the uncertainty in counting; 



