Astronomy. — " The distance-correctioii for the platen oj the 

 Harvard Map of the Sky". By Dr. H. Noht. (Communicated 

 by Prof. J. C. Kapteyn). 



(Communicated in llie meeting of January 25, 1919). 



It is a well-known fact that the limiting magnitude at the centre 

 of a celestial pliotograph differs from that near its margin. If, e.g., 

 the centre of a plate shows stars down to the photographic magni- 

 tude 11.0, this plate will at a certain distance from the centre show 

 stars not fainter than, say, 10"\8. In the work of star-counts on 

 photographic plates, therefore, it will be necessary to know for 

 every plate the limiting magnitude as a function of the distance 

 from its centre — or, as it is usually expressed: the distance-cor- 

 rection should be determined for each of the plates separately. 



If we knew the photographic magnitude of a sufKiciently large 

 number of stars, then we should be able to determine directly the 

 limiting magnitude for all parts of the plate and the distance-cor- 

 rection could easily be found. But, for a long time to come, photo- 

 graphic standards will be wanting and therefore, generally speaking, 

 a direct determination of the distance-correction for plates covering 

 a considerable part of the sky, is impossible. For such plates the 

 only way is to use an indirect method, but this leads to difïiculties, 

 all of which have not yet been overcome. It is the aim of the 

 present paper to deal with some of these difficulties. 



In a previous investigation^) I have deduced the distance-correc- 

 tion for the Harvard Map of the Shj, a collection of 55 negatives 

 on glass, on which Hknie had made star-counts *). In this research 

 I used the following method: firstly the variation of the star-density 

 on each plate with the distance from the centre has been examined. 

 It was tacitly assumed — a»id with regard to the following pages 

 I want to emphasize this especially — that this density, without the so- 

 called "'Bihboölhnny" and apart from local irregularities would have 

 the same value all over the plate; in other words that a decrease 



M H. NoRT. The Harvard Map of the Sky and the Milky Way. Recherches 

 astronomiques de I'Observaloire d'Ulrecht, Vol. VII, 1917. 



') H. Henie. The Distribution of the Stars to tlie eleventh Magnitude. Lunds 

 Universitets Arsskritt. N F. Afd. 2. Bd. 10. Nr. 1. 



