713 
account of the inequality and uncertainty of the limiting magnitude). 
The normal density AN, was adopted from the lists “Groningen 
Publications 18”; the argument, the limiting magnitude after the 
scale of Groningen 18, was taken, according to SELIGER, dependent 
on the star-density, and was for the lowest limit still corrected 
by 0,11, *') which gave 
6,56—0,023(D—0,7); 8,12—0,068(D—0,7); 9,36—0,246(D—0,7). 
On the average these limits in photometric scale are 6.6, 8.1 
and 9.4. ; 
6. Two of Kaprnyn’s “Selected Areas” come within this region: 
N°. 47 and N°. 48; N°. 48 is situated closer to the centre, but according to 
the chart of Dyson and Mrrorrr just outside a region with strong 
absorption; N°. 47, though more distant, comes just within the dark 
field S.W. from § Persei. In the ‘Durchmusterung of Selected 
Areas” ”) the numbers of stars were counted up to 12,0, 13,0, 14,0, 
15,0, and on Area 47 up to 16,0 (faintest stars 15,96 resp. 16,49). 
c. From the Dyson and Menorre chart, for every part of the 
region from 3) to 5"30m and 20° up to 35° we could draw the 
star-density per 100 square minutes on the FRANKLIN-ÂDAMS plates, 
already reduced to a common system. Regarding the limiting mag- 
nitude, for which these densities count, the authors say: ‘The 
limiting magnitude is not accurately fixed, but may be taken at 
about 15,8 and should be within 0™,25 of this figure” *). I have 
tried to control these data by making use of the three “Selected Areas” 
(47, 48, 49) falling within this region. To this end the log N’ for 
these places, as deduced from the Dyson and Merorrr chart, was 
compared to that of Kapreyn for m= 13, 14, 15 (and 16) and 
thus, through interpolation or extrapolation of the deviations from 
the normal /oy N the limiting magnitude was deduced, The values 
thus obtained are 16,02, 15,83 and 15,90: their average 15,9 has 
been adopted. For the rest a mistake of 0,1 in this value gives a 
mistake in the log N of 0.03 only. 
d. The data of the photographic “Carte du Ciel” cannot in gene- 
ral be used here. The great accidental irregularities in the limiting 
magnitude of the separate plates does not prevent the fixing of 
average densities and an average limiting magnitude, it is true, but 
in this case it is the separate plates that count, and these can be 
1) See with regard to this A. PANNEKOEK, Researches into the structure of the 
Galaxy. These Proceedings, Vol. XIII, p. 254. 
3) Annals of Harvard College Observatory. Vol. Cl. 
$) l. c. page. 4. 
