1325 



on plate B some of the densei- galactic regions are as it were dotted 

 with scarcely perceptible spots, so that it often seems entirely arbi- 

 trary whether some of them are to be considei-ed as belonging 

 togethei', and to be connted as stars. In deciding whether a scar- 

 cely visible 3''^ or 4^'' image existed alongside of tiie brighter images, 

 the snbjective certainty was considerably greater. 



In the case of the brighter stars another nncertainly presented 

 itself. It sometimes happened that of some star, which in the large 

 images was decidedly fainter than another, moie faint images conld 

 nevertheless be discerned, as here the fainter images were small 

 and sharp, and with the others they were large and diffuse. This 

 is caused by the uncommon achromatisation of the Zeisstriplet ^), 

 which renders the yellow stars large and hazy, and the white staj-s 

 bright and small. This cii'cumstance, which may be of use in deci- 

 ding the colour of such weak stars, often rendered the counting 

 troublesome; as a rule the visibility of the weakest image was raken 

 as a criterion for the classifying. 



The region counted comprises 100 squares (in AR. of — 7 to -|-3, 

 in decl. of + 5 to —5). The centre of the i)late lies on 279°30' -f 

 -|-11°30'; the side of each square is 15', 28, so the surface is 

 0,0649 = '/i5Mi square degrees. Tlie cornerpoints of the region 

 explored are situated at 



277°40'6-|-12°44',8; 277°4J',6 -f 10°12',4; 280°17',0 + 12°46',3; 



280°16',5 -f 10°13',B. 



The countings have been executed by means of the microscope 

 of the Repsold-ai)parafns for rectangular coordinates at the Ley den 

 observatory, fitted out with the weakest ocular; the enlargement 

 was tenfold, rather too strong for the purpose. The results of the 

 countings have been collected in Table I ; each square contains 

 successively the number on plate B, the number on plate A, the 

 numbers on A with at least 3 and 4 images, and the numbers on 

 A with at least 5 and 6 images. 



2. The scale of magnitudes. In order to find the limiting mag- 

 nitudes for which these numbers stand, th^ magnitude of a number 

 of stars had to be ascertained. This part of the investigation present- 

 ed the greatest difticulties, as it had to be elfected with somewhat 



'i The focal distance is minimum for 394 [xfi (Hkrtzsprung A. N. 4951. Vol. 

 207. 88). 



86 

 Proceedings Royal Acad. Amsterdam. Vol. XXI. 



