Vol. 6. 1920 ASTRONOMY: SHAPLEY AND DAVIS 
487 
accidental and systematic errors; the average probable error of a mag- 
nitude is about 0.04. Since the distance of Messier 3 appears to be known 
with higher accuracy than that of any other stellar cluster, we have in 
this photometric catalogue a useful collection of. data on the absolute 
C.I 
=-+1 
.00 
V 
/ 
V 
- — 
1 
/ 
4 
ft 
( 
:.!.+ 
1.00 
).40 
1 
f- 
/ 
II 
/ 
( 
:.!.< 
; +0. 
40 
/ 
FIG. 1 
Frequency of absolute photovisual magnitudes 
(abscissae) in Messier 3. Upper, middle, and 
lower curves are for red, yellow, and blue stars, 
respectively. Ordinates are numbers of stars for 
intervals of 0.4 in absolute magnitude. 
brightness of stars — data that will become still more valuable as spectral 
observations are extended. 
3. Von Zeipel's catalogue of positions^ is used as the basis of the sys- 
tem of coordinates and of the system of numbering. For 340 stars fainter 
than the limit of his catalogue approximate positions have been determined 
from Mount Wilson plates. 
The magnitudes^of stars within I'.S of the center have not been mea- 
