ASTRONOMY: H. SHAPLEY 
269 
ently is much more concentrated. Accordingly it becomes appropriate 
to investigate clusters in the Milky Way and the faint stars in the 
galactic clouds. The presence of negative color indices, or a normal 
range of color, in such regions, assuming of course the distances to be 
great, will indicate the essentially complete transparency of space 
throughout the whole stellar system. 
The position, with respect to the galactic plane, of the fields in which 
magnitudes and colors have been studied in the course of this work are 
listed in the following table. With two or three exceptions the regions 
are in or around stellar clusters. The fourth and fifth columns give 
for each region the limiting values of the color indices. In several of 
these fields the survey is not yet complete, and values somewhat larger 
and smaller may be found later. This applies especially to Messier 5 
Preliminary Results Bearing on the Extent or the Galactic System 
REGION 
GALAC- 
TIC 
LONGI- 
TUDE 
GALAC- 
TIC 
LATI- 
TUDE 
OBSERVED 
COLOR INDEX 
AVER- 
AGE 
Py 
MAG. 
AVER- 
AGE 
COLOR 
INDEX 
PARALLAX 
Largest 
Smallest 
M = 0 
M = -f2 
12*^ 
+78'' 
+ 1.77 
-0.39 
15.1 
-0.16 
0''0001 
0^00024 
27 
+40 
+ 1.42 
-0.52 
16.54 
-0.34 
0.00005 
0.00012 
Messier 15* 
33 
-28 
+ 1.50 
-0.21 
16.0 
-0.14 
0.00006 
0.00016 
XX Cygni** 
60 
+ 13 
+ 1.33 
-0.14 
12.64 
-0.02 
0.0003 
0.0007 
69 
- 3 
+ 1.58 
-0.37 
12.65 
-0.01 
0.0003 
0.0007 
139 
+ 1 
+2.12 
-0.45 
13.5 
-0.16 
0.0002 
0.0005 
142 
+ 1 
+ 1.50 
-0.30 
12.5 
-0.23 
0.0003 
0.0008 
154 
+ 3 
+ 1.31 
-0.15 
11.5 
-0.07 
0.0005 
0.0013 
189 
- 1 
+2.00 
-0.04 
12.3 
-0.02 
0.0003 
0.0009 
Messier 5* 
332 
+46 
+ 1.67 
-0.11 
14.6 
-0.10 
0.0001 
0.0003 
Near Mil 
354 
- 3 
+2.23 
-0.42 
14.2 
-0.14 
0.00015 
0.00036 
355 
- 3 
+2.06 
-0.16 
14.32 
-0.08 
0.00014 
0.00034 
Near Mil 
356 
4 
+ 1.81 
-0.14 
13.9 
-0.01 
0.0002 
0.0004 
*Globular cluster. **Three stars. 
and Messier 3, for which the magnitudes of the fainter and probably 
more t3^ical b- class stars are not yet available. The sixth and seventh 
columns give for each region the mean photovisual magnitude and mean 
color index of a group of the faintest blue stars. The average number 
in each group is ten. 
The accidental errors affecting the colors are so small that the dis- 
persion of color index and the negative and small positive values of the 
index cannot be attributed to observational uncertainty. The syste- 
matic errors have been carefully examined for several of these regions and 
they are similarly inadequate, at least as far as our present knowledge 
of them extends. The blueness of the faint and distant stars, thus 
