ASTRONOMY: H. SHAPLEY 
13 
intrinsic luminosity on color and the possibility of the predominance of 
redder spectral types among the faint stars and the more distant ones. 
In all previous special studies of the problem, the chief difhculties 
have been the uncertainty of the distances (so few of which have been 
directly measured) and their relatively small values. For a given spec- 
tral type the observed color excess of the more distant stars is also small. 
Consequently, though positive values for the absorption coefhcient have 
always been derived, they have in no case been considered as definitely 
final proof of the existence of an appreciable space absorption. The 
four most recent determinations (all of which depend upon studies of the 
colors of relatively nearby stars) give the following values for the in- 
crease of the color index for each unit of distance (32.6 light-years): 
Observer King Kapteyn Jones Van Rhijn 
Absorption coefficient 0.019 0.0031 0.0047 0.0015 mag. 
Since the color effect is cumulative, an increase in the length of the 
base line increases greatly the accuracy of a measurement of absorption. 
If instead of using stars in our immediate stellar system, we extend the 
study of colors to much more distant objects, then we can readily decide 
whether light scattering is to play an important part in stellar investi- 
gations. The advantage of great distances is taken in the present study 
of globular clusters, and already it is possible to make a definite contri- 
bution to the problem of the existence of light scattering. 
At the beginning of a systematic study of magnitudes and colors of 
stars in clusters, the great globular system in Hercules, Messier 13, has 
been observed photographically, both with ordinary plates sensitive to 
blue and violet light and with isochromatic plates which respond chiefly 
to the most effective visual light when the shorter wave-lengths are cut 
off by means of a yellow color filter. Photographic and photovisual 
magnitudes for about 1300 stars have been determined. The color in- 
dices thus secured are compared with those for nearby stars with iden- 
tical spectra (whenever the spectra of the cluster stars can be directly 
observed), and the excess of redness for the cluster stars indicates the 
amount of space absorption. When the spectra can not be directly ob- 
served the color indices in the cluster still may serve to solve the prob- 
lem. For, if the stars in the distant cluster are even approximately 
similar to those near the sun, the absolute values of the color indices will 
tell at once whether there is practically no absorption or whether there 
is a really appreciable amount. 
The detailed discussion of the magnitudes in Messier 13, which will 
appear as a Contribution from the Mount Wilson Solar Observatory, will 
