270 
ASTRONOMY: H. SHAPLEY 
shown to exist for numerous directions in space, may be accepted, there- 
fore, as an indication of the general ineffectiveness of differential Hght 
scattering. This result also naturally implies the absence of all dim- 
inution of the intensity of Kght through the agency of interstellar media. 
The daily encounter of the earth with millions of meteors is frequent- 
ly cited as evidence of the prevalence of highly efficient light-scattering 
and Kght-obstructing material throughout interstellar space. If, as 
seems likely, the meteoric dust, which is closely allied to the meteoric 
streams and comets, is a definite and original part of the solar system 
and is not cosmic in origin and motion, this argument loses its force; 
and whatever extinction of light such matter produces, either in the 
solar system or in immediate gravitational field of other stars, is wholly 
unimportant in its bearing on stellar distribution and distances. 
Disregarding light absorption, we may compute for each region the 
parallax of stars of the tabulated mean photovisual magnitude, as soon 
as we know the mean absolute luminosity corresponding to the average 
color. Upon the basis of the researches of Kapteyn, Russell, Charlier, 
and others, we make two assumptions for the mean absolute magnitude 
of the blue stars, M 0 and M = +2, and compute the results given 
in the last two columns of the table. For the globular clusters the 
smaller parallaxes are undoubtedly preferable, since in these systems 
we are certainly treating only the brightest of the bluer stars. More- 
over, the smaller value is in keeping with results derived by means of 
variables and other stars of high luminosity. For the open clusters 
the larger values of the parallax are probably better. In a few of the 
regions, however, the magnitudes and colors are provisional; and the 
adopted mean absolute magnitudes of B-type stars, though not im- 
probable, are of course somewhat uncertain. Hence the estimates of 
the extent of the galactic system (including its open clusters), and the 
indications of the relatively greater distances of the globular clusters, 
are here proposed as only preliminary results ; their main value lies in 
the definite CAddence of the vast dimensions of the visible stellar universe 
and in the problems suggested for further investigations along this line. 
1 Shapley, H., these Proceedings, 2, 1916, (12-15). 
2 Seares, F. H., Ihid., 2, 1916, (521-525). 
3 Shapley, H., Mt. Wilson Contrih. No. 116, 1915, (1-92), pages 52-58 and 88. 
Ihid., pages 58-61. 
