12 
ASTRONOMY. H. SHAPLEY 
fruitful to suggest the possible practical use of these magnesian clays in 
cases in which a tight tamping is necessary where water has access, as in 
shutting off crevices in bored wells when, as in those yielding oil, undesir- 
able flows of water interfere, and where tamping with ordinary clays 
fails to tighten sufficiently and permanently. Other uses for such a pro- 
digiously swelling material readily suggest themselves. 
STUDIES OF MAGNITUDES IN STAR CLUSTERS, I. ON THE 
ABSORPTION OF LIGHT IN SPACE 
By Harlow Shapley 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Received by the Academy, November 17, 1915 
All researches on the structure of the stellar universe must take into 
account the probable and possible effects of the scattering or obstruction 
of light in its passage through interstellar space. It is recognized, for 
instance, that if the loss due to absorption, or scattering, by the free 
molecules of matter in space totals as much as a millionth of one per 
cent of the visual light in a journey of a hundred million miles, then, as- 
suming the effect proportional to the distance, every star 3500 light- 
years away would be observed about two magnitudes fainter than its 
true brightness. As a consequence any uncertainty in the coefficient of 
scattering, especially if it is large as cited above, is very serious in studies 
of the distance of the faint stars and particularly in considerations of the 
stellar densities in various parts of the galactic system. In fact, the 
hypothesis that light extinction is imperceptible is prerequisite to the 
conclusion that the stellar universe is finite in extent. 
Because of the importance of the subject several extensive investiga- 
tions have been undertaken in recent years for the purpose of determin- 
ing the amount of absorption. It is generally assumed that if any 
dimming of a star occurs it will be apparent as selective molecular scat- 
tering, which varies as the inverse fourth power of the wave length of 
the light. 
As the effect for blue light would be about double that for yellow, an 
obvious method of detecting and measuring selective absorption is 
through the study of the colors of faint and distant stars. By means of 
measures of color indices various investigators have found that the 
fainter stars are on the average redder than the brighter ones, and as the 
fainter stars are also on the average, the more distant, this excess of red- 
ness might be and often is accepted as an indication of space absorption. 
There are, however, other possible interpretations, including the eft'ect of 
