ASTRONOMY IN A WORLD AT WAR—DOUGLAS 159 
more than two centuries, to receive its first modification. Nor was the personal 
interest wanting: a great adventure in thought had at length come safe to shore. 
[A. N. Whitehead.] 
De Sitter’s expanding universe suggested an outward motion of 
the stellar bodies within the framework of space as defined by his 
modification of the Einstein equation of spacetime geometry. Ten 
years later, Lemaitre, who had fought with the Belgian army in the 
war years and afterward entered Louvain University, brought for- 
ward his theory of expanding space. This made the radius of cur- 
vature of space a function of time, and gave a new stimulus to the 
astronomers in those great observatories equipped to probe most 
deeply into space. In the following years, at Mount Wilson and 
Harvard particularly, the exploration of space was carried on with 
vigor, and methods were found of estimating the distances of the 
remote galaxies. A special lens was designed to obtain their spectra 
at Mount Wilson, and thanks to the broad, strong H and K lines of 
ionized calcium, red shifts could be measured to distances estimated 
as 250,000,000 light-years. The correlation between distance and red 
shift has provided a remarkable confirmation of the theory of the 
expanding universe. Recessional velocities up to one-seventh the 
velocity of light have now been observed. In the years between the 
wars a few voices were heard to question the interpretation of the red 
shift as a Doppler displacement, but since no alternative explanation 
suggested itself without postulating some entirely new law of Nature, 
the expanding universe remained as a working hypothesis in the back- 
ground of most astronomers’ minds. 
One of the interesting things that these recent war years have 
brought is the reopening of this question by E. P. Hubble. Is the 
universe expanding? Is the red shift actually indicative of motion? 
Or is the framework of the universe static? And if static, what is 
the explanation of the displacement of all spectrum lines to the 
red for distant galaxies? Hubble’s analysis of all available data 
based on the assumption that the universe is expanding, necessitates 
the calculation of a dimming factor due to recession. When cor- 
rection is made for this in the estimation of distances, he claims 
that a map results which is not of homogeneous density, which 
implies an increasing rate of expansion with distance, and therefore 
an “age” of the universe totally inadequate. On the other hand 
when he assumes a static framework for the universe, the analysis 
of all the data gives a map that shows a linear relation between 
red shift and distance, and a homogeneity of density. This map 
has more to commend it than has the former map, and hence the 
assumption of a static framework appears to be favored. But, as 
various astronomers have pointed out, the weakness of this result 
lies in the large probable errors of the quantities involved, so that 
