184 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
not universally accepted, but even the most cautious of us admit that 
red-shifts are evidence either of an expanding universe or of some 
hitherto unknown principle of nature. 
The two observed characteristics of the observable region, namely, 
the approximately uniform distribution and the approximately linear 
law of red-shifts, must be satisfied by any theory of the universe. 
They are the only observational results on the grand scale that can 
be used as tests. They serve to eliminate many theories formerly 
developed on insufficient data, but several modern theories survive 
the tests. These latter theories all permit the observed features in 
a limited region near the observer but they predict that departures 
from the simple approximate laws of distribution and of red-shifts 
will be found when the measures are extended to greater distances. 
These departures differ from theory to theory, and, if the measures 
can be extended to the necessary distance, will distinguish the correct 
theory from the false. 
Thus the most important observational problems in cosmology may 
be described as the small, second-order effects of great distances. 
The nebulae appear to be distributed in a roughly uniform manner 
and the red-shifts appear to be roughly proportional to distance, out 
to the limits of the 100-inch. The next step is to determine these 
features more precisely over the limited range of the 100-inch and 
approximately out to far greater distances. 
Attempts have been made to attain the necessary precision with 
the 100-inch, and the results appear to be significant. If they are 
valid, it seems likely that red-shifts may not be due to an expanding 
universe, and much of the current speculation on the structure of the 
universe may require re-examination. The significant data, however, 
were necessarily obtained at the very limit of a single instrument, 
and there were no possible means of checking the results by inde- 
pendent evidence. Therefore the results must be accepted for the 
present as suggestive rather than definitive. 
The problem is essentially one for the 200-inch. This new telescope 
will penetrate into space out to a thousand million light-years, and 
the second-order effects of great distance will be so conspicuous that 
they cannot be missed. 
As a particular and final example, let me mention the effects of 
increasing red-shifts on apparent brightness. It is well known that 
a rapidly receding light appears fainter than a similar, but stationary, 
light at the same momentary distance. The reason is that the stream 
of light-quanta from the moving light is thinned out by the recession 
so that fewer quanta per second reach the observer. Since brightness 
is measured by the rate of arrival of quanta, the receding light appears 
abnormally faint. 
