STRUCTURE OF THE UNIVERSE—HEAPS 173 
at a time when all matter in the universe was assembled and ready for. 
a cosmic blow-out of such tremendous proportions. 
So much for objection number one. The second objection arises as 
follows. When a source of light moves away from an observer there 
are two effects produced. The first, the Doppler effect, has been 
mentioned as a change of color, a reddening of the light. A second 
effect is a decrease of brightness, known as the “dimming factor.” It 
is easy to see why a light should appear to be dimmer when the source 
moves away from the observer. Suppose a stationary machine gun 
is firing bullets at a fixed target at the rate of five per second. Then 
every second five bullets hit the target. However, if the gun is moving 
away from the target, still firing five shots a second, there will not be 
five bullets hitting every second. The bullet discharged from the gun 
at the end of a given second will have had to traverse a greater distance 
than the bullet which was fired at the beginning of the second, so it 
will take a longer time to reach the target. Perhaps only four bullets 
will hit the target in one second. The extra bullet has gone to fill the 
extra space in the bullet stream—the extra space created by the reces- 
sion of the gun. The case of a hight source is exactly analogous. 
Now in estimating the distance of a nebula its brightness is taken as 
a criterion of the distance. The question arises as to whether the dim- 
ming factor should be applied when making the distance estimates. 
If the nebulae are actually moving away from us then the factor must 
certainly be applied. If the reddening of the light is not caused by 
a velocity of recession then the dimming factor must not be applied. 
With such tremendous speeds of recession this factor makes quite a 
big difference in results. 
The following discussion is very largely quoted from the annual 
Sigma Xi lecture delivered in December 1941 at Dallas by E. P. 
Hubble of the Mount Wilson Observatory. Dr. Hubble is one of the 
world’s foremost authorities on the subject of nebulae. 
Let us first suppose that the reddening of the light is not caused by a 
velocity of recession. It may be due to some hitherto undiscovered and 
unknown phenomenon. We can then estimate distances without any 
dimming factor and a survey can be made to find out how the nebulae 
are distributed throughout the region of space within our present 
range of view. Such surveys have been made at Mount Wilson and 
Mount Hamilton, out to a distance of 420 million light-years. Data 
have also been obtained and analyzed at Harvard, and the net result 
indicates a fairly uniform distribution of nebulae throughout the 
observable regions of space. There are, on the average, just as many 
per unit volume at great distances as in the immediate neighborhood 
of our own group. 
