STRUCTURE OF THE UNIVERSE—HEAPS lw | 
Suppose the lights of a very distant city are observed at night 
through a telescope. The various spots of light all look much alike. 
However, they are not all the same in character. Some may be caused 
by incandescent lamps, some by neon signs, some, perhaps, may be 
due to the newer type of yellow sodium lamp used for illuminating 
highways. 
We now put a glass prism in front of the telescope objective. The 
telescope must be deviated sideways, if we are to see the city through 
the prism and the telescope. When we do see it, each spot of light 
appears to be smeared out into a band of color. The colors present 
in each spot of light are separated and spread out and we can see 
just what colors are present in the light from each source. The neon 
signs are characterized by definite colors in the orange and red; the 
sodium lamps can be recognized by the fact that only one cae 
yellow, is visible. 
If we were to photograph the lights of an enormous city from an 
enormous distance the whole city would appear as a small, luminous 
spot. The prism would smear out the separate lights of which the 
spot is composed, but they would all be superposed in a single 
smeared spot for the whole city. However, if there were a large 
number of sodium lamps one point in the smear would be brighter 
than the rest because there would be an excess of the yellow sodium 
light. 
A nebula, consisting of millions of suns a long distance away, be- 
haves like our hypothetical city except for one small difference. 
Light from a sun has dark absorption lines or bands from which 
color is missing as a result of absorption in the sun’s atmosphere. 
There is a dark line in the spectrum of our own sun, corresponding 
to absorption of hydrogen in the sun’s atmosphere. This dark line 
always appears at the same place in the spectrum no matter what 
kind of a source, and always means that hydrogen is present. Dark 
lines appear in the nearer nebulae about where they should be in the 
spectrum. For the more distant nebulae, however, they are shifted 
toward the red end of the spectrum. 
There is only one known explanation for such a shift of a spectral 
line. If the source is moving away from an observer the light re- 
ceived appears redder than when the source is stationary. This 
phenomenon is called the Doppler effect. It is a matter of common 
experience in the field of sound. The pitch of an automobile horn 
is lowered as the horn passes rapidly by an observer and recedes 
from him. 
The photographs of the nebulae show that the hydrogen absorption 
line is shifted farther and farther away from the normal position as 
