384 
POPULAR SCIENCE REVIEW. 
with this known rate the rate at which they successively 
reach him. If they come in quicker succession than from a 
luminous body at rest, he will know that the source of light is 
approaching as certainly as our observer down-stream would 
know that his friend was sauntering towards him if the corks 
came two feet apart instead of three feet. If, on the contrary, 
the light-waves of a particular kind come in slower succession 
than from a body at rest, the observer will know that the source 
of light is receding, precisely as the river-side observer would 
know that his friend was travelling away from him if the corks 
came past him four feet apart instead of three. 
Now, the stellar spectroscopist can distinguish among *the 
light waves of varied length which reach him, those which have 
a particular normal length. He analyses star-light with his 
spectroscope, and gets from it a rainbow-tinted streak crossed 
by dark lines. These dark lines belong to definite parts of the 
spectrum ; that is, to such and such parts of its red, or orange, 
or yellow, or green, or blue, or indigo, or violet portion. Thus 
they correspond to light having a particular wave-length. And 
many of these lines in stellar spectra are identifiable with the 
lines due to known elements. For instance, in the spectrum of 
Sirius there are four strong dark lines corresponding to the 
known bright lines of the spectrum of hydrogen. Thus the 
wave-length corresponding to any one of these dark lines is 
perfectly well known to the spectroscopist from what he has 
already learned by examining the bright lines of hydrogen. 
Now, if Sirius were receding very rapidly, the wave-length cor- 
responding to one of these lines would be lengthened ; it would 
correspond, in fact, to a part of the spectrum nearer the red end 
or the region of longer light waves, and thus the dark line 
would be shifted towards the red end of the spectrum ; whereas, 
on the contrary, if Sirius were very rapidly approaching, the 
dark line would be shifted towards the violet end of the spectrum. 
All that would be necessary would be that the rate of approach 
or recession should bear an appreciable proportion to the rate 
at which light travels, or 185,000 miles per second. For, re- 
verting to our cork-thrower, it is clear that if he travelled up- 
stream or down-stream at a rate exceedingly minute compared 
with the stream’s rate of flow, it would he impossible for the 
observer down stream to be aware of the cork-thrower’s motion 
in either direction, unless, indeed, he had some very exact 
means of measuring the interval between the successive corks. 
Now the spectrum of a star can he made longer or shorter 
according to the dispersive power employed. The longer it is, 
the fainter its light will be ; but, so long as the dark lines can 
be seen, the longer the spectrum is, the greater is the shift due 
to stellar recession or approach ; and, therefore, the more readily 
