226 THE POPULAR SCIENCE MONTHLY. 



can be equally sure that his friend is sauntering up-stream, if the corks 

 come past with a common interval exceeding three feet; and that he 

 is sauntering down-stream, if the common interval is less than three 

 feet. And, if, by some process of measuring, he can find out exactly 

 how much greater or how much less than three feet the interval is, he 

 can tell exactly how fast his friend is sauntering up-stream or down- 

 stream. It would not matter how far down-stream the observer might 

 be, so long as the stream's rate of flow remained unchanged ; nor, in- 

 deed, would it matter, even though the stream flowed at a different 

 rate past the observer than past the cork-thrower, so long as neither 

 of these two rates was liable to alteration. 



Now, we may compare the emission of light-waves by a luminous 

 object to the throwing of corks in our illustrative case. The rate of 

 flow for light-waves is indeed infinitely faster than that of any river, 

 being no less than 185,000 miles per second. The successive light- 

 waves are set in motion at infinitely shorter time-intervals, since for ex- 

 treme red light there are no less than 458,000,000,000,000 undulations 

 per second, and for extreme violet no less than 727,000,000,000,000 ; 

 but these specific differences do not affect the exactness of the illus- 

 tration. It is obvious that all that is necessary to make the par- 

 allel complete is that the flow of light-waves shall reach the observer 

 at a constant rate (which is the actual case), and that he shall know, 

 in the case of any particular and distinguishable kind of light, what 

 is the rate at which the wave-action is successively excited, and be 

 able to compare with this known rate the rate at which they succes- 

 sively reach him. If they come in quicker succession than from a lu- 

 minous body at rest, he will know that the source of light is approach- 

 ing, as certainly as our observer down-stream would know that his 

 friend was sauntering toward him if the corks came two feet apart in- 

 stead of three feet. If, on the contrary, the light-waves of a particu- 

 lar kind come in slower succession than from a body at rest, the ob- 

 server 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 analyzes 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 identifi- 

 able with the lines due to known elements. For instance, in the spec- 

 trum 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 



