Photography of the Skies v 



Let us now return to the sister phenomena of 

 light. At one end of the visible spectrum the 

 violet rays have about half the length of the red 

 rays at the other end of the scale; accordingly 

 about twice as many violet as red rays enter 

 the eye in a second. Let us imagine a star like 

 Betelgeux, which, at rest, would emit red rays 

 solely. If such a star were to dash toward the 

 earth with the speed of light, 186,400 miles a 

 second, its rays would be so much shortened 

 as to be halved in length, and the star would 

 appear violet its characteristic lines and hues 

 showing themselves at one extreme of the visible 

 scale instead of at the other. Of course, no star 

 moves toward the earth with more than a small 

 fraction of the speed of light, and yet so refined 

 is the measuring of the displacement of spectral 

 lines that a motion toward the earth of some- 

 what less than one mile in one second can be 

 readily determined. In the case of Betelgeux 

 its movement toward the earth is known to be 

 seventeen and six- tenths miles a second, about 

 one eleven-thousandth part, of the velocity of 

 light, the displacement of its red lines toward the 

 violet end of the scale being about one eleven- 

 thousandth part of the whole length of the spec- 

 trum. If, in a contrary case, a star is receding 

 from the earth, its spectroscopic lines will be 

 shifted toward the red end of the scale, just as a 

 locomotive whistle falls to a lower pitch as the 

 engine moves away from a listener standing still. 

 By this method Gamma Leonis is known to be 



