THE MOVEMENTS OF THE STARS. 291 



star. Here comes in the important principle on which 

 I have laid so much stress, that the dark line spec- 

 trum produced by the cold hydrogen would be identi- 

 cally situated line for line with the bright line spectrum 

 generated in the incandescent gas. We arrange, in fact, 

 that two spectra shall be produced by the same system of 

 prisms, so as to give every facility for the comparison. If 

 the star were at rest relatively to the earth, each delicately 

 bright line should be strictly continuous with each delicate 

 dark line. If, however, the star be in relative movement 

 to or from the earth, then the system of lines which 

 it radiates will exhibit a shift relative to the system of 

 lines produced by the incandescent hydrogen which is at 

 rest with respect to the apparatus. Here, then, we have 

 the means of measuring the velocity of the star along the 

 line of sight. Take any one of the lines, the F line, for 

 instance, its displacement relatively to the adjacent bright 

 line can be measured. It can be measured with accuracy 

 too, when the apparatus has been constructed with a 

 delicacy that optical skill has now rendered possible. 



We are gradually learning the full significance of the 

 word light. We are now aware that the undulations which 

 we see are only a couple of octaves, so to speak, out of an 

 immense volume of undulations, which we have no senses 

 for perceiving. We are thus at last attaining some concep- 

 tion of the majesty of these words, " Let there be light." 



