DRIFTING LIGHT- WA VES. 91 



manner than Doppler could have hoped for, is overlooked 

 on the other. Both points are noted in the article above 

 referred to, in the same paragraph. "We may dismiss," I 

 there stated, "the theory started some years ago by the 

 French astronomer, M. Doppler." But, I presently added, 

 " It is quite clear that the effects of a motion rapid enough 

 to produce such a change " (i.e. a change of tint in a pure 

 colour) " would shift the position of the whole spectrum 

 and this change would be readily detected by a reference to 

 the spectral lines." This is true, even to the word "readily." 

 Velocities which would produce an appreciable change of 

 tint would produce "readily" detectible changes in the posi- 

 tion of the spectral lines; the velocities actually existing 

 among the star-motions would produce changes in the posi- 

 tion of these lines detectible only with extreme difficulty, or 

 perhaps in the majority of instances not detectible at all. 



It has been in this way that the spectroscopic method 

 has actually been applied. 



It is easy to perceive the essential difference between this 

 way of applying the method and that depending on the 

 attempted recognition of changes of colour. A dark line in 

 the spectrum marks in reality the place of a missing tint. 

 The tints next to it on either side are present, but the tint 

 between them is wanting. They are changed in colour 

 very slightly, in fact quite inappreciably by motions of re- 

 cession or approach, or, in other words, they are shifted in 

 position along the spectrum, towards the red end for recession, 

 towards the violet end for approach ; and of course the 

 dark space between is shifted along with them. One may 

 say that the missing tint is changed. For in reality that is 

 precisely what would happen. If the light of a star at rest 

 gave every tint of the spectrum, for instance, except mid- 

 green alone, and that star approached or receded so swiftly 

 that its motion would change pure green light to pure yellow 

 in one case, or pure blue in the other, then the effect on the 

 spectrum of such a star would be to throw the dark line 

 from the middle of the green part of the spectrum to the 



