DRIFTING LIGHT WA VES. 89 



orange became yellow, the yellow green, the green blue, 

 the blue indigo, the indigo violet, while the violet waves 

 became too short to affect the sense of sight Then, if that 

 were all, that sun, being deprived of the red part of its 

 light, would shine with a slightly bluish tinge, owing to the 

 relative superabundance of rays from the violet end of the 

 spectrum. We should be able to recognize such a change, 

 yet not nearly so distinctly as if that sun had been shining 

 with a pure green light, and suddenly beginning to approach 

 us at the enormous rate just mentioned, changed in colour 

 to full blue. Though, if that sun were all the time approach- 

 ing us at the enormous rate imagined, we should be quite 

 unable to tell whether its slightly bluish tinge were due to 

 such motion of approach or to some inherent blueness in 

 the light emitted by the star. Similarly, if a white sun 

 suddenly began to recede so rapidly that its violet rays were 

 turned to indigo, the indigo to blue, and so on, the orange 

 rays turning to red, and the red rays disappearing altogether, 

 then, if that were all, its light would become slightly red- 

 dish, owing to the relative superabundance of light from the 

 red end of the spectrum; and we might distinguish the 

 change, yet not so readily as if a sun shining with pure green 

 light oegan to recede at the same enormous rate, and so 

 shone with pure yellow light Though, if that sun were all 

 the time receding at that enormous rate, we should be quite 

 unable to tell whether its slightly reddish hue were due to 

 such motion of recession or to some inherent redness in its 

 own lustre. But in neither case would thai be all. In the 

 former, the red rays would indeed become orange ; but the 

 rays beyond the red, which produce no effect upon vision, 

 would be converted into red rays, and fill up the part of the 

 spectrum deserted by the rays originally red. In the latter, 

 the violet rays would indeed become indigo ; but the rays 

 beyond the violet, ordinarily producing no visible effect, 

 would be converted into violet rays, and fill up the part of 

 the spectrum deserted by the rays originally violet Thus, 

 despite the enormous velocity of approach in one case and 



