DRIFTING LIGHT-WAVES. 85 



I can form no other opinion than that these arguments 

 amount really but to an expression of inability to understand 

 the matter. This may seem astonishing, but is explained 

 when we remember that some mathematicians, by devoting 

 their attention too particularly to special departments, lose, 

 to a surprising degree, the power of dealing with subjects 

 (even mathematical ones) outside their department. Apart 

 from the soundness of the reasoning, the facts are unmis- 

 takably in accordance with the conclusion to which the 

 reasoning points. Yet some few still entertain doubts, a 

 circumstance which may prove a source of consolation to 

 any who find themselves unable to follow the reasoning on 

 which the effects of approach and recession on wave-lengths 

 depend. Let such remember, however, that experiment in 

 the case of the aerial waves producing sound, accords per- 

 fectly with theory, and that the waves which produce light 

 are perfectly analogous (so far as this particular point is con- 

 serned) with the waves producing sound. 



Ordinary white light, and many kinds of coloured light, 

 may be compared with noise that is, with a multitude of 

 intermixed sounds. But light of one pure colour may be 

 compared to sound of one determinate note. As the aerial 

 waves producing the effect of one definite tone are all of one 

 length, so the ethereal waves producing light of one definite 

 colour are all of one length. Therefore if we approach 01 

 recede from a source of light emitting such waves, effects 

 will result corresponding with what has been described above 

 for the case of water-waves and sound-waves. If we ap- 

 proach the source of light, or if it approaches us, the waves 

 will be shortened ; if we recede from it, or if it recedes from 

 us, the waves will be lengthened. But the colour of light 

 depends on its wave-length, precisely as the tone of sound 

 depends on its wave-length. The waves producing red 

 light are longer than those producing. orange light, these aie 

 longer than the waves producing yellow light ; and so the 

 wave-lengths shorten down from yellow to green, thence to 

 blue, to indigo, and finally to violet Thus if a body shining 



