DRIFTING LIGHT- WA VES. 8 1 



cork-thrower were at rest, and the observer moved up-stream 

 that is, towards him the corks would pass him at a 

 greater rate than ten a minute ; whereas, if the observer were 

 travelling down-stream, or from the thrower, they would 

 pass him at a slower rate. If both were moving, it is easily 

 seen that if their movement brought them nearer together, 

 the number of corks passing the observer per minute would 

 be increased, whereas if their movements set them further 

 apart, the number passing him per minute would be 

 diminished. 



These illustrations, derived from the motions of water, 

 suffice in reality for our purpose. The waves which are 

 emitted by luminous bodies in space travel onwards like the 

 water-waves or the corks of the preceding illustrations. If 

 the body which emits them is rapidly approaching us, the 

 waves are set closer together or narrowed ; whereas, if the 

 body is receding, they are thrown further apart or broadened. 

 And if we can in any way recognize such narrowing or 

 broadening of the light-waves, we know just as certainly that 

 the source of light is approaching us or receding from us (as 

 the case may be) as our observer in the second illustration 

 would know from the distance between the corks whether 

 his friend, the cork-thrower, was drawing near to him or 

 travelling away from him. 



But it may be convenient to give another illustration, 

 drawn from waves, which, like those of light, are not them- 

 selves discernible by our senses I refer to those aerial 

 waves of compression and rarefaction which produce what 

 we call sound. These waves are not only in this respect 

 better suited than water-waves to illustrate our subject, but 

 also because they travel in all directions through aerial space, 

 not merely along a surface. The waves which produce a 

 certain note, that is, which excite in our minds, through the 

 auditory nerve, the impression corresponding to a certain 

 tone, have a definite length. So long as the observer, and a 

 source of sound vibrating in one particular period, remain 

 both in the same place, the note is unchanged in tone, though 



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