THE MOVEMENTS OF THE STARS. 273 



what we mean by the wave length. We do not here refer 

 to the length of the wave in the same sense as we could 

 speak of the length of the beach ; we simply mean the 

 interval between one wave and the next, or, to speak more 

 definitely, the number of feet between the crest of one wave 

 and the crest of the wave following it ; nor will there 

 be any difficulty in understanding from the analogy of the 

 beach the difference between long waves and short waves. 

 It is true that in many respects the analogy of waves 

 on the beach is defective when we would seek to under- 

 stand the waves in the ether ; we must rather think of 

 the latter as a succession of throbs rapidly succeeding each 

 other throughout this invisible and intangible fluid. If 

 the throbs succeed each other at comparatively long 

 intervals, then the light of which they convey the impres- 

 sion is red ; the waves that are the shortest, so far as visual 

 rays are concerned, correspond to light of a violet hue. 



We thus assign a length to each wave, and we can 

 regard that length as in direct correspondence with the 

 colour of the light ; this may be submitted to strict 

 numerical expression, in fact, we know the wave lengths 

 of the principal rays of light with an accuracy which is 

 one of the most marvellous results of modern physical 

 research. These waves are very minute ; take the veiy 

 longest we can see, which corresponds to extremely deep 

 red at the very end of the spectrum, the wave length of 

 the ray identified by the Fraunhofer line known as A is 

 expressed as 7604'04 tenth-metres. Let me attempt to 

 illustrate what this length really corresponds to ; where 

 are we to find an object small enough for the comparison ? 

 If we divide an inch into 1,000 equal parts, yet each of 

 T 



