vii.] ANALOGIES IN SOUND. 87 



These numbers show that the difference between the blue 

 light at one end of the beautifully coloured band and the 

 red at the other, is nothing more nor less than a difference 

 almost identical with the difference between a high note and a 

 low note upon the piano. The reason why one end of the 

 spectrum is red and the other blue, is that in light as in sound 

 we have a system of disturbances or waves; we have long 

 waves and short waves, and what the low notes are to music 

 the red waves are to light. The dispersion of light, whether 

 effected by refraction or diffraction, is simply the sorting out, 

 and arranging in regular succession, of the various light-tones 

 in the order of their wave-lengths. 



We can now recognise the strict analogy between the world 

 of sound and the world of light. Ears are tuned to hear different 

 sounds some people can hear much higher notes than others, 

 and some people can hear much lower notes than others. 

 In the same way some people can see colours to which other 

 people are blind ; indeed, the more we go into this matter, and 

 the more complete we make our inquiries, the more striking 

 becomes the analogy between these two classes of phenomena. 



The latest measurements tell us that a light-producing dis- 

 turbance travels at the rate of 186,000 miles in a second of 

 time. Imagine the molecular agitation depending upon this 

 statement, and then remember that a glow-worm can set it all 

 going, and that, when once in full swing, the distance of the 

 most remote star is traversed, as it were, at a bound, and without 

 sensible loss of energy. 



Now as in 186,000 miles there are 298,000,000 metres, or 

 29,800,000,000,000,000,000 hundred-millionths of a millimetre, 

 and as all the waves must enter the eye in a second, we have, 

 for the number of wave crests per second, 



29,800.000000000,000,000 = 3 92>000i000)0 oo,000, 

 i ouu7 



1 A millimetre is '03937 of an inch. 



