THE WAVE THEORY OF LIGHT 27S 



red, orange, yellow, green, blue, indigo, and violet, accord- 

 ing to Newton's nomenclature. Newton never used a narrow 

 beam of light, and so could not have had a homogeneous 

 spectrum. 



This is a diagram painted on glass and showing the colours 

 as we know them. It would take two or three hours if I 

 were to explain the subject of spectrum analysis to-night 

 We must tear ourselves away from it. I will just read out 

 to you the wave-lengths corresponding to the different posi- 

 tions of the sun's spectim of certain dark lines commonly 

 called " Fraunhofer's lines.'* I will take as a unit the one 

 hundred thousandth of a centimetre. A centimetre is .4 of 

 an inch; it is a rather small half an inch. I take the 

 thousandth of a centimetre and the hundredth of that as a 

 unit. At the red end of the spectrum the light in the neigh- 

 bourhood of that black line A (FiG. 120) has for its wave- 

 length 7.6; B has 6.87; D has 5.89; the "frequency" for A 

 is 3.9 times 100 million million, the frequency of D light is 5.1 

 times loo million million per second. 



Now what force is concerned in those vibrations as com- 

 pared with sound at the rate of 400 vibrations per second? 

 Suppose for a moment the same matter was to move to and 

 fro through the same range but 400 million million times per 

 second. The force required is as the square of the number 

 expressing the frequency. Double frequency would require 

 quadruple force for the vibration of the same body. Sup- 

 pose I vibrate my hand again, as I did before. If I move it 

 once per second a moderate force is required; for it tQ 

 vibrate ten times per second 100 times as much force is re* 

 quired; for 400 vibrations per second 160,000 times as much 

 force. If I move my hand once per second through a space 

 of a quarter of an inch a very small force is required; it 

 would require very considerable force to move it ten times 

 a second, even through so small a range; but think of the 

 force required to move a tuning-fork 400 times a second, and 

 compare that with the force required for a motion of 400 

 million million times a second. If the mass moved is the 

 same, and the range of motion is the same, then the force 

 would be one million million million million times as great as 

 the force required to move the prongs of the tuning-fork it 



