270 KELVIN 



constituting light waves, I may say that 30 vibrations 

 per second is about the smallest number which will pro- 

 duce a musical sound; 50 per second gives one of the 

 grave pedal notes of an organ, 100 or 200 per second give 

 the low notes of the bass voice, higher notes with 250 per 

 second, 300 per second, 1,000, 4,000 up to 8,000 per sec- 

 ond give about the shrillest notes audible to the human 

 car. 



Instead of the numbers, which we have, say in the most 

 commonly used part of the musical scale, i. e. f from 200 

 or 300 to 600 or 700 per second, we have millions of millions 

 of vibrations per second in light waves: that is to say, 

 400 per second, instead of 400 million million per second, 

 which is the number of vibrations performed when we have 

 red light produced. 



An exhibition of red light travelling through space from 

 the remotest star is due to propagation by waves or 

 vibrations, in which each individual particle of the trans- 

 mitting medium vibrates to and fro 400 million million 

 times in a second. 



Some people say they cannot understand a million million. 

 Those people cannot understand that twice two makes four. 

 That is the way I put it to people who talk to me about 

 the incomprehensibility of such large numbers. I say fini- 

 tude is incomprehensible, the infinite in the universe is com- 

 prehensible. Now apply a little logic to this. Is the 

 negation of infinitude incomprehensible? What would you 

 think of a universe in which you could travel one, ten, 

 or a thousand miles, or even to California, and then find 

 it come to an end? Can you suppose an end of matter 

 or an end of space? The idea is incomprehensible. Even 

 if you were to go millions and millions of miles the idea 

 of coming to an end is incomprehensible. You can under- 

 stand one thousand per second as easily as you can under- 

 stand one per second. You can go from one to ten, and 

 ten times ten and then to a thousand without taxing your 

 understanding and then you can go on to a thousand million 

 and a million million. You can all understand it. 



Now 400 million million vibrations per second is the kind 

 of thing that exists as a factor in the illumination 



