3 I4 POPULAR LECTURES AND ADDRESSES. 



notes of the bass voice, higher notes with 250 per 

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

 per second give about the shrillest notes audible 

 to the human ear. 



Instead of the numbers, which we have, say in 

 the most commonly used part of the musical 

 scale, i.e., 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 per- 

 formed when we have red light produced. 



An exhibition of red light travelling through 

 space from the remotest star is due to pro- 

 pagation by waves or vibrations, in which each 

 individual particle of the transmitting 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 in- 

 comprehensibility of such large numbers. I say 



