342 POPULAR LECTURES AND ADDRESSES. 



something like the ii,oooth for the lowest of the 

 red light. 



Lastly, how do we know the frequency of 

 vibration ? 



Why, by the velocity of light. How do we 

 know that ? We know it in a number of different 

 ways, which I cannot explain now because time 

 forbids, and I can now only tell you shortly that 

 the frequency of vibration for any particular ray 

 is equal to the velocity of light divided by the 

 wave-length for that ray. The velocity of light 

 is about 187,000 British statute miles per secom 

 but it is much better to take the kilometre- 

 which is about six-tenths of a mile for the unil 

 when we find the velocity is very accurate!; 

 300,000 kilometres, or 30,000,000,000 centimetres, 

 per second. Take now the wave-length of 

 sodium light, as we have just measured it 

 means of the salted spirit lamp, to be on< 

 i/,oooth of a centimetre, and we find the fn 

 quency of vibration of the sodium light to be 

 510 million million per second. There, then, 

 you have a calculation of the frequency from 



