THE SIX GATEWAYS OF KNOWLEDGE. 281 



the limits of matter, as known to us, to vibrate. 

 Vibrations transmitted as waves through steel, or 

 air, or water, cannot be more frequent than a 

 certain number, which I cannot now put a 

 figure to, but which, I say, may be reckoned 

 in hundred-thousands or a few millions per 

 second. 



But now let us think of light. The sense of 

 sight may be compared to the sense of sound in 

 this respect that it also is a matter of vibration. 

 Light we know to be an influence on the retina of 

 the eye, and through the retina on the optic 

 nerve ; an influence dependent on vibrations, whose 

 frequency is something between 400 million millions 

 per second and 800 million millions per second. 

 Now we have a vast gap between 400 per second, 

 the sound of a rather high tenor voice, and 400 

 million millions per second, the number of vibra- 

 tions corresponding to dull red light the gravest 

 red light of the prismatic spectrum. Take the 

 middle of the spectrum yellow light the period 

 of the vibrations there is in round numbers 500 

 million millions per second. In violet light we 



