ch. i.] THE RELATIVITY OF KNOWLEDGE. 19 



rays. First, we have the visible rays of medium refrangi- 

 bility, ranging from red to violet, and sometimes called the 

 Newtonic rays. Beyond the violet, in the outlying portions 

 of the spectrum, lie the so-called Ritteric rays, of greatest 

 refrangibility, which are not visible, but are manifested 

 through their actinic or chemical effects ; these are the rays 

 with which we photograph. Beyond the red, at the other 

 end of the spectrum, lie the so-called Herschellic rays, of 

 least refrangibility, which also are not visible, but are mani- 

 fested through their thermal effects. These invisible rays 

 differ from the visible physically, only by their different 

 periods of motion or wave-lengths, in which respect the 

 visible rays differ also among themselves, as is indicated by 

 their different colours. Bearing this in mind, let us con- 

 template the remarkable series of effects produced in our 

 consciousness by gradually increasing rates of vibration in 

 the particles of matter. Vibrations occurring less frequently 

 than 16 times in a second, produce in us the consciousness of 

 a succession of noises. Vibrations which occur oftener than 

 16 times, but less often than 30,000 times, in a second, 

 produce in us the consciousness of musical notes, which are 

 higher and higher in pitch as the vibrations are more rapid. 

 Vibrations occurring oftener than 30,000 times, but less often 

 than 458,000,000,000,000 times, in a second, do not affect us 

 through the ears, but the more rapid ones affect us through 

 the nerves of the skin, and produce in us the consciousness of 

 heat. Vibrations occurring at the rate of 458,000,000,000,000 

 in a second, affect us through the eyes, and produce in us the 

 consciousness of red light ; at the rate of 577,000,000,000,000 

 in a second, they produce in us the consciousness of green 

 light ; at the rate of 727,000,000,000,000 in a second, they 

 produce in us the consciousness of violet light. At still 

 higher rates than this, they cease to affect us through the 

 •iyea, and indeed produce in us no definite state of conscious- 

 ness at all, though they may be remotely concerned in keep- 



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