LIGHT, COLOR, AND SOUND 219 



Now you can understand that the apparent merging of 

 light and heat is more than an appearance; it is an actual 

 fact, for there are waves of radiant energy of certain lengths 

 that do actually produce upon our sense-organs the effects 

 of both light and heat. Also you can see that, by inter- 

 fering with the waves of light or heat rays, and thereby 

 altering their length, heat may be changed to light, or 

 light to heat. Similarly, as you have doubtless noted, 

 the little waves on water change their length when they 

 strike a rock or are otherwise interfered with. 



So we may go back to our question, what is light? We 

 find that it really isn't a thing, but it is an effect. That 

 is, it isn't a thing any more than pain or pleasure are real 

 things. It is simply an effect produced on us, through the 

 instrumentality of our eyes, by waves of radiant energy 

 of certain lengths. Eyes different from ours may get 

 entirely different sensations of light from ours. Thus 

 we say that a cat can see at night, while to an insect this 

 undoubtedly appears to be a very different world from 

 what it appears to us. You know that a tuning-fork 

 across the room will respond to sound vibrations of a cer- 

 tain pitch which are set up by striking a note on the piano. 

 Similarly, we may regard some of our sense-organs as in- 

 struments tuned up to certain pitches; to certain kinds of 

 vibrations they respond, to others they do not. If there 

 were no eyes to see, or ears to hear, or skin to feel, would 

 the things we call light, sound, and heat then really exist? 



Light-waves are shorter than those of heat, and travel 

 faster. They are very much shorter than those of sound, 

 and travel correspondingly faster. This fact you have 

 witnessed many times. You have seen the steam rise 

 from a locomotive-whistle before you have heard the 



