ECHOES OF BATS AND MEN 



and sound is in some ways even more useful. For exam- 

 ple, we can see somewhat less than 1 octave of frequen- 

 cies, or wave lengths, roughly from 4 to 7.5 X 10""^ centi- 

 meter. Our sense of hearing, on the other hand, extends 

 from about 20 to 20,000 c.p.s., a range of a thousand- 

 fold, or approximately 10 octaves. Audible sound can 

 thus contain a much richer variety of frequencies than 

 visible light, and this is partly why sound rather than 

 Ught is used for speech. Of course, there are other rea- 

 sons; for instance, living organisms cannot generate 

 hght, except for a few luminescent animals and plants. 



The sharp shadows cast by light make it less useful as 

 a vehicle for speech and short-range communication. 

 Just because sound does go around comers, it is useful 

 in calling and signaling, particularly when almost every 

 motion and contact between a person or animal and the 

 physical world around it generates some sound. The 

 great advantage of light to us is that it has short wave 

 lengths and, consequently, objects of small size give off 

 specular reflections. It is for this reason that eyes and 

 lenses can focus sharp images. Only when one tries to 

 use a microscope to see objects about the size of the 

 wave length of light does that wave length become an 

 important Hmitation. An object must be smaller than 

 one micron (one millionth of a meter) before it scatters 

 light rather than reflecting it. 



If sound waves and light waves did not already exist, 

 we well might find scientists trying to invent them, one 

 to form sharp images and permit accurate observations 

 of small details, the other with a wide-frequency spec- 

 trum to convey complex information with a minimum 

 of interference from shadow-casting obstacles. The two 

 types complement each other, and while the loss of our 

 sense organs for either is a major handicap, there is 



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