PREFACE 



echoes a short time later. It is especially stimulating to 

 examine the many effective ways in which animals make 

 use of echoes, and to compare these with artificial de- 

 vices which operate on the same basic principles. This 

 comparison illustrates the important fact that some of 

 the most difficult scientific questions have been solved 

 by co-operation between different branches of science 

 or technology. A century-old mystery of zoology was 

 largely dispelled by one afternoon in the appropriate 

 physics laboratory. And physicists faced with discourag- 

 ing practical problems are inspired to believe that their 

 tasks are not quite hopeless when they consider the 

 accomplishments of even the smallest living brains. 



Finally, there is the hope, still far from realization, 

 that full and proper use of the physics and biology of 

 echoes may serve to lessen the handicap of blindness. 

 For what blind men attempt crudely, in fijiding their way 

 about in a world of darkness, specialized animals ac- 

 complish widi truly marvelous skill and eflBciency. Elec- 

 tronic instruments also accomplish the seemingly impos- 

 sible by detecting invisible targets at great distances. 

 There is an important unity in the role which echoes play 

 in the biology, psychology, and physics of orientation. 



This account of fruitful co-operation among many dif- 

 ferent kinds of scientists has drawn upon much of their 

 published work. Some of this material is discussed at 

 greater length in Listening in the Dark, and I am grate- 

 ful to the editors of the Yale University Press for per- 

 mission to use part of its subject matter. Readers in- 

 terested in more detailed information will also find 

 helpful references in the short bibliography on page 147. 

 I have received many helpful suggestions from the staff 

 of the Physical Science Study Committee. A large num- 

 ber of companions and colleagues have participated in 

 my own observations and experiments, and their aid and 



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