ECHOES OF BATS AND MEN 



practical problem in testing the "skin pressure theory" 

 was to shield the subjects' skin from any possible in- 

 fluence that might be arriving from the obstacle, and 

 this was doubly difi&cult because no one could say what 

 this might be— air currents, electromagnetic radiation, 

 heat or cold, or possibly some sort of energy not known 

 to physics. To test the sound theory, the logical proce- 

 dure was to prevent sounds from reaching the subjects' 

 ears without interfering with whatever the skin might be 

 feeling as a result of proximity to the obstacles. The 

 covering of the skin clearly had to be accomplished with- 

 out interfering with the subjects' hearing, and vice versa. 

 The final outfit that the subjects were obliged to wear 

 consisted of a long veil of thick felt which covered the 

 head and shoulders, plus heavy leather gloves to shield 

 the hands. Ordinary clothing covered the rest of the body 

 surface. Such was the protection that they could not feel 

 even the air current of an electric fan directed at their 

 heads. After some preliminary trials to accustom them 

 to walking about in this "armor," the subjects found 

 they could detect the screen almost as well as ever. The 

 average distance of first detection had been 6.9 feet with 

 no veil or gloves, and it was now reduced only slightly 

 —to 5.25 feet. This seemed to dispose of the possibility 

 that obstacles were detected by feehng them through the 

 skin, despite the fact that originally some of this group 

 of subjects, like many blind people, were certain that 

 they felt the obstacles with the hands or face. 



The next experiment was to leave the hands and face 

 completely free but to cover the subjects' ears. Earlier 

 experiments of this kind had given conflicting results; 

 sometimes the detection of obstacles was impaired, 

 sometimes not. Complete exclusion of sound by ear- 

 plugs is not possible, but Supa, Cotzin, and Dallenbach 

 wished to be sure that as httle sound as possible reached 



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