the two Ears in the Perception of Space. 407 



distance. The third of them does not depend upon the pos- 

 session of two eyes, or upon "binocular parallax/' but con- 

 sists in the perception by the muscular sense of the amount of 

 effort which the ciliary muscle must exert to focus the eye for 

 rays proceeding from an object at a given distance. The 

 author has elsewhere * shown how this last mode of perception 

 is complicated by the question of the colour of the rays 

 in consequence of the imperfect achromatization of the eye. 

 To these three a fourth, and purely associative, kind of 

 perception of distance must be added, in the apparent mag- 

 nitude of objects of known size. Of these modes of per- 

 ception, the two depending on binocular parallax are of 

 greatest importance; without them our optical conceptions of 

 solidity and distance would be extremely imperfect. The 

 theory of binocular vision was practically complete when the 

 invention of the stereoscope, and of its reductio ad absurdum 

 the pseudoscope, proved the general correctness of Wheat- 

 stone's views, though several corollaries to the theory have 

 since been elaborated by younger workers. 



(c) The auditory sense may in like manner afford us per- 

 ceptions of linear and of angular magnitude — that is to say, of 

 distance and of direction. But the theory of the acoustical 

 perception of space is strangely incomplete. Just as the pos- 

 session of two eyes gives us by binocular parallax the most 

 important of the optical means of perception of space, so the 

 possession of two ears is found, from the researches of Mach, 

 Luca, Steinhauser, Graham Bell, and of the present writer, 

 to give us, by binaural parallax, important means for receiving- 

 acoustical perceptions of space. Besides these binaural per- 

 ceptions, it is undeniable that imperfect monaural perceptions 

 of space exist. These will be noticed in passing. 



2. Any comparison of the ear and eye in respect of their 

 power of affording perceptions of space would be incomplete 

 if three points of essential difference in the arrangements of 

 these organs were not adverted to: — 



(i) The ear has no lens, and nothing equivalent to a lens 

 by which a sound-wave proceeding in any given direction 

 could be focussed on the receptive mechanism. The pinna or 

 auricle of the ear and its auditory meatus do not even serve, 

 as a hollow parabolic reflector might serve for light, to focus 

 rays coming in different directions on different points of the 

 receptive membrane. 



(ii) The receptive nerve-structure of the ear is differentiated 

 in a wholly different manner from that of the eye, so that its 

 different parts are not sensitive to different directions in the 

 * Phil, Mag. July 1877, [5] vol, iv. p. 48. 



