406 Prof. S. P. Thompson on the Function of 



trace any electricity whatever to this source. The problem of 

 the source of the electricity of the atmosphere is still unsolved. 

 Evaporation, first proposed by Volta, whose theory until now 

 has been better supported by experiment than any other, fails 

 to account even for a small portion of it; and no other source 

 has been proposed which can yet be considered sufficient. 



LI. On the Function of the two Ears in the Perception of 

 Space. By Professor Silvants P. Thompson, D.Sc, B.A. 



1. rf^HE conceptions formed in our minds of the exten- 

 J- sion of space maybe resolved into two parts: — (1) 

 the conception of distance independent of direction; (2) the 

 conception of direction independent of distance. 



These conceptions are founded upon the perceptions of three 

 separate senses, each of which, though distinct from the 

 others, enables us to form conceptions both of linear magni- 

 tude (or distance) and of angular magnitude (or direction). 

 These three senses are: — 



(a) The muscular sense; 



(b) The optical sense; 

 (r) The auditory sense. 



(a) The muscular sense gives us direct perceptions of linear 

 magnitude and of angular magnitude through the sensations 

 produced by extending the limbs and by twisting the frame 

 and head. 



(b) The optical sense gives us perceptions of angular direc- 

 tion of two kinds: — the first of them depending upon the optical 

 formation of images of external objects upon the differentiated 

 nerve-structures of the retina of the eye ; the second of them 

 being partly optical, partly muscular, and derived from the 

 sense of muscular effort required to move the eye-balls, or the 

 whole head, into the position in which light from an object 

 situated in any given direction shall be most easily observed. 

 The optical perception of distance is of three kinds — one kind 

 being indirect and associative, the other two muscular. The 

 first of these depends upon the possession of two eyes, giving, 

 in consequence, two retinal pictures with slight differences, 

 which we gradually learn to associate with varying conditions 

 of distance. The second, and principal, of them depends also 

 upon the possession of two eyes, the greater or lesser amount 

 of muscular effort required to converge their optic axes in 

 viewing an object affording a basis for a mental estimate of 



* Communicated by the Author, having- been read before Section D 

 of the British Association at York, September 1881. 



