238 RECENT PROGRESS OF THE THEORY OF VISION. 



relations of locality. No other sense can be compared with the 

 eye in this respect. The sense of touch, it is true, can distin- 

 guish relations of space, and has the special power of judging 

 of all matter within reach, at once as to resistance, volume, and 

 weight ; but the range of touch is limited, and the distinction 

 it can make between small distances is not nearly so accurate 

 as that of sight. Yet the sense of touch is sufficient, as experi- 

 ments upon persons born blind have proved, to develop complete 

 notions of space. This proves that the possession of sight is 

 not necessary for the formation of these conceptions, and we 

 shall soon see that we are continually controlling and correcting 

 the notions of locality derived from the eye by the help of the 

 sense of touch, and always accept the impressions on the latter 

 sense as decisive. The two senses, which really have the same 

 task, though with very different means of accomplishing it, 

 happily supply each other's deficiencies. Touch is a trustworthy 

 and experienced servant, but enjoys only a limited range, while 

 sight rivals the boldest flights of fancy in penetrating to illimit- 

 able distances. 



This combination of the two senses is of great importance 

 for our present task ; for, since we have here only to do with 

 vision, and since touch is sufficient to produce complete concep- 

 tions of locality, we may assume these conceptions to be already 

 complete, at least in their general outline, and may, accordingly, 

 confine our investigation to ascertaining the common point of 

 agreement between the visual and tactile perceptions of space. 

 The question how it is possible for any conception of locality to 

 arise from either or both of these sensations, we will leave till 

 last. 



It is obvious, from a consideration of well-known facts, that 

 the distribution of our sensations among nervous structures 

 separated from one another does not at all necessarily bring 

 with it the conception that the causes of these sensations are 

 locally separate. For example,, we may have sensations of light, 

 of warmth, of various notes of music, and also perhaps of an 

 odour, in the same room, and may recognise that all these agents 

 are diffused through the air of the room at the same time, and 



