ON THE PSYCHO-PHYSICAL VIEW OF NATURE. 505 



volved in the manner in which our senses of sight and 

 touch combine and arrange simple sensations into the 

 whole of a well-ordered perception of space ; for we do 

 not become able to appreciate the fact of the slow 

 and gradual growth of this perception, which takes 

 place in the early days of our infancy, till long after 

 we have actually gained full possession of it. Some- 

 thing similar exists with regard to language and 

 thought : we only hear of grammar and logic long 

 after the main difficulties of speech and thinking have 

 been unconsciously mastered, and if it were not for 

 the existence of other languages than our own, and 

 of an erroneous logic as exemplified in errors of cal- 

 culation and of measurement, it is doubtful whether 

 grammar and logic would have been so early developed. 

 As it is, the physiological problem of the formation of 

 our space perception was actually first forced upon 

 naturalists by the observation of pathological cases, such 

 as the acquisition of sight in later life through couching, 

 the existence of colour blindness, and a variety of optical 

 delusions which still serve as indispensable test cases for 

 the various theories that have been propounded. Only 

 when something turns out to be palpably wrong do we 

 begin to inquire what constitutes the right side of many 

 things. 



Thus the cases of Cheselden and Wardrop and the 

 colour blindness of Dalton set physiologists thinking 

 about the genesis of our space and colour perceptions. 

 A very great impetus perhaps the most valuable of all 27. 

 was given by Wheatstone's invention of the stereo- stone's 



stereoscope. 



scope in 1838 ; an instrument which, as it were through 



