THE SENSES 



927 



specially instructed, carries his belief with him to his grave, that 

 when he looks at an apple he sees a round, smooth, tolerably hard 

 body, of definite size and colour ; while in reality all that the sense of 

 sight can inform him of is the difference in the intensity and colour 

 of the light falling on his retina when he turns his head in a particular 

 direction. 



An interesting illustration of the role of experience in shaping 

 our visual judgments is found in the sensations of persons born 

 blind and relieved in after-life by operation. A boy between 

 thirteen and fourteen years of age, operated on by Cheselden, 

 thought all the objects he looked at touched his eyes. ' He 

 forgot which was the dog and which the cat, but catching the 

 cat (which he knew by feeling), he looked at her steadfastly 

 and said, " So, puss, I shall know you another time." f Pictures 

 seemed to him only parti-coloured planes ; but all at once, two 

 months after the operation, he discovered they represented 

 solids.' Nunnely, perhaps remembering the dictum of Diderot, 

 true as it is in the main, though tinged with the exaggeration 



FIG. 404. ILLUSION OF PARALLEL LINES (HERING). 



of the Encyclopedic, that ' to prepare and interrogate a person 

 born blind would not have been an occupation unworthy of the 

 united talents of Newton, Des Cartes, Locke, and Leibnitz/ 

 made an elaborate investigation in the case of a boy nine years 

 old, on whom he operated for congenital cataract of both eyes, 

 and, what is of special importance, instituted a set of careful 

 experiments and interrogations before the operation, so as to 

 gain data for comparison. Objects (cubes and spheres) which 

 before the operation he could easily recognise by touch were 

 shown him afterwards, but although ' he could at once perceive 

 a difference in their shapes, he could not in the least say which 

 was the cube and which the sphere.' It took several days, 

 and the objects had to be placed many times in his hands before 

 he could tell them by the eye. ' He said everything touched 

 his eyes, and walked most carefully about, with his hands held 

 out before him to prevent things hurting his eyes by touching 

 them.' 



Many other illustrations might be given of the fact that 

 ' seeing ' is largely an act of reasoning from data which may 

 sometimes mislead. Thus in Figs. 404 and 405 the long hori- 



