VISION 



tool 



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." Pictures seemed to him only 

 parti-coloured planes ; but all at once, two months after the opera- 

 tion, he discovered they represented solids. 1 Nunnely, perhaps 

 remembering the dictum of Diderot, that ' to prepare and interro- 



Fig. 420. Illusion of Parallel Lines (Hering). 



gate a person born blind would not have been an occupation un- 

 worthy 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 recognize by touch were shown him 



afterwards, but al- 



though * he could at 

 once perceive a differ- 

 ence in their shapes, 

 he could not in the 

 least say which was 

 the cube and which 



XXXUXUIXIUXUXU 



xxxxxxxxuxuuxu) 

 '//XT////////;////// 



Fig. 421. Illusion of Parallel Lines (Zollner). 



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. 420 and 421 the long horizontal lines are 

 really parallel, but do not appear so owing to the confusion of 

 judgment produced by the short sloping lines. In Fig. 422 the 

 spaces covered by A, B, and C are equal squares, but A appears 



