THE SENSES 809 



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 operation, he discovered they repre- 

 sented solids.' Nunnely, perhaps remembering the dictum of 

 Diderot, true as it is in the main, though tinged with the exaggera- 

 tion 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 thingshurtinghis eyes by touching them. 1 



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. 307 and 308 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. 309 the spaces covered by A, B, and C are equal squares, 

 but A appears taller than B, and C smaller than either A or B. In 

 the same figure the lines D and E are of the same length, but E 

 seems considerably longer than D. 



The apparent size and form of an object is intimately related to 

 the size, form, and sharpness of its image on the retina. We are, 

 therefore, able to discriminate with great precision the unstimulated 

 from the excited portions of that membrane, especially in the fovea 

 centralis, and also the degree of excitation of neighbouring excited 

 parts. But instead of localizing the image on the retina as we localize 

 on the skin the pressure of an object in contact with it, we project 

 the retinal image into space, and see everything outside the eye. In 

 vision, in fact, we have no conception of the existence oi either retina 

 or retinal image ; and even the shadows of objects within the eye are 

 referred to points outside it. Thus, for instance, an opacity or a 

 foreign body in any of the refractive media and no eye is entirely 

 free from relatively opaque spots can be detected, and its position 

 determined by the shadow which it casts on the retina when the eye 

 is examined by a pencil of light proceeding from a very small point. 

 Let a diaphragm with a small hole in it be placed in front of the 

 eye at such a distance that a pencil diverging from the hole will pass 

 through the vitreous humour as a parallel beam, equal in cross- 

 section to the pupil (Fig. 310), and let the aperture be illuminated 

 by focussing on it the light of a lamp placed behind a screen. The 



