278 RECEXT PROGRESS OF THE THEORY OF VISION. 



These two difficulties do not apply to the Empirical 

 Theory, since it only supposes that the actual sensible 

 ' sign,' whether it be simple or complex, is recognised as 

 the sign of that which it signifies. An uninstructed 

 person is as sure as possible of the notions he derives 

 from his eyesight, without ever knowing that he has two 

 retinae, that there is an inverted picture on each, or that 

 there is such a thing as an optic nerve to be excited, or a 

 brain to receive the impression. He is not troubled by 

 his retinal images being inverted and double. He knows 

 what impression such and such an object in such and 

 such a position makes on him through his eyesight, 

 and governs himself accordingly. But the possibility of 

 learning the signification of the local signs which belong 

 to our sensations of sight, so as to be able to recognise 

 the actual relations which they denote, depends, first, on 

 our having movable parts of our own body within sight ; 

 so that, when we once know by means of touch what rela- 

 tion in space and what movement is, we can further 

 learn what changes in the impressions on the eye cor- 

 respond to the voluntary movements of a hand which we 

 can see. In the second place, when we move our eyes 

 while looking at a field of vision filled with objects at 

 rest, the retina, as it moves, changes its relation to the 

 almost unchanged position of the retinal picture. We 

 thus learn what impression the same object makes upon 

 different parts of the retina. An unchanged retinal 

 picture, passing over the retina as the eye turns, is like a 

 pair of compasses which we move over a drawing in order 

 to measure its parts. Even if the 'local signs' of sensa- 

 tion were quite arbitrary, thrown together without any 

 systematic arrangement (a supposition which I regard as 

 improbable), it would still be possible by means of the 

 movements of the hand and of the eye, as just described, 



