244 RECENT 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 retime, 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 bring inverted and double. He knows what im- 

 pression such and such an object in such and such a position 

 makes on him thrcmgh 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, de- 

 pends, first, on our having movable parts of our own body with- 

 in sight ; so that, when we once know by means of touch what 

 relation in space and what movement is, we can further learn 

 what changes in the impressions on the eye correspond 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 sensation were qxiite 

 arbitrary, thrown together without any systematic arrangement 

 (a supposition which 1 regard as improbable), it would still be 

 possible by means of the movements of the hand and of the eye, 

 as just described, to ascertain which signs go together, and which 

 correspond in different regions of the retina to points at similar 

 distances in the two dimensions of the field of vision. This is 



fibres of the left and the inner of the right retina ; just as the inner reins of 

 both horses cross, so that the outer rein of the off horse and the inner of the 

 near one run together to the driver's right hand, while the inner rein of the off 

 and the outer of the near horse pass to his left hand. Tn. 



