242 RECENT PROGRESS OF THE THEORY OF VISION. 



that we are able to distinguish local differences in the field of 

 vision. 



The difference, therefore, between the two opposing views is 

 as follows. The Empirical Theory regards the local signs 

 (whatever they really may be) assigns the signification of which 

 must be learnt, and is actually learnt, in order to arrive at a 

 knowledge of the external world. It is not at all necessary to 

 suppose any kind of correspondence between these local signs 

 and the actual differences of locality which they signify. The 

 Innate Theory, on the other hand, supposes that the local signs 

 are nothing else than direct conceptions of differences in space as 

 such, both in their nature and their magnitude. 



The reader will see how the subject of our present inquiry 

 involves the consideration of that far-reaching opposition 

 between the system of philosophy which assumes a pre-existing 

 harmony of the laws of mental operations with those of the 

 outer world, and the system which attempts to derive all 

 correspondence between mind and matter from the results of 

 experience. 



So long as we confine ourselves to the observation of a 

 field of two dimensions, the individual parts of which offer no, or, 

 at any rate, no recognisable, difference in their distances from the 

 eye so long, for instance, as we only look at the sky and distant 

 parts of the landscape, both the above theories practically offer 

 an equally good explanation of the way in which we form con- 

 ceptions of local relations in the field of vision. The extension 

 of the retinal image corresponds to the extension of the actual 

 image presented by the objects before us; or, at all events, there 

 are no incongruities which may not be reconciled with the Innate 

 Theory of sight without any very difficult assumptions or 

 explanations. 



The first of these incongruities is that in the retinal picture 

 the top and bottom and the right and left of the actual image 

 are inverted. This is seen in Fig. 30 to result from the rays of 

 light crossing as they enter; the pupil the point a is the retinal 

 image o(A,bo. This has always been a difficulty in the theory 



