276 RECENT PROGRESS OF THE THEORY OF VISION. 



The reader will see how the subject of our present 

 enquiry involves the consideration of that far-reaching 

 opposition between the system of philosophy which as- 

 sumes 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 land- 

 scape, both the above theories practically offer an equally 

 good explanation of the way in which we form concep- 

 tions 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 of A, b of B. 

 This has always been a difficulty in the theory of vision, 

 and many hypotheses have been invented to explain it. 

 Two of these have survived. We may, with Johannes 

 Muller, regard the conception of upper and lower as only 

 a relative distinction, so far as sight is concerned — that 

 is, as only affecting the relation of the one to the other; 

 and we must further suppose that the feeling of corre- 

 spondence between what is upper in the sense of sight and 

 in the sense of touch is only acquired by experience. 



