262 RECENT PROGRESS OF THE THEORY OF VISION. 



Let us now put together the results to which our enquiry 

 into binocular vision has led us. 



I. The excitement of corresponding points of the two 

 retinae is not indistinguishably combined into a single impres- 

 sion ; for, if it were, it would be impossible to see Stereoscopic 

 Lusti'e. And we have found reason to believe that this effect 

 is not a consequence of Retinal Rivalry, even if we admit the 

 latter phenomenon to belong to sensation at all, and not rather 

 to the degree of attention. On the contrary, the appearance of 

 lustre is associated with the restriction of this rivalry. 



II. The sensations which are produced by tne excitation of 

 corresponding points of each retina are not indistinguishably 

 the same ; for otherwise we should not be able to distinguish 

 the true from the inverted or ' pseudoscopic ' relief, when two 

 stereoscopic pictures are illuminated by the electric spark. 



III. The combination of the two different sensations received 

 from corresponding retinal points is not produced by one of 

 them being suppressed for a time ; for, in the first place, the 

 perception of solidity given by the two eyes depends upon our 

 being at the same time conscious of the two different images, 

 and, in the second, this perception of solidity is independent of 

 any movement of the retinal images, since it is possible under 

 momentary illumination. 



We therefore learn that two distinct sensations are trans- 

 mitted from the two eyes, and reach the consciousness at the 

 same time and without coalescing; that accordingly the com- 

 bination of these two sensations into the single picture of the 

 external world of which we are conscious in ordinary vision is 

 not produced by any anatomical mechanism of sensation, but by 

 a mental act. 



IV. Further, we find that there is, on the whole, complete, 

 or at least nearly complete, coincidence as to localisation in the 

 field of vision of impressions of sight received from correspond- 

 ing points of the retinae ; but that when wo refer both impres- 

 sions to the same object, their coincidence of localisation is much 

 disturbed. 



If this coincidence were the result of a direct function of 



