318 Notes on Physiological Optics. 



-with it. If it be said that we unconsciously perceive them 

 and intuitively distinguish between the two kinds, homony- 

 mous and heteronymous, this conclusion cannot be confirmed 

 or disproven, except so far as experiments like those just 

 detailed may be accepted as having some bearing upon the 

 subject. The -writer's disposition is to discard intuition 

 entirely, and, with Helmholtz*, to regard the degree of atten- 

 tion bestowed upon objects pictured at the same moment on 

 different parts of the two retinas as an element of more import- 

 ance than either play of the eyes or the perception of double 

 images. The point in the field of view to which most atten- 

 tion is habitually given is that pictured upon corresponding- 

 retinal parts ; but the attention is at the same moment divided, 

 being given in less degree to many other parts of ihe field of 

 view as simultaneously perceived with each eye. The mental 

 suggestion due to the impression of non-corresponding parts 

 is that of the third dimension in space. If this be called the 

 perception of double images, their effect seems to be dependent 

 upon their not emerging into consciousness. Add to this the 

 fact that the gradation between single and double vision is 

 wholly imperceptible, and hence that for infinitesimal depar- 

 tures from single vision there can be no demonstrable distinc- 

 tion between the two kinds of double images. In the inter- 

 pretation of our sensations we are certain that experience is 

 our habitual guide, though by no means always a reliable one. 

 Whether intuition can be accepted as an additional guide at 

 all, it is not easy to pronounce. The debate between the 

 advocates of the empiristie and nativistic theories is doubtless 

 like the well-known quarrel about a certain shield, and may be 

 continued indefinitely. The domain of intuition is certainly 

 far more limited than was thought a few generations ago ; 

 whether it can be reduced to zero may perhaps be decided a 

 few generations hence. In all ordinary cases of binocular 

 vision the effect is cumulative. The judgment quickly reached 

 is a product not only of difference in the degree of attention 

 given at the same moment to objects seen by direct and by 

 indirect vision respectively, but also of variation in attention 

 to different points directly viewed in succession, of the mus- 

 cular sense while free play is given to the eyes, and of all the 

 elements available in monocular vision, which have been 

 grouped together under the name of physical in contrast with 

 p hy siological perspective. 



* Helmlioltz, Ojjtique Physiologique, p. 1009. 



