THE PERCEPTION OF SIGHT. 243 



of vision, and many hypotheses have been invented to explain it. 

 Two of these have survived. We may, with Johannes M tiller, 

 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 sup- 

 pose that the feeling of correspondence between what is upper 

 in the sense of sight and in the sense of touch is only acquired 

 by experience, when we see the hands, which feel, moving in 

 the field of vis-ion. Or, secondly, we may assume with Fick ' 

 that, since all impressions upon the retina must be conveyed to 

 the brain in order to be there perceived, the nerves of sight and 

 those of feeling are so arranged in the brain as to produce a 

 correspondence between the notion they suggest of upper and 

 under, right and left. This supposition has, however, no pre- 

 tence of any anatomical facts to support it. 



The second difficulty for the Intuitive Theory is that, while 

 we have two retinal pictures, we do not see double. This diffi- 

 culty was met by the assumption that both retinae when they 

 are excited produce only a single sensation in the brain, and 

 that the several points of each retina correspond with each 

 other, so that each pair of corresponding or 'identical' points 

 produces the sensation of a single one. Now there is an actual 

 anatomical arrangement which might perhaps support this 

 hypothesis. The two optic nerves cross before entering the 

 brain, and thus become united. Pathological observations make 

 it probable that the nerve fibres from the right-hand halves of 

 both retinse pass to the right cerebral hemisphere, those from 

 the left halves to the left hemisphere. 2 But although corre- 

 sponding nerve fibres would thus be brought close together, it 

 has not yet been shown that they actually unite in the 

 brain. 



1 Ludwig Fick, late Professor of Medicine in the University of Marburg, 

 the brother of Prof. Adolf Fick, of Zurich. 



2 We may compare the arrangement to that of the reins of a pair of horses : 

 the inner fibres only of each optic nerve cross, so that thoi-e which run to the 

 right half of the brain are the outer fibres of the right and the inner of the left 

 retina, while those which run to the left cerebral hemisphere are the outer 



