CHAP, iir.] SIGHT. 119 



retina stimulated by the light, over the white half of the field 

 of vision, this visual substance was wholly exhausted, while over 

 the half not so stimulated some remained, sufficient to give rise 

 to a fugitive visual sensation when that half was stimulated by 

 light. But the result may be explained without reference to any 

 special visual substance ; it is only natural that some part or the 

 whole of the retinal nervous apparatus should be sooner exhausted 

 when stimulation is added to deprivation of nourishment than 

 when stimulation is absent. We may here remark that pressure 

 on the eye-ball gives rise to temporary colour-blindness like 

 that existing in the periphery of the retina ; as the pressure 

 is continued red and green pass through yellow into white, 

 while yellow and blue pass directly into white. 



We have then no satisfactory evidence of the existence of any 

 visual substance or substances, of a photochemical or other nature, 

 lying outside the nervous elements. There may be such, but we 

 have at present no proof of their existence. And it must be 

 remembered, with regard to the hypothetical ' visual substances ' 

 of Hering's theory, or of other theories involving the existence of 

 visual substances, that it is not necessary that these should lie 

 outside the nervous conducting elements, or indeed should lie 

 exclusively in the retina. The visual substance is merely supposed 

 to be the basis of visual sensations, it is introduced to explain 

 these sensations ; but those sensations are developed in the brain, 

 and as we have already more than once insisted, it is always 

 difficult, and in many cases impossible to distinguish in a sensation 

 between central and peripheral events. All our knowledge goes 

 to shew that sensations, like other nervous processes, are the 

 outcome of the metabolism of nervous substance ; and it is at 

 present quite open to us to suppose that the visual substances, 

 changes in which we recognize as the basis of colour and other 

 visual sensations, are substances forming part of the nervous 

 material of the central organs of vision, each substance being 

 affected in its own way by nervous impulses generated in the 

 retina by rays of light of certain wave-lengths. The various 

 exhaustions, mixtures and the like, supposed by the theories, 

 would on this view take place in the central organ. On the 

 other hand the visual substances may reside in the retina and 

 the central organ may do little more than, so to speak, record 

 the changes which had already taken place in the retina. Or 

 perhaps we ought to make no such sharp distinction between 

 retina and central organ. Our present knowledge is unable to 

 decide these matters ; but the acceptance or rejection of the 

 theories of colour vision is quite independent of any view as to 

 the exact nature or position of the visual substances. 



775. Whatever view we adopt, whether photochemical or 

 other, as to the changes which lead to stimulation of the real 

 endings of the retinal nervous mechanism, we cannot at present 



