22 Foster, Physical Basis of Psychical Events. 



be the natural one of light or an artificial one, such as 

 pressure, an electric current, a chemical agent and the like, 

 we do get psychical effects, we do produce visual sensations, 

 and the visual sensations resulting from an artificial stimu- 

 lation are, in their essential features, identical with those 

 resulting from natural stimulation. But if the optic fibres be 

 stimulated directly, no psychical effect is produced, neither 

 sensations of light, nor sensations of pain, nor sensations 

 of any kind. We infer that the natural nervous impulses 

 which sweep along the optic fibres when the retina is 

 stimulated are of a different nature from those which sweep 

 along it when it is directly stimulated by artificial means, 

 though both give the same token of currents of action. 

 The former are differentiated impulses, and it is differ- 

 entiated impulses alone which can so act upon the central 

 cerebral portion of the visual mechanism as to give rise 

 to psychical effects. 



I may here remark in passing that a similar arrange- 

 ment probably obtains in regard to the psychical effects 

 of sensation, of touch proper, and of temperature, as 

 developed in ordinary sensory cutaneous nerves ; artificial 

 stimulation will produce pain, but not the specific tactile 

 or temperature sensation. The case, however, is not so 

 clear here as it appears to be in vision, and I only 

 mention it incidentally. 



To return to visual matters. The optic impulses, as 

 they travel along the optic nerve, have already become 

 differentiated. Though we possess no experimental 

 evidence on the matter, we may infer that the differentia- 

 tion is effected by successive steps at the several linkages. 

 We do not know the details of the changes which take 

 place in the cone cell, in the primary visual unit when 

 light falls upon it ; somehow or other the disturbances of 

 the ether give rise to material changes, changes which we 

 may probably characterise as nervous, travelling along 



