82 Royal Society : — 



further ; for it has been evinced in manifold experiments that when 

 the axes of all the pencils of objective light which concur in imaging 

 a picture upon the retina are normals to its surface, any point in 

 the picture may be perceived as lying in successive directions, form- 

 ing very variable angles with and round about its normal. The 

 retina cannot inform us of the visible direction of any point painted 

 on its surface. 



This being so, there is no alternative but to seek for a solution 

 of the mystery in the structure out of which the retina proceeds, 

 for the property in question is plainly inherent in the visual ner- 

 vous apparatus. 



The author recalls that he had long ago pointed out that, though 

 the optic nerve in its orbital course and its fibres in their retinal 

 course are obnoxious to mechanical pressure, no visual sensation 

 can be immediately produced by such pressure on nerve-trunks or 

 branches. Sensation can only be produced by pressure through the 

 sclerotic by affecting the rods and cones of the bacillar layer, and 

 then only when the flexure of the retina crowds together the internal 

 (as to the eye) ends of the bacillar elements. He cites his former 

 words : — " When we turn in the dark the eyeballs sharply, or even 

 mildly, a couple of white circular rings, brighter at one margin than 

 the other, enclosing a paler area with a central dark spot, flash forth, 



the diameter extending an angle of several degrees The 



phenomenon is plainly the result of flexure of the retina where the 

 nerve runs into it, as the eye is pulled round in its socket until it 

 drags upon the nerve ; and it is to be noted that it is again where 

 the inner retinal elements are squeezed laterally that the pheno- 

 menon is disclosed." The absence of the tough and dense sclerotic 

 where the nerve penetrates it, as well as of the choroid, indicates 

 how readily the nerve must yield to the slightest traction. 



In these previously recorded facts the author feels assured that 

 he had, unwittingly, provided himself with a key to the secret of 

 visible direction. 



For it has been shown by diversified experiments that whenever 

 there is a parallax in visible direction it is accompanied by a dis- 

 placement of the base of the optic nerve in the same direction — that 

 is to say, by traction upon the nerve-stem, tending to carry its 

 distal extremity that way. The " white circular rings, brighter at 

 one margin than the other," have been instanced as proclaiming 

 that such traction cannot occur without flexure between the nerve- 

 -stem and the eye-apple, which displays itself at the junction of the 

 optic disk with the surrounding retinal expansion. In other words, 

 under the concordant action of the orbital muscles, all the move- 

 ments of the globe are so equably coordinated that the nerve-stem 

 is never subjected to unwonted traction, and consequently always 

 emerges through the ocular tissues to open out into the retina as a 

 normal to their surfaces, in which case no visual parallax appears. 

 But no sooner is there lateral traction than the axis of the emergent 

 nerve-stem, or of the optic disk, deviates from the said normality ; 

 and were that disk impressible by objective light, its central point 



