THE PERCEPTION OF LIGHT. , 17 



call be marked. It is then only a question of similar 

 triangles to determine how far behind the blood vessels lies 

 the percipient layer, and the distance thus calculated is 

 found to agree, within the limits of errors of observation, 

 with the distance of the bacillary layer as determined by 

 microscopic examination of a dissected eye. 



I have said as you go backward from the centre of the eye- 

 ball, you have, in front of the rest of the retina, a plexus, iis 

 it is called, of nerve-fibres lying side by side, something like 

 the threads in a skein of silk, but gradually leading onwards 

 to the optic nerve. Light passes across these, but it does not 

 excite the nerves in passing through them. The nerves are 

 transparent, and the light produces no effect upon them 

 directly. If it did, your whole field of view would be con- 

 fused, because it is known that when a nerve is excited the 

 sensation is referred to a particular part no matter Avhere the 

 nerve may be affected. Suppose you could isolate, say in the 

 thigh, a particular nerve leading to the great toe, and pinch 

 it Avithout hurting its neighbours, you would feel the pincli 

 not where the nerve is pinched, but in the great toe. So, 

 here, if these nerve-fibres were excited by the passage of 

 light through them, then the sensation corresponding to the 

 excitement of a particular nerve-fibre, Avhicli would be that 

 of a definite point in the field of view, would be excited by 

 an external luminous point lying anywhere in the curve in 

 which the surface generated by a straight line passing 

 through the optical centre and intersecting the fibre in 

 question would cut what we may call the celestial sphere, 

 and the correspondence between the subjective points in the 

 field of view and objective external points would be lost. 

 And the fact that the visual nerves are not affected by light 

 which passes across them is further shown by the well-known 

 experiment of the bhnd spot, where the optic axis passes out 

 of the eye-bail, not in the axis of vision but to one side, 

 towards the nose, so that an object Avliose image falls on the 

 blind spot of one eye is seen by means of the other. 



But now comes a question, and here we enter on uncertain 

 and debated ground — How is it that the nerves are stimu- 

 lated by the light at all 1 



We have reason to believe that these rods and cones form 

 the means by which the light, acting on them, causes the 

 stimulation of the nerve. As 1 have said they consist of two 

 elements, an inner and outer; the outer from the centre of 

 the eye, i.e., the inner as regards the body, being of that 



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