THE E7E AS A SENSORY APPARATUS. 535 



point d on the wall, on the prolongation of the line joining 

 the nodal point, k, of the eye with c. If the candle be moved 

 so as to illuminate the point V of the sclerotic, the shadow of 

 v will be cast on c' and will accordingly seern on the wall to 

 move from d to d' . It is clear that if we know how far b is 

 from b', how far the wall is from the eye, and how far the 

 nodal point is from the retina (15 mm. or 0.6 incJa}, and 

 measure the distance on the wall from d to d', we c^f calcu- 

 late how far c is from c' : and then how far the vessel throwing 

 the shadow must be in front of the retinal parts perceiving 

 it. fin this way it is found that the part seeing the shadow, 

 that is the layer on which light acts, is just abouJMs far be- 

 hind the retinal vessels as the main vascular trumts of the 

 retina are in front of the rod and cone layer. It is, there- 

 fore, in that layer that the light initiates those changes which 

 give rise to nervous impulses ; which is further made obvious 

 by the fact that the seat of most acute vision iaihefovea cen- 

 tral is, where only this layer and the cone-fibres diverging 

 from it are present. When we want to see anything dis- 

 tinctly we always turn our eyes so that its image shall fall on 

 the centres of the yellow spots.J 



The Vision Purple. How light acts in the retina so as to 

 produce nerve stimuli is still uncertain. Recent observations 

 show that it produces chemical changes in the rod and cone 

 layer, and seemed at first to indicate that its action was to 

 produce 'substances which were chemical excitants of nerve- 

 fibres ; but although there can be little doubt that these 

 chemical changes play some important part in vision, what 

 their role may be is at present quite obscure. If a perfectly 

 fresh retina be excised rapidly, its outer layers will be found 

 of a rich purple color. In daylight this rapidly bleaches, but 

 in the dark persists even when putrefaction has set in. In 

 pure yellow light it also remains unbleached a long time, but 

 in other lights disappears at different rates. If a rabbit's eye 

 be fixed immovably and exposed so that an image of a window 

 is focused on the same part of its retina for some time, and 

 then the eye be rapildy excised in the dark and placed in 

 solution of potash alum, a colorless image of the window is 

 found on the retina, surrounded by the visual purple of the 

 rest which is, through the alum, fixed or rendered incapable 

 of change by light. Photographs, or optograms, are thus ob- 

 tained which differ from the photographer's in that he uses 



