512 THE HUMAN BODY. 



When we want to see anything distinctly we always turn 

 our eyes so that its image shall fall on the centre of the 

 yellow spot. 



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

 produce nerve stimuli is still uncertain. Recent observa- 

 tions show that it produces chemical changes in the rod and 

 cone layer, and seemed at first io 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. JHL_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 putre- 

 faction lias 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 

 rapidly 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 obtained 

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

 to produce chemical changes which give rise to colored 

 bodies, while here the reverse is the case. If the eye be 

 not rapidly excised and put in the alum after its exposure, 

 the optogram will disappear; the vision purple being rapidly- 

 regenerated at the bleached part. This reproduction of ft 

 is due mainly to the cells of the pigmentary layer of the 

 retina, which in living eyes exposed to light thrust long 

 processes between the rods and cones. Portions of frogs' 

 retinas raised from this bleach more rapidly than those left 

 in contact with it, but become soon purple again if let fall 

 back upon the pigment- cells. Experiments show, however, 

 that animals (frogs) exposed for a long time to a bright light 

 may have their retinas completely bleached and still see 

 very well; they can still unerringly catch flies that come 



