536 THE HUMAN BODY. 



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 ut in the alum after its exposure, the 

 optogram will disappear ; the vision purple being rapidly re- 

 generated at the bleached part. This reproduction of it is 

 due mainly to the cells of the pigmentary layer of the retina, 

 which in living eyes exposed to light thrust long processes 

 betwee^he 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) exited for a long time to a bright light may have 

 their retinW completely bleached and still see very well; they 

 can still unerringly catch flies that come within their reach ; 

 and they can also distinguish colors, or at least some colors, 

 as green. Moreover, the vision purple is only found in the 

 outer segments of the rods ; there is none in the cones, and 

 yet these alone exist in the yellow spot of the human eye, 

 which is the seat of most acute vision; and animals, such as 

 snakes, which have only cones in the retina, possess no vision 

 purple and nevertheless see very well. 



It may be that other bodies exist in the retina which are 

 also chemically changed by light, but the changes of which 

 are not accompanied by alterations in color which we can see; 

 and, in the absence of the vision purple, seeing might be 

 carried on by means of these, which may be less quickly 

 destroyed by light and so still persist in the bleached retinas 

 of the frogs above mentioned. For the present, however, the 

 question of the part, if any, played in vision by such bodies 

 must be left an open one : and the possibility that the rods 

 and cones form an apparatus which directly converts ethereal 

 vibrations into nerve stimuli without any intervening chemi- 

 cal process must be borne in mind. 



The Intensity of Visual Sensations. Light considered as 

 a form of energy may vary in quantity ; physiologically, also, 

 we distinguish quantitative differences in light as degrees of 

 brightness, but the connection between the intensity of the 

 sensation excited and the quantity of energy represented by 

 the stimulating light is not a direct one. In the first place, "" 

 some rays excite our visual apparatus more powerfully than 

 others : a given amount of energy in the form of yellow light, 

 for example, causes more powerful visual sensations than the 



