802 THE EYE AND VISION [CH. LVI. 



visual sensation is perceived by the cerebrum, is not yet understood. 

 It is supposed that the change effected by the agency of the light 

 which falls upon the retina is in fact a chemical alteration in the 

 protoplasm, and that this change stimulates the optic nerve-endings. 

 The discovery of a certain temporary reddish-purple pigmentation of 

 the outer limbs of the retinal rods in certain animals (e.g., frogs) 

 which had been killed in the dark, forming the so-called rhodopsin or 

 visual purple, appeared likely to offer some explanation of the 

 matter, especially as it was also found that the pigmentation dis- 

 appeared when the retina was exposed to light, and reappeared when 

 the light was removed, and also that it underwent distinct changes 

 of colour when other than white light was used. It was also found 

 that if the operation were performed quickly enough, the bleached 

 image of a bright object (optogram) might be fixed on the retina by 

 soaking the retina of an animal which has been killed in the dark, in 

 alum solution. 



The rhodopsin is derived in some way from the black pigment 

 (melanin or fuscin) of the polygonal epithelium of the retina, since 

 the colour is not renewed after bleaching if the retina is detached 

 from its pigment layer. 



Certain pigments, not sensitive to light, are contained in the 

 inner segments of the cones. These are oil globules of various 

 colours, red, green, and yellow, called chromophanes, and are found 

 in the retinae of marsupials (but not other mammals), birds, reptiles, 

 and fishes. Nothing is known about the yellow pigment of the 

 yellow spot. 



Another change produced by the action of the light upon the 

 retina is the movement of the pigment cells. On being stimulated by 

 light the granules of pigment in the cells which overlie the outer 

 part of the rod and cone layer of the retina pass down into the 

 processes of the cells, which hang down between the rods: these 

 melanin or fuscin granules are generally rod-shaped, and look almost 

 like crystals. In addition to this, a movement of the cones and possibly 

 of the rods occurs, as has been already mentioned ; in the light the 

 cones shorten, and in the dark they lengthen (Engelmann). 



Dewar and McKendrick were the first to show that the chemical 

 changes in the retina are accompanied with an electrical change. 



Red light has no action on visual purple ; the maximum bleaching effect takes 

 place in greenish-yellow light. Now, when the living eye is brought into a condition 

 of "dark adaptation," that is, when the retina has become adapted to light of low 

 intensity, the colours of the spectrum alter in brightness ; the red end becomes 

 shortened and much darker; the blue end becomes brighter, and the region of 

 maximum brightness is in the green. This change of brightness with change of 

 adaptation is absent in the fovea, where there are no rods. The selective action of the 

 colours of the spectrum on the visual purple is so strikingly similar to the altered 

 conditions of brightness just described, that changes in the visual purple of the rods 

 have been supposed to be the cause of sensations excited by feeble illumination (i. e. , 



