THE EYE AND VISION. [CH. LV. 



These observations were confirmed by examining in a similar 

 way the colour sensations of seventy other people, but there are 

 individual differences in the extent to which the colour sensations 

 overlap. 



Changes in the Retina during activity. 



The method by which a ray of light is able to stimulate the 

 endings of the optic nerve in the retina in such a manner that a 

 visual sensation is perceived by the cerebrum is not yet under- 

 stood. 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 disappeared when the retina was ex- 

 posed 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 visual purple cannot however be absolutely essential to the 

 due production of visual sensations, as it is absent from the retinal 

 cones, and from the macula lutea and fovea centralis of the human 

 retina, and does not appear to exist at all in the retinae of many 

 animals, e.g., bat, dove, and hen, which are, nevertheless, possessed 

 of good vision. 



However the fact remains that light falling upon the retina 

 bleaches the visual purple, and this must be considered as one of 

 its effects. If it produces chemical changes in other substances, 

 these must be colourless and so extremely difficult to discover. 

 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 coloured 

 bodies are oil globules of various colours, red, green, and yellow, 

 called chromophanes, and are found in the retinas of marsupials 

 (but not other mammals), birds, reptiles, and fishes. Practically 

 nothing is known about the yellow pigment of the yellow 

 spot. Another change produced by the action of the light 



