516 VISUAL SENSATIONS. [BOOK in. 



tion of a portion of protoplasm into a pigment at once capable of 

 absorbing light, and sensitive to light, i.e. undergoing decomposition 

 upon exposure to light. An organism, a portion of whose proto- 

 plasm had thus become differentiated into such a pigment, would 

 be able to react towards light. The light falling on the organism 

 would be in part absorbed by the pigment, and the rays thus 

 absorbed would produce a chemical action and set free chemical 

 substances which before were not present. We have only to sup- 

 pose that the chemical substances are of such a nature as to act as 

 a stimulus to the protoplasm of other parts of the organism, (and 

 we have manifold evidence of the exquisite sensitiveness of proto- 

 plasm in general to chemical stimuli,) in order to see how rays of 

 light falling on the organism might excite movements in it, or 

 modify movements which were being carried on, or might other- 

 wise affect the organism in whole or in part. 



Such considerations as the foregoing may be applied to even 

 the complex organ of vision of the higher animals. If we suppose 

 that the actual terminations of the optic nerve are surrounded by 

 substances sensitive to light, then it becomes easy to imagine how 

 light falling on these sensitive substances should set free chemical 

 bodies possessed of the property of acting as stimuli to the actual 

 nerve-endings and thus give rise to visual impulses in the optic 

 fibres. We say "easy to imagine/' but we are, at present, far from 

 being able to give definite proofs that such an explanation of the 

 origin of visual impulses is the true one, probable and enticing as 

 it may appear. 



One of the most striking features in the structure of the retina 

 is the abundance of black pigment in the retinal, or as it is some- 

 times called choroidal, epithelium. It is difficult to suppose that 

 the sole function of this pigment is to absorb the superfluous rays 

 of light, and that the rays thus absorbed are put to no use but 

 simply wasted. And indeed it has been shewn that the pigment is 

 sensitive to light; but the changes in it induced by light are ex- 

 cessively slow. Moreover its presence cannot be of fundamental 

 importance, since vision is not only possible but fairly distinct with 

 albinos in which -this pigment is absent. 



Then again, in the vast majority of vertebrate animals, the 

 outer limbs of the rods are suffused with a purplish-red pigment, 

 the so-called visual purple, which is so eminently sensitive to light 

 that images of external objects may by appropriate means be 

 photographed in it on the retina. When the eye of a frog or of a 

 rabbit is examined in an ordinary way, with full exposure to light, 

 the retina appears colourless. But if the eye be kept in the 

 dark for some time before it is examined, the retina, if removed 

 rapidly, will be found to be of a beautiful purplish-red colour. 

 Upon exposure to light the colour changes to yellow and then 

 fades away, leaving however the retina, not only white but more 

 opaque than it was before. Upon examination with the microscope 



