VISUAL SENSATIONS. 763 



falling on even undifferentiated protoplasm might set up a decomposition of 

 that protoplasm, and so inaugurate a molecular disturbance ; in other words, 

 that light might act as a direct stimulus to protoplasm. As a matter of fact, 

 however, such evidence as we at present possess goes to show that native 

 undifferentiated protoplasm is, as a rule, not sensitive to light (that is, to 

 those particular waves which, when they fall on our retina give rise in us to 

 the sensation of light), though in the case of some lowly organisms, whose 

 protoplasm exhibits very little differentiation and in particular contains no 

 pigment, a sensitiveness to light has been observed. Nor can we be surprised 

 at this indifference of protoplasm when we reflect that what we may call pure 

 protoplasm is remarkable for its transparency, that is to say, the rays of light 

 pass through it with the slightest possible absorption. But in order that 

 light may produce chemical effects, it must be absorbed ; it must be spent 

 in doing the chemical work. Accordingly, the first step toward the forma- 

 tion of an organ of vision is the differentiation of a portion of protoplasm 

 into a pigment at once capable of absorbing light and sensitive to light, 

 i. e.j undergoing decomposition upon exposure to light. An organism, a por- 

 tion of whose protoplasm had thus become differentiated into such a pig- 

 ment, would be able to react toward light. The light falling on the organ- 

 ism 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 suppose that the chemical sub- 

 stances 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 exqui- 

 site sensitiveness of protoplasm 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 otherwise 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 ter- 

 minations of the optic nerve are surrounded by substances sensitive to light, 

 then it becomes easy to imagine how light falling on these sensitive sub- 

 stances 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. 



651. 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 sometimes 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 

 shown that the pigment is sensitive to light ; but the changes in it induced 

 by light are excessively slow. Moreover, its presence cannot be of funda- 

 mental 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 expos- 

 ure to light, the retina appears colorless. 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 color. Upon exposure to light the 



