CHAP, in.] SIGHT. 1253 



light are able to bring about the decomposition of very many 

 chemical substances; and we accordingly speak of these substances 

 as being sensitive to light. All the facts dwelt on in this book 

 illustrate the great complexity and corresponding instability of the 

 composition of living matter. And we might reasonably suppose 

 that living matter itself would be sensitive to light ; that is to 

 say that rays of light falling on even undifferentiated protoplasmic 

 substance might set up a decomposition of that substance and so 

 bring about a molecular disturbance ; in other words, that light 

 might act as a direct stimulus to living matter. As a matter of 

 fact, however, we meet with very little evidence of this, especially 

 when we make a distinction between thermic rays, rays which 

 though they produce physical results are to us invisible, and 

 luminous rays which alone when they fall on our retina give rise 

 in us to the sensation of light. Nor can we be surprised at this 

 apparent indifference of living matter towards light when we 

 reflect that living matter in what we may call its purest form is 

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

 pass through it with exceedingly little absorption. But in order 

 that light may produce chemical effects, it must be absorbed ; its 

 energy must be spent in doing the chemical work. Accordingly 

 the first step towards the formation of an organ of vision, that is 

 to say an organ through which the body of a living being 

 reacts towards light, is the differentiation of a portion of the sub- 

 stance of the body 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 body 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 suppose that the chemical substances thus produced 

 are of such a nature as to induce other chemical changes, or in 

 some way or other to act as a stimulus to other parts of the 

 organism, (and we have manifold evidence of the exquisite sensi- 

 tiveness of living matter 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. 

 A comparatively simple illustration of this is afforded by some of 

 the lowly organisms called bacteria, especially by the one which 

 has been called bacterium photometricum. This organism is 

 remarkably sensitive to light, and especially reacts towards certain 

 rays of light. It is coloured with a purple pigment, apparently 

 allied to chlorophyll ; and the rays of light, to which it is espe- 

 cially sensitive, are just those which are absorbed by the pigment. 

 773. Photochemistry of the Retina. Such considerations as 



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