CHAP, in.] SIGHT. 921 



On the other hand we may perhaps more naturally turn to a 

 chemical explanation. We are familiar with the fact that rays of 

 light are able to bring about the decomposition of very many 

 chemical substances ; and we accordingly speak of these sub- 

 stances 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 undif- 

 ferentiated 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 ab- 

 sorption. But in order that light may produce chemical effects, 

 it must be absorbed ; its energy must be spent in doing the chemi- 

 cal 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 substance 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 pig- 

 ment, 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. \Vr 

 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 or- 

 ganism, (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 



