RECEPTOR ORGANS 



523 



a 



of the complexity of the vertebrate effect may be due to the nervous elements 

 of the retina. 



If we regard it as probable that the electrical response is connected with a 

 photo - chemical re- 

 action, we may con- 

 sider for a moment 

 such a reaction as 

 that of the decom- 

 position of silver 

 chloride by light in 

 the simplest condi- 

 tions, so that the 

 products are not 

 removed from the 

 sphere of action by 

 other reactions. In 

 the dark, we know 

 that silver and 

 chlorine reunite to 

 form the chloride 

 again, and at a 

 rate controlled, in 

 the main, by mass 

 action. As soon as 

 light begins to act 

 on the chloride, a 

 portion is decom- 

 posed and the pro- 

 ducts begin to 

 recombine again 

 owing to their own 

 affinity and inde- 

 pendently of the 

 action of light. It 

 will be clear, there- 

 fore, that, according 

 to the intensity of 

 the illumination, an 

 equilibrium will be 

 established at such 

 a position that the 

 rate of recombina- 

 tion is equal to that FIG. 165. ELECTRICAL CHANGES PRODUCED BY LIGHT IN THE 

 of decomposition RETINA. String galvanometer. The white areas are 



periods of illumination, the black areas are periods of 



darkness. 



Time in O'l second at the top. 



d 



e 



As soon as illumina- 

 tion ceases, recom- 

 bination begins at 

 its own proper rate, 

 since the opposing 

 reaction no longer 



O 



exists. Further par- 

 ticulars of such 

 photo-chemical 

 changes will be 

 found in Chapter 



a, Eye of pigeon. Brief illumination (0'23 second). Negative and positive 



variations with light, positive with subsequent darkness. 



b, Eye of pigeon. Longer period of action of light (072 second). 



<j, Effect of brief periods of darkness (O'l second each) on the pigeon's eye 

 previously exposed to continuous illumination. 



d, Eye of rabbit. Illumination for 0'5 second. Positive variation with light 



and secondary slow rise. Small, slow negative variation, after short 

 latent period, with subsequent darkness. 



e, Eye of Cephalopod (Eledone). Simple positive variation with light, after 



latent period of 0'023 second ; remaining practically constant during 

 illumination. Return to original value with darkness, also after a latent 

 period. 



(After Piper. ) 



XIX., and we shall 



see that there are other reactions which would illustrate the point more accurately, 

 since the decomposition of silver salts by light is not quite so simple as assumed 

 above. It serves, however, to illustrate the nature of the apparent equilibrium 



