500 



Dr. G. J. Burch. On Light- 



[Apr. 19 ! 



continuous illumination the sensation of light at any instant is not clue 

 exclusively to the light falling on the eye at that instant, hut includes also- 

 the remainders left over from previous illumination. 



Now in the electrical stimulation of nerve, and nerve-muscle preparations, 

 after a quite moderate strength of stimulus has been reached no further 

 increase causes any modification of the response. This was abundantly 

 evident in the experiments by Gotch and myself with the capillary electro- 

 meter — and, moreover, there was no sign of " fatigue " of nerve unless the 

 excitation was excessively great. It is fair to conclude that when, under the- 

 action of light, the exciting decomposition products reach a certain concen- 

 tration, no further increase adds anything to the intensity of the resultant 

 light-sensation. 



This may be far stronger than is pleasant, just as the muscular contractions 

 during cramp may be excessively painful, but it does not seem probable that 

 any compound produced as the result of a normal physiological process can 

 be of such character or of such concentration as to destroy the tissues in 

 which it originates. We should thus have a maximum limit to the possible 

 intensity of the light-sensation — a limit independent of the particular 

 physiological condition of the eye at the moment. Were it not for this the 

 theory of forced vibrations would indicate that with intense and long- 

 continued illumination the several colour-sensations must stand out more and 

 more distinctly instead of becoming paler and tending towards white. 



But there is evidence of another factor, purely physiological, which I may 

 term the shunt-factor, whereby the strength of the sensation is governed. 



I am inclined to think that this factor, manifesting itself under various, 

 conditions, affords the explanation of several quite different phenomena. 



Thus Charpentier's bands may be taken as evidence that the sudden onset 

 of a fairly bright illumination over a large surface results in a sensation of 

 intermittent intensity. Shelford Bidwell's experiments with pigments and 

 my own with spectral colours show that each colour-sensation acts inde- 

 pendently of the others in this respect. Purkinje's recurrent images, especially 

 in the striking form described by McDougall * exhibit the same thing in 

 connection with the positive after-effect. Some controlling mechanism is- 

 set in action whereby the positive after-effect, as it dies away, is periodically 

 shut off like the sound in the swell box of an organ. And this action, like 

 that of the bands of Charpentier, gives fairly rapid alternations. I take it 

 to be a spasmodic excitation of the negative after-effect. 



A similar but much slower periodicity may be observed in connection with 

 the fusion of binocular images, especially when both are needed to complete 

 * ' Journ. Psychol., 1904, vol. 1, part 1, p. 91. 



