132 On Areul Induction. 



they exercise a momentary control over the visual impulses from A], 

 Ao, and A 3 , transmitting them through C to B with an intensity con- 

 siderably reduced at first in the case of a bright flash. Perhaps even 

 the apparent delay of T \/' may be due to this initial reduction. 



Next suppose a second flash falling on AI DI and A 3 D 3 . Owing to 

 the cross connections from DI and D 3 to Do, the activity of D 2 is 

 aroused again, and the visual impulse from A 2 is blocked once more, so 

 that little or none of it gets through to Bo. 



If we suppose with Young that each colour-sensation results from 

 the activity of an independent peripheral system, it is easy to explain 

 the experimental phenomena. Let the second stimulus be white and 

 the first green. DI and D 3 , having been made to act strongly by the 

 repeated excitation, will block all visual impulses of green from AI and 

 A 3 by direct action, and from Ao by virtue of their cross-connections 

 with Do. The result will be that the components of purple, i.e., 

 (W G) = { (K + G + B + V) - G} will reach B t and B 3 , and nothing at 

 all will reach Bo exactly as in Dr. Shelford Bidwell's experiment. 



It is evident from a multitude of experiments that we have to deal 

 with two classes of contrast effects, namely, those which are induced 

 gradually and pass off slowly, and others which are momentary and 

 suggest the term spasmodic. The latter require for their production 

 a sharp and sudden stimulus, and one such stimulus may even be 

 followed by a multiple response, as in the recurrent images of 

 Purkinje or Charpentier's bands. To myself these phenomena recall 

 very forcibly the multiple electrical response of a nerve-muscle prepara- 

 tion, or of an electrical organ, to a single strong stimulus, and I suggest 

 that the " recurrent image " is caused by intermittent blocking of the 

 positive after-effect. The failure of Talbot's law for low speeds is 

 easily explained on this hypothesis. The ratio of the magnitude of 

 the response of the " D " structures to the magnitude of the stimulus 

 is greater when the stimulus is repeated a certain number of times per 

 second, i.e., a larger percentage of the total visual impulse from A is 

 blocked by D when there are fifty flashes per second than when there 

 are only five, and consequently the mean brilliancy appears less until 

 the limit is reached, beyond which Talbot's law holds. 



The phenomenon is manifested in greatest intensity when the 

 stimulus consists of two or three flashes close together followed by 

 an interval of rest. Possibly the reason of this is partly that the 

 activity of D may be maintained, and partly that A may recover from 

 its fatigue ; however that may be, it is essential in all experiments on 

 the reversal of the image that the eye should be in darkness for one- 

 half of the cycle. 



Another phenomenon explicable on this hypothesis is the flickering 

 of certain spectral colours. If a small weight is held out at arm's 

 length, after a time the muscles begin to shake and quiver. And if a 



