RECURRENT VISION 93 



It has been seen that McDougall arrived at the conclusion that 

 the relationship S oc 7 . 8t is probably correct. Viewed from the purely 

 physical standpoint this relationship and hence the Talbot-Plateau law 

 are such as might be anticipated 1 . From the physiological standpoint 

 one might rather have anticipated that the resultant sensation from 

 intermittent stimuli would follow the analogy of muscle tetanus, 

 i.e., that the application of new stimuli in the course of the curve of 

 sensation produced by the primary stimulus would suffice to prolong the 

 sensation curve at or near the maximum of the primary curve. It 

 appears therefore that the superposition of fresh stimuli during the 

 progress of the sensation curve of the primary stimulus produces 

 positive or inhibitory effects of such a nature that the resultant sensa- 

 tion curve shows rapid oscillations about a mean sensation intensity. 

 If the oscillations are sufficiently rapid a continuous mean sensation 

 results, in accordance with the Talbot-Plateau law. If the oscillations 

 are less rapid the sensation of flicker is felt. The analogy to muscle 

 tetanus is therefore not one to be pressed, but physiologists will have no 

 difficulty in finding analogies in the domain of neurology to the in- 

 hibitory effect of superposed stimuli. One may direct attention 

 particularly to Sherrington's work 2 . 



THE FLICKER PHENOMENON. 



If the oscillations produced by intermittent stimulation are not 

 sufficiently rapid to cause complete fusion, a sensation of flickering is 

 felt. If black and white sectors are rotated with gradually increasing 

 velocity there is first separate vision of the individual sectors. This is 

 followed by a peculiarly unpleasant coarse flickering, which passes into 

 a fine tremulous appearance, after which complete fusion occurs. Care- 

 ful observation reveals further interesting, though complicating details. 

 At a certain stage there is a peculiar glittering, the brightness being 

 greater than that of the continuous sensation after complete fusion. 

 Briicke 3 found the maximum with 17 '6 stimuli per second. It may be 

 explained by an absence of those inhibitory effects referred to above, 

 or by reinforcement of each white stimulus by the recurrent image of 

 the preceding sector (Briicke), or by temporal induction (vide infra), 

 or by a combination of such causes, v. Helmholtz noticed that at one 



1 Cf. v. Kries, in Nagel's Haiidb. d. Physiol. d. Menschen, p. 231. 



- The I nt-egralive Action of the Nervous System, London, 1906; of. also McDougall, 

 Brain, xxvi. 153, 1903. 



3 Sitz. d. k. Akad. d. Wissensch., Wien, SLIX. 2, 128, 1864. 



