140 WILLIAM L. DOLLEY, JR. 



1/1, and decreases from about 50 to 34.04 flashes per second, 

 as the ratio between the duration of the flashes and of the 

 interval changes from 1/1 to 1/35. Schenck (1897, p. 54) 

 states that if in intermittent light a grey period is interposed be- 

 tween the periods of darkness and of light the fusion-frequency 

 increases, while Helmholtz (1896, p. 489) and Plateau maintain 

 that the fusion-frequency is not affected by changes in the rela- 

 tive duration of the light and dark periods of intermittent light. 



According to von Kries the results of Baader and Schenck show 

 that in intermittent light consisting of alternate strong and weak 

 flashes the fusion-frequency depends on the relative brightness of 

 the flashes. As the relative brightness decreases the fusion-fre- 

 quency decreases no matter whether the average illumination is 

 unchanged or increases. 



The first to investigate the shortest perceptible time-interval 

 between two flashes of light was Exner who, working with elec- 

 tric sparks, found visual time-thresholds at 44/1000 seconds at 

 280 mm. distance and 21/1000 seconds at 640 mm. Weyer 

 (1899), who also used electric sparks, states that the visual time- 

 threshold is only 12/1000 seconds. If a series of stimulations 

 is used, he holds that according to adaptation and other condi- 

 tions, the flicker-threshold varies from 25/1000 to 87/1000 seconds 

 and the threshold for separation of a series varies from 42/1000 

 to 105/1000 seconds. 



Easier (1911), using a rotating black disk on which were painted 

 white sectors states that the time-threshold is about 40/1000 

 seconds with two visual stimulations and for a series of stimula- 

 tions it is about one-third as much. Dunlap, whose careful in- 

 vestigations have shown the difficulties of this problem, states 

 that with two stimulations the time threshold decreases with 

 increase in the length of the flashes and that this decrease seems 

 to be altogether a function of the duration of the first flash. He 

 also comes to the following conclusions as to the visual time- 

 threshold (1915, pp. 247-248): 



1. The effects of brightness of the light are variable, depending on 

 the other factors in such a way that no conclusion can be drawn as yet 

 concerning their effects. 



