RELATIVE STIMULATING EFFICIENCY OF LIGHT 139 



Most of the investigators mentioned above used a fairly high 

 flash-frequency. Lummer and Brodhun used a frequency of in- 

 terruption of from 27 to 200 per second. Parker and Patten 

 tested the effects of flash-frequencies of from 36 to 196 per second 

 but in most of their tests used a flash-frequency of over 116 per 

 second. Hyde states that in his work no attention was paid to 

 the number of flashes per second except to prevent all possibility 

 of a flicker. The flash-frequencies used by the other workers 

 are not available. 



The flash-frequency at which the "flicker" of intermittent 

 light becomes imperceptible, the "Verschmelzungsfrequenz" of 

 von Kries, depends upon the illumination. Baader (1891) states 

 that at a flash-frequency of 18.96 per second the flicker of inter- 

 mittent light of an illumination designated as " 1" cannot be per- 

 ceived and that the fusion-frequency rises gradually with in- 

 creasing illumination until at an illumination of "1800" the fre- 

 quency of interruption must be 50.24 per second before the 

 flicker disappears. According to Helmholtz the fusion-frequency 

 in strong lamp light is 24 per second, while that in full moon light 

 is only 10 per second. Kleiner (1874) also concluded that the 

 frequency at which the flashes of light fuse into a continuous 

 impression varies with the illumination. 



The fusion-frequency has also been shown to depend on the 

 condition of adaptation of the eye. Schaternikoff (1902) and 

 Charpentier (1887) state that with increasing dark adaptation 

 the fusion-frequency rises from about 10 to about 17 per second 

 if weak lights are used. The reverse is true if strong lights are 

 used, according to von Kries and Schaternikoff. In this connec- 

 tion it is interesting to note that the fusion-frequency for totally 

 color-blind individuals does not rise higher than 20 per second 

 (von Kries, 1905, p. 255). 



The fusion-frequency depends also upon the relative duration 

 of the periods of light and darkness in intermittent light accord- 

 ing to Dunlap, who maintains (1915, p. 230) that for the dark 

 adapted eye the fusion-frequency rises from 28.17 to about 50 

 flashes per second, as the ratio between the duration of the flashes 

 and of the interval between the flashes changes from 35/1 to 



