RELATIVE STIMULATING EFFICIENCY OF LIGHT 141 



2. The threshold is lower for the light-adapted eye than for the dark- 

 adapted eye. This holds, at least, for certain light-adaptations. 



3. The threshold is lower for an interval marked by flashes added 

 to a continuous stimulation, than flashes in a dark field. This holds 

 for a wide range of constant illumination, the threshold varying usually 

 with the brightness of the constant illumination up to the point where 

 the additions lose in distinctness. 



The fourth question mentioned above, as to the relative ap- 

 parent brightness of light acting for various lengths of time, has 

 been answered differently by different investigators, who have 

 also revealed several other interesting phenomena. The chief 

 attempts in this direction have centered around the " action- 

 time" of light, i.e., the time during which light must act in order 

 to produce its maximum effect in point of apparent intensity. 

 Exner (1868, p. 601), Martius (1902), and McDougall (1904, 

 pp. 151-189) maintain that the action-time of light varies in- 

 versely with the illumination, while Swan maintains that it does 

 not. According to McDougall the action-time of light is 0.2 

 seconds when the stimulus is so weak as to be barely perceptible, 

 and decreases to 0.03 seconds when the intensity of the stimulus 

 is sufficiently increased. He also states that the action-times of 

 red, green, and blue lights of the same intensity differ but little 

 or not at all. Kunkle (1874, p. 197) however holds that the 

 action-time of red, green, and blue light differs. 



The effect of light acting for longer than its action-time has 

 been investigated by McDougall (1904), who concludes that 

 when light of a certain illumination acts upon the human retina 

 for longer than its action time the apparent brightness of the light 

 remains equal to, and then becomes less than that of the same 

 light acting for only its action-time. McDougall also states that 

 when two lights of the same illumination act for different periods 

 both however for longer than the action-time, the one of the 

 shorter duration seems to the human eye to be the brighter. 



The effect of light acting for less than its action-time seems to 

 vary directly with the duration of the action of the light. This 

 contention is supported by the work of McDougall, Swan (1849), 

 Bloch (1885), and Charpentier (1887), McDougall says (1904, 

 p. 177): 



