608 BIOLOGICAL EFFECTS OF RADIATION 



conclusion. They maintain, moreover, that this frequency is "a sigmoid 

 function of the logarithm of the intensity." 



Dolley (50, 51, 52), Mast (148), and Mast and Dolley (157, 158), in 

 observations on several different insects, discovered that as the frequency 

 decreases from the critical frequency, the stimulating efficiency increases, 

 and that the stimulating effect of a given quantity of intermittent light at 

 optimum frequency is, under some conditions, more than 16 times as 

 great as that of the same quantity of continuous light. They maintain 

 that the optimum frequency and the stimulating efficiency at this fre- 

 quency vary directly with the intensity, and that stimulating efficiency 

 at optimum frequency is practically independent of the ratio between the 

 length of the light and the dark periods, if this ratio is < 1, i.e., if the dark 

 period is longer than the light period, but that it varies inversely with 

 this ratio if it is >1. They conclude that "the light receptors in the 

 eye of an insect are not continuously stimulated when they are continu- 

 ously illuminated; that there are periods during which the light acts, 

 followed by periods during which it does not act; sensitive periods 

 followed by non-sensitive (refractory) periods; periods in which light 

 induces, in the receptors, photochemical changes in one direction followed 

 by periods in which the reverse (restitution) occurs; so that, when in 

 intermittent illumination, the light and the dark periods harmonize 

 with the sensitive and the refractory periods, respectively, all of the light 

 received is used in stimulation, while in continuous illumination that 

 received during the refractory period is not used." They hold that 

 light induces, in the photoreceptors, the formation of certain substances 

 which, as soon as a given amount has accumulated, produces an impulse, 

 that the substances produced are then in some way converted, so as to 

 restore the system to the original condition, after which the light acts 

 again, etc. 



If a dark-adapted eye (insect, crab, eel) is suddenly illuminated, 

 the action current in the optic nerve increases rapidly to a maximum and 

 then decreases rapidly, after which it decreases slowly or remains nearly 

 constant for some time. The duration of the increase in the action 

 current varies inversely with the intensity of the light, but it is at best 

 only a small fraction of a second. There is no indication of regular 

 periodicity in continuous illumination (Adrian, 2; Hartline, 78, 80; 

 Prosser, 188). 



These results indicate that the difference between the stimulating 

 efficiency of continuous and intermittent light described above is corre- 

 lated with spurts of increase in impulses after the flashes in the intermit- 

 tent light rather than with periodicity in sensitiveness in the continuous 

 light; and that the decrease in stimulating efficiency with increase in 

 flash-frequency, is due to the decrease in the time for restitution during 

 the dark periods. 



