142 WILLIAM L. DOLLEY, JR. 



I am strongly disposed to believe that the law that the intensity of 

 the sensation varies directly with the duration of the action of the light 

 of given intensity holds good for all durations less than the action-time 

 of the light. 



In this connection and in view of the facts described in this 

 paper it is also of especial interest to note the following results. 

 Flickering lights of certain flash-frequencies seem to have a 

 greater stimulating efficiency for the human eye than have the 

 lights of the same illumination but of different flash-frequencies, 

 von Kries says (1905, p. 232) : 



Hat man auf einer rotierenden Scheibe Hinge mit verscheidenen Zah- 

 len schwarzer und weisser Sektoren, so bemerkt man bei passenden 

 Rotationsgeschwindigkeiten, dass ein stark flimmernder Ring im gan- 

 zen betrachtlich heller erscheint als ein vollkommen stetig gesehener. 

 Briicke gab an, dass bei einer Frequenz der Reizanstosse von etwa 17,5 

 in der Sekunde die Helligkeit am grossten erscheine. 



Hyde and Cady (1906, pp. 415-437) confirm these results, for 

 in an investigation of the mean horizontal intensity of incandes- 

 cent lamps by the rotating lamp method they found that when 

 certain types of incandescent lamps were rotated at three revolu- 

 tions per second a flickering light was produced, which to some 

 observers appeared of an intensity 4 per cent too high and to 

 others 3 per cent too low. Ferry also confirms these results for 

 he says (1894, p. 344) : "But it was noticed that if the (rotating 

 sectored) disc did not revolve rapidly enough to produce a per- 

 fectly steady illumination of the photometer screen, more light 

 appeared to go through the sectored disc than theoretically should." 

 Thus, apparently to the human eye intermittent light in which 

 the flashes of a certain duration are perceptible seems stronger 

 than it really is. 



The effect of intermittent light upon organisms other than man 

 has been studied but little and most of the investigators have 

 been interested in the first of the problems mentioned above, 

 i.e., the applicability of Talbot's law. Loeb, Ewald (1914), and 

 Wastenays (1917) state that this law holds for Eudendrium; 

 Ewald (1914) maintains that it holds for Daphnia; Loeb and 



