RELATIVE STIMULATING EFFICIENCY OF LIGHT 159 



Cady. These investigators contend that the flickering light ob- 

 tained by rotating certain types of incandescent lamps at a rate 

 of 3 revolutions per second seems to some observers to be brighter 

 than continuous light of the same illumination. Moreover, it 

 is supported by the results obtained by Ferry on the human eye, 

 which have been presented previously. 



VI. THE RELATION BETWEEN STIMULATING EFFICIENCY AND THE 



RATIO BETWEEN THE DURATION OF THE LIGHT AND DARK 



PERIODS OF INTERMITTENT LIGHT 



In the preceding section it was demonstrated that the stimu- 

 lating efficiency of intermittent light varies with the flash-fre- 

 quency. In this section it will be demonstrated that it also 

 varies with the ratio between the duration of the flashes and 

 the dark periods between the flashes. 



In the experiments described below the butterflies were 

 exposed at the intersection of two beams of light of equal illumi- 

 nation arranged as in the preceding experiments, the light in one 

 beam being continuous and that in the other intermittent. 

 They were given tests with the flash-frequency of the intermittent 

 light 30, 20, 15, 10, 5 and 2 per second and with the ratio between 

 the light and dark periods of the intermittent light 1/3, 1/1, and 

 3/1. The intermittent light used was produced by means of 

 three different rotating sectored disks, one in which one-fourth 

 of the disk was removed, one in which one-half of the disk was 

 removed, and one in which three-fourths of the disk was removed. 

 In every test made the total illumination of the intermittent 

 light was equal to that of the continuous light. In this way the 

 stimulating efficiency was ascertained of light of various flash- 

 frequencies in which the length of the light and dark periods 

 were in the following ratios respectively, 1 to 3; 1 to 1; and 3 to 1. 

 Fourteen butterflies were tested in all, some under nearly all of 

 the above conditions and others under only a few of them. The 

 results obtained varied considerably in different individuals. 

 In some respects, however, there was but little variation. 



