﻿Brightness of Intermittent Illumination. 649 



When the speed of the disk was not great enough the 

 appearance o£ the photometer was flickering. It was found 

 possible to obtain consistent values when the flickering was 

 not too pronounced. In these cases, there was no overlapping 

 of the visual impressions. Those observations in which the 

 appearance was flickering are marked with an/. 



Table I. shows results obtained with a disk having 4 shutters 

 and 4 openings all equal in size. The fraction of light trans- 

 mitted by the disk was *500. 



Table I. 



Time of total 

 darkness. 



Time of full 

 brightness. 



I, 



Kl 2 



K. 



•0144 sec. 

 •0116 



•00774 

 •00620 



■0144 sec. 

 •0116 



•00774 

 •00620 



•7694 



•7723 

 •7736 

 •7649 



1-549 

 1-556 

 .1-534 

 1-511 



Mean ... 



•497/ 

 '497/ 

 •504 

 •506 



•501 



Table II. shows results obtained with a disk having 

 8 apertures and 8 shutters of equal size. The fraction of 

 light transmitted is '500. 



Table II. 



Time of total 

 darkness. 



Time of full \ 

 brightness. I 2 



V. 



kI 2 



i 



K. 



•00847 sec 

 •00653 



•00847 sec. 

 •00653 



•8211 

 •8211 



1-6255 

 1-646 

 Mean . . . 



•504/ 

 •497 



•501 



In the two above sets of observations, no reappearance of 

 flicker was observed as the speed of the disk was increased ; 

 but in both cases the time of darkness was the same as the 

 time of full brightness, and the duration of visual impression 

 might be dependent upon the time for which the eye is ex- 

 posed to light, in such a way that, if the latter were diminished. 

 the former would be also, and there would be less chance of 

 overlapping with shutters and apertures of equal size. A 

 disk in which the apertures were 3 times the size of the 

 shutters was therefore used, there being 4 apertures and 

 4 shutters. The fraction of light transmitted by this disk 

 was *750. 



