﻿650 On the Brightness of Intermittent Illumination. 



The results are shown in Table III. 

 Table III. 



Time of total 

 darkness. 



Time of full 

 brightness. 



I, 



I 2 ' 



/cl 2 " 



K. 



•0064 sec. 

 •00273 



•00241 



•0292 sec. 



•01269 



•01483 



•8185 

 •8185 

 •7904 



1105 



1-090 

 1-054 



Mean . . . 



•741 / 



•751 



•749 



•747 



If the apertures are smaller than the shutters there might 

 be less chance of overlapping: The results in Table IV. 

 were obtained with a disk with 4 shutters and 4 openings, 

 the shutters being 3 times the openings; the fraction of light 

 transmitted through this disk is '250. 



Table IV. 



Time of total 

 darkness. 



Time of full 

 brightness. 



I- 



It 

 kI 2 



K. 



•0326 sec. 



•0135 



•0118 



•00748 sec. 



•00310 



•00270 



•7904 3-175 

 •7904 3-148 

 •7904 3-144 



•249/ 



•251 



•251 





Mean . . . 



■2503 



No reappearance of flicker was observed. 



It will be noticed in the tables that the time of total 

 darkness, when the appearance is flickering, becomes less 

 when the fraction of light transmitted through the disk, and 

 therefore the illumination, is greater. This is in agreement 

 with the results obtained by Porter*. 



Conclusion. 



From the above values of tc it can be concluded that the 

 residual visual impression carried on from one time of 

 illumination to the next and overlapping it makes no difference 

 to the apparent brightness. 



I should like to record my thanks to Dr. Womack, of 

 Bedford College, for suggesting this matter as a subject for 

 investigation. 



Bedford College, 

 Regent's Park, N.W., 

 March 9, 1915. 



* Porter, Proc. Roy. Soc. 1902. 



