1902.J 



Contributions to the Study of Flicker. 



323 



on (1) ), but also on the angular breadth of the white sector itself. If 

 the words " in sense (1) " occur hereafter, they refer to the first of the 

 above. 



In the experiments which have just been described, it will be under- 

 stood that since the disc remained throughout them half white, half 

 black, its brightness when rotating was never disturbed by any altera- 

 tion in the width of the white sector, and the results obtained connect 

 " I" the " last " of the impression, with I, the brightness of the white 

 sector at rest, or the degree of its illumination by the external source. 



In what follows, the illumination afforded by the external illuminant 

 is kept constant for each series of experiments, and the " last " is 

 determined for the various brightnesses of the rotating disc caused by 

 altering the angular magnitude of the white sector (and necessarily, of 

 course, the angle of the black sector also). 



The experimental details for the purpose are precisely the same as 

 before described, save that separate black and white discs, dovetailed 

 after Maxwell's method, were used instead of the single disc, half white 

 half black. The white disc was gradated about the circumference in 

 lengths subtending at the centre angles of 5°, and a concentric syren 

 card mounted on the axis of the same motor, indicated by its note the 

 speed of rotation. 



The general character of the results may be, to some extent, foreseen 

 from the results already obtained ; if, as seems likely, the increase of 

 stimulus given by increasing the width of the white sector is essentially 

 of the same nature as that given by increasing the external illumina- 

 tion, though keeping the white sector constant, we should expect that 

 the connection between n and the angular magnitude of the white 

 sector w (measured here and elsewhere in degrees) would be of the 

 form n = /(log w) ; and since n = when w = 0° or w = 360°, n is 

 likely to be a function also of the product w (360 - w). Experiment 

 shows that for any constant illumination, using the word in sense (1), 

 n = a + b log w (360 - w), where a and b are constants, b changes if 

 the illumination (in sense (1)) changes. The results of the experiments 

 are most clearly exhibited by the curves in fig. 3. Along the axis of X 

 are plotted the notes given by the syren with the number of revolu- 

 tions per second of the disc to which they correspond. The ordinates 

 are proportional to the angular magnitude of the white sector, measured 

 in degrees, from 0° to 360°. As it was proved that, within the errors 

 of experiment, each curve was symmetrical about a line drawn parallel 

 to the axis of X through the point corresponding to 180° on the axis 

 of Y, only the lower halves of the curves given by experiment are 

 drawn, and these are indicated in continuous lines. The true origin of 

 these curves lies 55*73 of the paper-scale degrees to the left of the 

 point B corresponding to 20 rotations of the disc per second. These 

 five continuous lined curves give the relation ascertained experiment^ 



z 2 



