1902.] 



Contributions to the Study of Flicker. 



313 



" Contributions to the Study of Flicker. Paper II." By T. 0. 

 Porter, M.A., Eton College. Communicated by Lord Eay- 

 leigh, F.RS. Eeceived May 14, — Read June 5, 1902. 



In the first paper,* curves are given which show the rate at which a 

 disc half white half black must be rotated in order that flicker may just 

 vanish when the disc is illuminated by the different colours of a diffrac- 

 tion spectrum. Also curves showing how the rate of rotation alters with 

 the alteration in the angular width of the coloured sector when placed 

 in the different colours of the same spectrum. These last are repro- 

 duced with some additions and corrections in the present paper (fig. 4). 

 It was shown in that paper (a) that as the stimulus applied to the 

 retina increases in intensity, the impression produced retains its maxi- 

 mum value for a shorter and shorter time, (b) that a stimulus requires 

 a finite time to produce its maximum effect, (c) that the duration of 

 the impression on the retina undiminished decreases as the time during 

 which the stimulus is applied increases, one of these quantities being 

 roughly inversely proportional to the other. 



There were also certain conclusions which confirm the work of 

 others, and which it seems unnecessary to repeat. Since this first 

 paper was written, continued experiments have suggested fresh means 

 and precautions for making the observations still more accurate, and 

 these must be first described. 



(1.) The quantity of white light reflected from the black sector 

 hitherto used was measured, by comparing its luminosity with that 

 of a disc having two narrow white tapering projections, subtending 

 known angles at the centre. This was made to rotate, and viewed 

 when flickerless, against a pitch dark background. It was thus proved 

 that the quantity of white light reflected by the Indian ink with 

 which the black part of the disc was painted varied, when the disc 

 was regarded at different angles, from 4 per cent, to 10 per cent, of 

 that reflected by the white cardboard used, and although in the 

 experiments already described, the disc had always been regarded 

 from very nearly the same standpoint, when it reflected 7 per cent., 

 nevertheless it seemed better to use a black pigment which would 

 diffuse what light it reflected more evenly. The disc was therefore 

 coated with a mixture of lampblack and alcohol, shellac varnish being 

 added in just sufficient quantity to prevent the lampblack from rub- 

 bing off. On careful measurement by the method already described, 

 the quantity of white light reflected from this very " dead " black 

 proved sensibly constant for a comparatively large variation in the 

 angle of reflection, and equal to 4*1 per cent. 



It seems well at this point to state how the want of blackness in 



* ' Eoy. Soc. Proc.,' vol. 63, p. 347. 



