1902.] 



Contributions to the Study of Flicker. 



327 



flicker may just vanish. This, so far as the writer is aware, has never 

 been given before, and it seems remarkable that n should prove a 

 logarithmic function of the illumination (whether the variation of illu- 

 mination be due to the external source of light, or whether it be due 

 to alteration in the magnitude of the white sector, the external 

 illuminant remaining constant). It is also of interest to examine 

 in the light of the results just mentioned the curves expressing 

 the rate at which a disc of variable white sector must be rotated in 

 the different colours of the same spectrum in order that nicker may 

 just vanish. The figure 3* gives these curves, as found under the 

 conditions there described. Since the publication of that paper, the 

 experiments there described have been repeated, with the additional 

 precautions mentioned in the present paper, with the result that whilst 

 the general form of the curves has been confirmed, the central part of 

 each, represented in fig. 3 as a vertical straight line, has been proved 

 to be slightly curved, with the concave side facing the axis of Y. 

 These redetermined curves are shown in fig. 4, for the principal 

 colours of a spectrum (obtained by means precisely similar to those 

 used in the construction of the curves in the first paper, and which need 

 scarcely be repeated here), the dotted curves on the same figure being 

 those derived from x = b log w (360 - w\ and y = w, where b is a 

 constant for each curve, and w is the angular magnitude of the white 

 sector, measured in degrees. The seven constants for these curves 

 are — 



It will at once be evident that the curves for the different colours 

 coincide, within the errors of experiment, with the dotted curves, and 

 this fact proves that the duration of the impression of the different 

 spectral colours, undiminished, depends solely on the luminosity of the 

 colours, and not on their wave frequency. This has been suspected 

 before,! but it has never, so far as the writer knows, been definitely 

 proved. The proof here given is, that, so far as flicker phenomena 

 go, we obtain exactly the same curves, whether we place the disc in 

 the different colours of the spectrum, or subject it to white light, and 



* 1 Roy. Soc. Proc.,' vol. 63, p. 352. 



f Vide a paper by Professor E. L. Nichols, ' American Journal of Science,' 1884, 

 No. 28, pp. 243 to 252. 



For the violet 



0-2660459 x 100 

 0-2837823 x 100 

 0-2993016 x 100 

 0-3148209 x 100 

 • 3303403 x 100 

 0-3547722 x 100 

 0-3747256 x 100 



blue- violet 



blue 



blue-green and crimson 

 full green and vermilion 

 yellow-green and scarlet 

 yellow 



