Contributions to the Study of " Flicker" 347 



pressed into good contact with the aluminium discs, served as a 

 means of transmitting current through the electrolyte between the 

 discs. The frequency was 73, the temperature 18 C., and square root 

 of mean square value of current about 1 ampere. No perceptible 

 difference was observed in phase difference between potential and 

 current when the discs were rotated and at rest in the electrolyte. 



The conclusion is that the effect investigated in this paper takes 

 time to develop, and is not fully developed with alternate currents of 

 frequencies sixteen and'ninety-eight complete periods per second. It 

 can be increased by increasing the current density for a given film, and 

 is greatly influenced by temperature. The metal aluminium with its 

 film is suitable for use as the plates of condensers, if due regard be 

 given to current density and temperature. It might in some cases 

 be found useful as an equivalent to a metallic resistance. 



Messrs. Simpson, Greenbank, and Davy, Student Demonstrators in 

 the Siemens Laboratory, King's College, London, have given me 

 valuable assistance in the experimental part, and in the working out 

 of results. To these gentlemen I tender my thanks. 



4i Contributions to the Study of < Flicker.' " By T. C. PORTER, 

 Eton College. Communicated by LORD RAYLEIGH, F.R.S. 

 Received May 13, Read May 26, 1898. 



Much work has already been done on this subject, though little of a 

 quantitative character. Many observers have described the curious 

 colour sensations which rapid alternations of light and darkness can 

 excite under certain conditions, admirably exemplified in the " spec- 

 trum tops." Foremost amongst those whose experiments and writings 

 have led to the present very general interest in the subjects of flicker, 

 and of the sensation of light and colour, may be mentioned Helmholtz, 

 Silvanus Thompson, Shelford Bidwell, Henry, Charpentier, and 

 Rood ; whilst the first to try experiments on the relative sensitive- 

 ness of the eye to flicker in light of different colours, seems to have 

 been J. Plateau, who, however, employed pigments, and not the 

 colours of the spectrum. 



The writer's first experiments were made to ascertain the exact 

 relative rotations at which the flicker just vanishes in the different 

 colours of the same spectrum, and were carried out (a) as suggested 

 by Professor Rood in his ' Modern Chromatics,' with a balanced, 

 blackened, opaque disc, having a broad semicircular arc removed, 

 and (6) on a cardboard disc, half black, half white, viewed in the 

 different colours of the spectrum of the second order of a Rowland's 

 plane diffraction grating of 14,434 lines to the inch. Two sources of 

 light were employed, (a) direct sunshine, (b) lime-light. The results 



