Theory of the Flicker Photometer. 321 



under-rated. This form of flicker photometer is less sensitive 

 than the ordinary equal exposure arrangement. 



7. Mechanical imperfections in the flicker-photometer 

 field are shown to seriously shift the equality point even 

 with no colour difference, emphasizing the necessity for 

 strictly substitution methods in flicker photometry. 



Theoretical. 



The behaviour of the visual apparatus toward intermittent 

 light is closely parallel to the action of a layer of matter, 

 obeying the Fourier conduction law, in which the diffusivity 

 varies as the logarithm of the intensity of the illumination, 

 this layer being exposed to the intermittent light on one 

 side, while its condition on the other side is measured by 

 an instrument whose sensibility is governed by the Weber- 

 Fechner law. 



Or. given the experimental data on the behaviour of the 

 eye in perceiving flicker with equal dark and light sectors 

 for different colours at various intensities, it is possible 

 from the most general law of conduction, on the basis of 

 the simple assumptions here made, to predict with con- 

 siderable accuracy the phenomena occurring with disks of 

 varying openings, at both high and low illuminations, the 

 phenomena with superposed flickering and steady illumina- 

 tions, the sensibility characteristics of the flicker photo- 

 meter, the behaviour of the flicker photometer toward 

 different colours at various illuminations, the occurrence of 

 colour flicker and brightness flicker, the effect of unequal 

 exposures of the coloured lights under comparison, and the 

 disturbing effect of mechanical defects in the photometric 

 field. 



The phenomena explained or predicted by the theory 

 constitute, in fact, all the known experimental facts in con- 

 nexion with the flicker photometer. Where the corre- 

 spondence between theory and experimental fact is more 

 qualitative than quantitative, we have advanced reasons for 

 believing these differences to be due to the approximate 

 nature of the solutions given of the mathematical work, 

 or the insufficiency of our experimental knowledge of: certain 

 factors. 



Physical Laboratory, 



The United Gas Improvement Company, 



Philadelphia, Pa., Oct. 1915. 



Phil. Mag. S. 6. Vol. 31. No. 184. April 1916. 



