The Theory of the Flicker Photometer. 291 



compared colours are exposed for equal times, and the only 

 setting of the flicker photometer which was the subject of 

 numerical study was the equality point. 



In the present paper a study is made, on the basis of the 

 same assumptions, of certain unsym metrical conditions. 

 These conditions are much more difficult to treat. They 

 necessitate, for instance, definite knowledge of the value, 

 under various unusual conditions, of the Fechner fraction, 

 and of the relative effect of impressions of different wave- 

 form on the ultimate receiving apparatus. Their complete 

 treatment is in some cases practically impossible without 

 certain simplifying assumptions, which are not in entire 

 accord with known facts. 



Partly — perhaps, principally — on this account the further 

 development of the theory presented here is not entirely 

 satisfactory. But, while it fails to fit the experimental facts 

 quantitatively as closely as we would hope, it does handle 

 the principal phenomena qualitatively in a very striking 

 minner, and, moreover, has prompted experimental work 

 whose results must receive consideration in the development 

 of any theory of flicker photometry. 



2. The Sensibility of the Flicker Photometer. 



Of the two methods of utilizing flicker for the measure- 

 ment of light, one — the frequency of disappearance of 

 flicker — is quite insensitive, the other — the point sit which 

 two alternated illuminations show a minimum of flicker — 

 is quite sensitive. It is for this reason that the flicker photo- 

 meter proper is deserving of consideration as an instrument 

 of precision, while the critical frequency phenomena are of 

 little value in measurement. 



The low sensitiveness of the critical frequency method is, 

 as far as the mode of treatment here followed goes, simply 

 a matter of experimental fact. The error of setting for 

 critical speed amounts to one or two cycles per second, 

 which means for ordinary intensities 25 to 50 per cent, in 

 illumination. Add to this the fact that variations in con- 

 ditions of adaptation, &c, cause uncertainties fully as large, 

 and the unsatisfactory character of the method for measure- 

 ment is clear. 



How, then, can the flicker photometer, whose behaviour 

 we have endeavoured to predict from the critical frequency 

 phenomena, possess the sensibility it does ? This greater 

 sensibility we find is shown by the straightforward appli- 

 cation of the theory as presented in the former paper, as 

 follows : — 



X2 



