Theory of the Flicker Photometer 



311 



As in the earlier paper, we may calculate the position of 

 setting of the flicker photometer by equating the two ranges, 

 as given by equation (10). Doing this we obtain the equation 



F F 

 log I 2 _ = log I 2 - -- . (14) 



Va^ogl.cb + b, ° ^a s logI s (l-<£)+& 3 



(assuming the substantial correctness of the relation used 

 to connect difTasivity and illumination), where F, a 1 , a 2 , 

 b^ b 2 are constants. 



Inspection of this equation shows that, as was the case 

 with different colours equally exposed, the first term on each 

 side becomes relatively more important the higher the illu- 

 mination, that is, the position of setting becomes more and 

 more nearly the equality of brightness point. But in the 

 present case the practical coincidence with the equality of 

 brightness setting is pushed to very much higher illu- 

 minations. 



We have taken numerical values for red and blue light 

 from the second of the series of papers on the experimental 

 work on the flicker photometer * and inserted them in the 

 equation (14), with the result shown in fig. 10. It is evident 



Fig. 10. 



J i 



1 



1 





// 









Lost? 



The Flicker Photometer with unequal exposures of the compared 

 lights. Kelative " slit widths " as calculated for different relative ex- 

 posures of red and blue light, for different illuminations, showing under- 

 exposed colour to be under-rated. 



from these curves that, if each colour acted on entirely 

 separate channels, unsymmetrical flicker photometers would 

 read very differently from the symmetrical one, the difference 

 amounting to from 20 to 50 per cent., even at high illu- 

 minations. 



* " Spectral Luminosity Curves obtained by the Method of Critical 

 Frequency/' Ives, Phil. Mag. Sept. 1912, p. 352. 



