Photometry of Lights of Different Colours. Ill 



The disagreement between observers will be less in comparing 

 the unsaturated colours of any acceptable illuminants than in 

 measuring bright spectral colours, but it is nevertheless 

 evident that in establishing standards of different coloured 

 lights, either the mean of a number of observers of " normal 

 colour vision " must be taken, or the luminosity curve of 

 each observer must be known in order that corrections may 

 be made to his readings — when it is learned how to apply 

 such corrections. 



Fig. 11 shows the distribution of energy in the source used 

 in the comparative measurements* (tungsten lamp through 

 prism, &c.) and the mean flicker curve of five observers at 

 250 I.U. reduced to an equal energy distribution. Since the 

 flicker curve closely agrees in shape with the equality of 

 brightness curve, but is more definitely determined, it may 

 be taken as the mean luminosity curve of these observers, 

 and probably lies closer to being the luminosity curve of the 

 normal or average eye than any heretofore obtained, its 

 maximum lies at\545/x. This is in very good agreement 

 with the high illumination curve obtained by Koenig for his 

 own eyef- 



Summary of results. 

 The chief new experimental results of this investigation 

 are : — 



1. The flicker method is more sensitive than the equality 



of brightness method, where different coloured lights 

 are compared. 



2. The results by the flicker method are more reproducible 



than those by the equality of brightness. 



3. Decrease of illumination shifts the maximum of lumi- 



nosity toward the blue, by equality of brightness 

 (Purkinje effect) ; toward the red by the flicker 

 method. 



4. Decrease of the size of the photometric fields at low 



illuminations shifts the maximum of luminosity toward 

 the red for the equality of brightness method (yellow 

 spot effect) ; toward the blue for the flicker method. 



5. The relative positions of the two kinds of spectral 



luminosity curves are in general different. 



* The distribution of energy in the source used for Plates I. and n. 

 (PL IV.) for an older tungsten lamp is slightly different from the lamp 

 used in the comparative measurements. 



t The value of -565 n for the maximum as obtained by Nutting for 

 Kcenig's observations left out of account the correction for the dispersion 

 of the prism. 



Phil. Mag. S. 6. Vol. 24. No. 139. July 1912. N 



