860 



Mr. H. E. Ives on the 



maximum unity, constituting a table of relative luminous 

 efficiencies of the spectral colours. In fig. 2 is plotted the 



Fig. 2. 











\ \\ 















// / 

 / ' 















i 



' 



\ \ 

 \ \ 



\ \ 













1/ 





\ \ 

 V 













f i 







\ 











11 

































/ 

















* 









\\ 





__,' 



^■*'" 













•40 -42 



•44 



•48 



•52 -54 



•56 



•58 -60 -6l 



•64 -66 -68 -70 -71 u 



Average Normal Spectrum Luminosity Curve of Eighteen Observers at 25 metre candles 



illumination of MgO surface. 



Dashed line — . . Koenig's data, for his own eye, equality of brightness method. 

 Dotted line Thurmel's „ „ flicker method. 



mean luminosity curve. Dotted in are the curves obtained 

 by Koenig *, for his own eye (equality of brightness method), 

 and by Thiirmel f (flicker method, also for his own eye). 

 The new curve, as it happens, is about midway in character 

 between the two latter. 



Fig. 3 gives all the eighteen curves, each plotted separately 

 along with the average curve. Study of these shows their 

 range in position and shape. It will be noted that certain of 

 the curves are comparatively broad, others narrow, A rather 

 common difference is in the relative value in the green, i. <?., 

 at \52/i. Many of the curves would be alike except for their 

 flatness or prominence at this point. These latter differences 

 can probably be ascribed to differences in the pigmentation 



* Data for High Illumination as reduced by Nutting, Bull. Bur. of 

 Stds. vol. vii. no. 2, p. 235. 



f Annalen der Physik, vol. xxxiii. p. 1154 (1910). 



