100 



COLOUR VISION 



is identical with the source of light of the spectrum. Ives obtained 

 a remarkable confirmation of the accuracy of the flicker method, and 

 arrived at the conclusion that it surpasses all other photometric methods 

 in sensitiveness and accuracy. 



Finally, Ives measured the luminosity curves of eighteen observers 

 with normal colour vision at 25 metre-candles illumination of a mag- 

 nesium oxide surface. The average curve deduced from these experi- 

 ments is shown in Fig. 39 and is compared with Konig's curve as 

 reduced by Nutting 1 and with Thurmel's curve 2 . 



. Konig's photopic luminosity curve (equality of brightness method). 

 Thiirmel's photopic luminosity curve (flicker method). (Ives.) 



In conclusion it may be stated that in the case of colours it is 

 generally agreed that the fusion frequency depends solely on the 

 luminosities of the alternating fields, and the coincidence of the equality 

 of brightness curve with the flicker luminosity curve (Figs. 38, 39) bears 

 out the assumption. We may at any rate assume with confidence 

 that both the methods measure the " something " (p. 44) which we 

 call brightness or luminosity. 



1 Bull, of the Bureau of Standards, vii. 2, 235. 



2 Ann. d. Physik. xxxm. 1154, 1910. 



