240 



COLOUR VISION 



Watson has adduced conclusive evidence in this direction, partly 

 from his own observations by the flicker method, and partly by calcula- 

 tion from Abney's published cases. Since the observations in Abney's 

 cases were all published before the calculated curves were obtained, 

 and some were indeed published before the sensation curves used in 

 the calculations had been obtained, they comprise an overwhelming 

 volume of evidence. 



If therefore we have the normal luminosity curve we can calculate 

 the luminosity curves for various grades of approximate dichromatism 

 (Fig. 68). If these are plotted on a large scale they will afford a means 



120 

 110 

 100 



90 



80 



70 



60 



50 



40 



30 



20 



10 



64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 



)i I I lecjoo' I I I 



op 



I I I I 



SOpOAU 



Fig. 69. Luminosity curves of a normal trichromat with excessive macular pigmentation. 

 + + + foveal ; O O O Q parafoveal. (Watson.) 



of measuring directly the amount of deficiency of red or green sensation 

 in any protanope or deuteranope, whether partial or complete. All that 

 is necessary is to obtain the examinee's flicker luminosity curve with the 

 same source of light and under as nearly identical conditions as possible. 

 The fact that all the curves intersect at a particular wave-length affords 

 one means of deciding that the case belongs to the group of approximate 

 dichromats. His average readings at this point should agree with the 

 average readings of the normal trichromat, and in every case the normal 

 readings should be taken at the same time. For purposes of diagnosis 



