166 



COLOUR VISION 



If however the apparatus is kept as constant as possible and the 

 same procedure is adopted in each series of observations concordant 

 results are obtained from the same individual and comparable results 

 from different individuals, v. Kries and Nagel compare directly the 

 results obtained from deuteranopes, using the mixtures of 645 /z/x, and 

 460-8 JJL^I lights, with those obtained from protanopes, using mixtures 



o 

 a> 



10 



Ol 



O 



01 CJ1 



IO 

 CT) 



CD 



f 



Fig. 45. Gauging curves for dichromats : " warm " curves. S and M, two protanopes ; 

 N and St, two deuteranopes. Abscissae, arbitrary scale of prismatic spectrum of 

 gaslight, indicating certain wave-lengths ; ordinates, arbitrary scale, (v. Kries.) 



of 589-2 pp, and 460-8 pp. lights. While it is legitimate to choose two 

 mixing lights of any wave-length so long as one is chosen from each of 

 the two end regions of the spectrum, the curves in all such cases having 

 the same characteristic form, the units are referred to different standards 

 and it is a somewhat doubtful assumption to suppose that the results 

 are directly comparable. 



