246 COLOUR VISION 



"The sums of the blue and red sensations are still equal but the 

 sum of the green sensation ordinates is greater, and hence the mixed 

 colour will not match the white but will appear to such a person too 

 green. If, however, the yellow slit is moved towards the red to c, 

 Fig. 74 (SSN 49-8, 5860 A.U.) and the width of the violet slit is made 

 2-72 times that of the yellow slit we get 



" The three sums are now equal and hence the mixed colour will 

 match the white to such an observer, although it will appear orange to 

 normal vision. 



" Hence if an observer does not agree with the normal when violet 

 and yellow are matched to form white, but requires that the yellow 

 slit be moved towards the red to form a match, we conclude that his 

 green sensation curve is displaced towards the red and vice versa." 



This explains the accuracy of the match which is made with the 

 potassium chromate method when there is a shift as compared with 

 cases where there is a reduction of one of the sensation curves {v. 

 p. 243). 



" In the case of a shift of the green sensation towards the red amount- 

 ing to 2 SSN as indicated by the dotted curves in Figs. 73 and 74 we 

 should expect the following effects to be produced : 



"1. The part of the spectrum which to the normal appears yellow 

 will appear greenish, for owing to the displacement the green sensation 

 excited will be greater than in the normal. In the same way what 

 appears orange to the normal will appear yellow, and so on. 



" 2. If we place three slits in the spectrum, one at the place of the 

 red lithium line, d, Fig. 74, another at the b magnesium line e, Fig. 74, 

 and the third in the violet at b, Fig. 74, and by varying the width of the 

 three slits produce a mixture which to the normal appears to match 

 the white, this match will not appear correct to the observer with the 

 green shift (whom, for short, we may designate by R., the normal being 

 mdicated by N.). To R. the match will be imperfect, for the green 

 sensation he receives from the light passing through the green slit at 

 e will not be as great as it is to N. By opening the green slit, we can, 

 however, obtain a match which is correct for R., but his match will 



