244 



COLOUR VISION 



spectrum. There is good reason to think that cases of the same nature 

 are not uncommon, but in the cases hitherto examined the shift has 

 always affected the green sensation curve and in the same manner. 



Figs. 73 and 74 show the normal sensation curves in terms of 

 luminosity, Fig. 73 being the equal areas curve. 



If an observer is an " approximate dichromat," e.g., a partial 



^ 22 24 26 ze 50 32 M 36 58 40 42 44464650525456566062 ^ 

 4^0 5/000 5500 6,000 6i500 zoooA.U 



Fig. 73. Normal sensation curves in terms of luminosity. (Abney and Watson.) 



;0I2I4.I6ie2O222426 28 50 52>t3636 4042444<i48| :i052 



4J00 



I I / laic I 1^1 I I I I 

 5500 6/900 6^500 



54 56 58 ^62 



r.oooAU. 



I I I I I I I I 

 4600 5,000 



Fig. 74. Normal sensation curves on equal area scale, (Abney and Watson.) 



deuteranope with half the normal green sensation, on the luminosity 

 scale the ordinates of his green sensation curve will be half those of the 

 normal curve. On the equal area scale, however, his green sensation 

 curve will be the same as the normal, for since the area of the green 

 sensation curve on the luminosity scale is now 124, to obtain the equal 

 area scale we must multiply by 579/124 = 4-42 (cf. p. 228). Hence, 



