250 COLOUR VISION 



the normal. The reason for this effect is at once apparent from a con- 

 sideration of the sensation curves. Consider a green at SSN 36 (5090 

 A.U.). At this point in the spectrum the red and blue sensation curves 

 for N. on the equal area scale intersect. Hence we may regard the effect 

 produced by light of this wave-length as an amount of green sensa- 

 tion (represented by the difference between the ordinate of the green 

 sensation curve and the ordinate of either the red or the blue sensation 

 curve) diluted by white light (this white light corresponding to the 

 equal amounts of red, green, and blue sensation excited). It will be 

 observed that the amount of the diluting ivhite is the same for the 

 normal and the person with the displaced green curve ; but to R., the 

 amount of residual green sensation is less than half that of the normal. 

 In other words the green perceived by R. when light of this wave-length 

 enters his eye is very much more diluted than it is to a person having 

 normal colour vision. As a certain amount of dilution with white 

 light will obliterate the perception of green in the coloured ray, it 

 follows that the amount of white light which will obliterate it is 

 considerably less for R. than it is for a normal vision." 



" Further, the want of saturation of the green light makes the 

 chromatic threshold for green much higher than normal, so that a green 

 light must be made brighter for a person with a green shift before it 

 can be distinguished from a white light than for the normal. 



" The above results both as to the effect of dilution with white and 

 as to the chromatic threshold are of great practical importance, for 

 they both affect the power of an observer to identify green lights such 

 as those used at sea. These lights are never pure spectral lights, though 

 they are equivalent to spectral colours diluted with white. Thus the 

 Board of Trade standard light-green light can be matched by a mixture 

 of spectral green at SSN 37-4 or 5115 A.U. with an equal amount of 

 white (arc light). Further, if the size of the image of the coloured patch 

 on the retina is diminished, it must be remembered that the amount of 

 white required to extinguish a spectral colour is very much reduced." 



