232 COLOUR VISION 



Experiments made by Abney support these conclusions. He has 

 shown how the factor of fatigue can be arrived at and how luminosity 

 curves can be obtained for the fatigued eye. He did not employ the 

 high intensities used by Burch. 



II. Dichromatic Vision 



Young himself^ suggested that Dalton's colour blindness was due 

 to absence of the red component, v. Helmholtz, Donders, Konig, and 

 others found that this hypothesis accounted well for the facts, and 

 further, that the second great type of colour blindness, now known as 

 deuteranopia, could be attributed to absence of the green component. 

 The rare cases of tritanopia were attributed to absence of the blue 

 component. 



If the red component is absent it will be seen from Fig. 64 that 

 from SSN 65 to 60 there will be no sensation of colour, nor indeed of 

 light. The red end of the spectrum will be shortened. 



From SSN 60 to 50 the green component only is present. It will 

 give rise to a sensation much exceeding in purity that of the normal 

 unfatigued eye. The sensation will differ only in intensity, not in hue, 

 just as in the red of the normal eye between SSN 65 and 60. As has 

 already been pointed out we have no certain knowledge of the actual 

 sensation which the protanope is conscious of, and there can be little 

 doubt that it is very different from what we call " green," though he 

 may call it green and often does ; yet he often, too, calls it red. 



At SSN 50 the third component is stimulated. At SSN 34-6 (500 /z/x) 

 the green and blue sensation curves intersect. As the ordinates are 

 equal he will have a sensation similar to that of the trichromat when all 

 three components are equally stimulated, i.e., he will have a sensation 

 of the same nature as that which he obtains from white light, and he 

 will therefore probably call it white. At any rate he will match it with 

 the white of the combined spectrum if the intensity is suitably arranged 

 for him. In other words SSN 34-6 is his neutral point {v. p. 163). 

 Between SSN 50 and 34-6, then, his colour-sensation, whatever it is, 

 is becoming less saturated. Beyond SSN 34-6 his colour sensation 

 becomes more and more saturated, until at SSN 16, where the green 

 component ceases to be stimulated, it reaches its full spectral saturation, 

 and remains the same, dift'ering only in intensity, to the end of the 

 spectrum. 



1 Lectures, ii. 315, 1807. 



