Dr. W. Pole on Colour-Blindness. 193 



beginning with rays of a smaller wave-length than corre- 

 spond to the extreme red. The longer waves have then little 

 or no yellow effect; and since the white effect is also wanting 

 there, the spectrum appears shortened at the red end. 



Kries and Krister found that the red end appeared more 

 saturated than the yellow : so that red-green blind persons 

 can distinguish rightly a saturated red of the hue of the 

 spectral red from yellow ; they understand by red a highly 

 saturated yellow. Such persons, if their spectrum is not 

 shortened, probably see, instead of the spectral red, a yellow 

 of higher saturation than normal eyes can ever see. For 

 with the latter the pure yellow hue is strongly mixed with 

 white ; and as they go to orange and beyond, the yellow 

 becomes more and more overpowered by the red sensation. 



Hering determined the hue of red which red-green blind 

 persons matched with grey. He took a paper of the hue of 

 spectral red, but which to him had clearly a yellow tinge, 

 and he found that this required, to render it neutral, blue 

 in different proportions for different patients; those with 

 full-length spectrum, and the neutral point nearer the line E, 

 required much blue; those with shortened spectrum, and the 

 neutral point nearer F, required less blue. 



The red thus obtained, and which the red-green blind eye 

 matches with a certain grey, will appear to a normal eye 

 sometimes more blue and sometimes more yellow. It tends 

 more to purple the farther the neutral point lies towards the 

 green, and more to the spectral red the farther it lies towards 

 the blue. The grey matching red will, for a shortened 

 spectrum, appear to the normal eye much darker than this 

 red. 



The neutral green and the neutral red of any red-green 

 blind person may be matched with each other by adding 

 white or black. And their mixture will also produce white 

 for the normal eye, which proves them to be true comple- 

 mentary colours. 



The author says: — " I have obtained above 100 equations in 

 this way, with ten different colours, from four red-green blind 

 patients, and have convinced myself that the consequences 

 drawn from the theory of opposite colours are throughout 

 correct. It is shown that these patients see only two colours. 

 That these are yellow and blue is, it is true, not absolutely 

 demonstrated, any more than that two normal persons both 

 see yellow exactly alike. If anyone chooses to assert 

 that one of them sees blue where the other sees yellow, we 

 cannot contradict him, nor can he, on the other hand, prove 

 his assertion. But it is only by the assumption that yellow 



