190 Dr. W. Pole on Colour-Blindness. 



The remainder of this section explains certain physiological 

 or psychophysiological functions of vision by which the author 

 conceives that the colour-sensations may be produced, and 

 their peculiar relations accounted for. For these the earlier 

 Memoir may be referred to. The most relevant proposition 

 is, that in any pair of opposite colours the two sensations 

 are produced by contrary actions on the same excitable 

 substance, The author's views on these points have been 

 much controverted, but the discussions upon them do not 

 immediately affect the general nature of the colour-blind 

 explanations. 



The Second Section is " On the optical powers ( Valenzen) 

 of homogeneous and mixed light." It contains the author's 

 views about the spectrum ; and for the purpose of making 

 them clear he has taken the German privilege of forming a 

 new word, Valenz. It is from the Latin valens, powerful, 

 efficacious; and is used to express the recognizable presence 

 of a certain colour in a mixture. Thus the hue of scarlet, 

 compounded of red and yellow, would be said to have both a 

 red and a yellow valenz ; crimson, compounded of red and 

 blue, would have a red and a blue valenz, and so on. There 

 is no good English equivalent for the word, and it will be 

 simpler to use the original than a circumlocution. 



The author holds that the whole of the spectral colours 

 contain white, and therefore have a white " valenz," which 

 is most distinct in the yellow and yellow-green, but less so 

 in the other parts. 



All rays from the extreme red or beginning of the spec- 

 trum as far as the pure or fundamental green (Ur-grun, or 

 original green, a term borrowed from Preyer) contain also 

 yellow, i. e. have a yellow valenz. All rays from that to the 

 violet end contain blue or have a blue valenz. These distinc- 

 tions divide the spectrum into two parts, one characterized 

 by yellow, the other by blue. At the beginning of the red 

 the yellow sensation is so weak that for the normal eye it is 

 overpowered by the red, and similarly in the strong gieen 

 it is overpowered by that colour ; it is only in a small stripe 

 that the pure yellow is seen, which corresponds to the Urgelb. 

 The variations in the blue half are similarly characterized. 



The pure yellow divides the yellow half of the spectrum 

 into two quarters, and the pure blue similarly divides the 

 blue half. The four quarters are therefore characterized as 

 follows : — 



