194 Dr. W. Pole on Colour-Blindness. 



and blue are the hues of the red-green blind that congenital 

 colour-blindness comes into analogy with the red-green blind- 

 ness of the retina periphery, and with acquired red-green 

 blindness, in both which there has been abundant proof that 

 yellow and blue are really seen, as the affected person knows 

 all colours by his own experience. The red-green blind 

 patients always point out pure yellow and blue correctly, and 

 no two colours in which blue on the one hand and yellow on 

 the other hand are prominent, are ever mistaken for each 

 other"* 



Hering then goes on to show, as many others have done, 

 the striking incompatibility, with the observed facts, of the 

 original explanation of colour-blindness by the absence of one 

 of the three fundamental colours of the Young-Helmholtz 

 doctrine. He takes, as an example, the assumed form called 

 green-blindness (implying the normal vision of red and 

 violet), and shows such inconsistencies in it as have almost 

 entirely destroyed its credibility f. 



He then alludes to the newer explanation introduced, to 

 save the theory, by assuming that the excitability curves of 

 the red and green have become identical f. This he con- 

 siders also unreasonable and inadmissible, as giving up the 

 fundamental principle of Young's theory, and as not dedu- 

 cible from any facts observed. He concludes, on the whole, 

 that if the Young-Helmholtz theory had not been bequeathed 

 to physiologists as a venerable legacy, it would certainly 

 never have been drawn from the examination of the colour- 

 blind. 



On the other hand, he calls attention to the fact that all 

 the phenomena of red-green blindness arise quite simply and 

 naturally out of the Theory of Opposed Colours. 



Since the above described Essay was written Professor 

 Hering has naturally matured his views, and has published 

 much further interesting matter on the subject : he has 

 engaged in sharp controversies on his colour theories gene- 

 rally ; but does not appear to have been led to make any 

 essential modifications in his explanations of dichromic vision. 



* The remarkable confirmation by cases of one-eyed red-green blind- 

 ness bad not become prominent at that time : they were noticed fully by 

 Hering at a later date. 



t The original words are as follows : — "Ich will das Bild der t Griin- 

 blindkeit/ wie es sich nach der urspriinglichen Theorie von Helmholtz 

 ergibt, hier nicht weiter skizziren, denn es glaubt, abgesehen von einigen 

 Nachziiglern, doch Niemand mehr daran." 



+ See Phil. Mag. Jan. 1893, p. 55. 



