Dr. W. Pole on Colour-Blindness, 191 



Vdlenzen. 



The First quarter has Red, yellow, and white. 



The Pure yellow „ Yellow and white. 



The Second quarter „ Yellow, green, and white. 



The Pure green ,, Green and white. 



The Third quarter „ Green, blue, and white. 



The Pure blue „ Blue and white. 



The Fourth quarter „ Blue, red, and white. 



The Third Section contains " General observations on Colour- 

 blindness." 



After remarking on various possible disturbances of colour- 

 vision, as to which theory can give no account, the author 

 states that it follows definitely from his doctrine that an eye 

 which has no sensation of red can have no sensation of green 

 and vice versa ; and similarly for yellow and blue. 



Now it appears that there are actually eyes for which the rays 

 of light give no red and. also no green sensations, while the 

 yellow, blue, and white colour-effects remain, either as in the 

 normal eye, or with modifications. Such eyes are red-green 

 blind. Whether there is a corresponding blue-yellow blind- 

 ness, with vision of red and green, is uncertain; but there 

 appear to be cases where both pairs fail, i. e. blindness to all 

 colour. 



The author adds : — "I possess at present too little personal 

 experience on colour-blindness to enable me to give a general 

 theory of it. But my studies for years in the literature of such 

 investigations are sufficient to show me that the phenomena of 

 colour-blindness are irreconcilable with the Young-Helmholtz 

 theory, and they especially convince me that all really red- 

 blind persons are also green-blind, and vice versa; and further 

 recent observations render this conclusion continually more 

 probable. When Helmholtz wrote his 'Physiological Optics ' 

 the circumstances were very different, and the proportionally 

 scarce material at his disposal appeared well calculated to 

 support his theory." 



The Fourth Section is devoted to the consideration of 

 " General or total Colour-blindness," and of a remarkable 

 case of the kind discovered by Becker. By a careful study of 

 this case, the author endeavours to show how difficult the 

 facts of it are to reconcile with the theory of the three 

 sensations, and how consistently it may be explained by his 

 own hypothesis. 



The Fifth Section treats of Red-green Blindness, which is the 

 most usual form of the defect, and with which we have chiefly 

 to do here. 



