376 Mr. H. R. Droop on 



fessor Hippel, of Giessen, has given an account (differing in 

 some respects) of the first or blue-yellow case in Graf's Archiv 

 fur Ophthalmologic, vol. xxvi. p. 176, vol. xxvii. pt» 3, p. 47. 



These two cases suggested to Professor Preyer of Jena a 

 theory (which he propounded in 1881 in Pfliiger's Archiv, 

 vol. xxv.) that ordinary eyes have two pairs of colour-sensa- 

 tions — (1) yellow and blue, and (2) red and bluish green, 

 and that the colour-blindness which consists in confusing red 

 and green, or, as the case may be, blue and yellow, is due to 

 the absence of one pair of these sensations. But Professor 

 Preyer does not deal with the difficulty that Helmholtz .and 

 Maxwell are supposed to have proved, that there cannot be more 

 than three colour-sensations, although that view is treated as 

 unquestionable by other recent writers, e. g. by Professor Don- 

 ders (Grafs Archiv fur Ophthalmologie, vol. xxvii.), and by 

 Professor v. Kries, of Freiburg im Breisgau (Die Gesichts- 

 empfindungen and ihre Analyse, Leipzig 1882, p. 33); and it 

 is naturally a serious obstacle to the fair consideration of 

 Professor Preyer' s theory. 



But inasmuch as Professor Preyer supposes that in each 

 pair of his colour-sensations the one sensation is complemen- 

 tary to the other, Ave have the equation 



R + G= White = Y + B, 

 a linear relation between the four colour-sensations ; and 

 therefore it follows, from what I have already proved, that 

 this hypothesis of two pairs of complementary colour-sensa- 

 tions is quite consistent with what Maxwell and Helmholtz 

 established. 



This theory of Professor Preyer's explains the leading 

 facts of colour-blindness, viz. that a colour-blind eye only per- 

 ceives two homogeneous colours, and that it is unable to dis- 

 tinguish between red and green, or, as the case may be, 

 between blue and yellow. It is impossible to ascertain with 

 absolute certainty that persons who are colour-blind with 

 both eyes see the same colours as the two persons who 

 have been discovered colour-blind of only one eye; but it is 

 noteworthy that when Dr. Pole made that minute examina- 

 tion of his colour-blindness, the results of which he gave in 

 the ' Philosophical Transactions 'for 1859, he came to the 

 conclusion that the colours he saw were yellow and blue and, 

 as the result of their mixture, white; and he only gave up this 

 view r in deference to the three-sensation theory then supposed 

 to be conclusively established. 



The following facts, not connected with colour-blindness, 

 seem to me to give considerable support to the hypothesis of 

 two pairs of complementary colour-sensations : — 



(1) Observations have been made as to the sensibility of 



