KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 463 



for all variations of position of the yellow one ; and if this is so, another curious question 

 arises. If the white is normal, and the yellow colour varies, the blue colour, being 

 complementary, ought to vary also. Possibly a small change in the blue may have been 

 overlooked, as its extremely low luminosity often interferes with its accurate observation, 

 and Helmholtz has shown (p. 318) that considerable variations in the appreciation of 

 complementaries occur in different individuals. See also " Data," W, for similar variabilit}^ 

 At any rate, this matter, though only a slight one, requires further investigation. 



Possible Explanation of the Dichromic Variability. 



Accepting, therefore, the facts that these variations exist in dichromic vision, 

 and that the old hypothesis of alternative blindness to red or green is no longer 

 tenable, we must now consider whether there is any other and better explanation to be 

 found ? 



The most likely place to search for it is in the phenomena of normal vision. Is any- 

 thing to be found there analogous to these variations, — anything which, mutatis 

 mutandis, will explain and account for them? I believe that there is, but that the fact 

 has escaped due attention, in consequence of the ever-depressing influence of pre- 

 conceived theory. 



It is nothing new, generally, that variations occur in normal vision. I had occasion 

 to say in 1859, on the authority of eminent artists (my old friend Thomas Creswick 

 being one), that there are few people who have a perfect appreciation of colour. And 

 Professor Clerk Maxwell, in his Paper of 1860, noticed in several places real differences 

 in the normal eyes of different individuals, producing constant and measurable differences 

 in the apparent colour of objects. 



But the first person, so far as I know, who has brought this fact out in due pro- 

 minence is Lord Rayleigh, as he, in 1881, mentions some striking facts in regard to 

 the visible effects of the red and green rays on different normal-eyed persons. The 

 extracts from his Paper given in " Data," letter V, will show the magnitude of the 

 variations, the difference between the chromic strength of the impressions of red on two 

 individuals being as much as 2 "6 to 1. 



In the same year Von Kries and Frey published a notice of experiments in the same 

 direction, finding the difference 20 to 50 per cent. 



These results were so surprising that the experiments were repeated by one of the 

 most able observers, Professor Donders, who published the particulars in 1884, confirm- 

 ing Lord Rayleigh's assertions. See "Data," letter X. 



The matter was also inquired into by Konig and Dieterici, who found variations 

 having the ratio of 3*25 to 071. See "Data," Y. They plotted the varying intensity 

 curves, and their results were incorporated in Helmholtz's work. 



In 1885 Hering wrote a long and able article "On Individual Variations in the 

 Colour-Sense," in which he produced similar examples. See " Data," letter Z. 



