KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 447 



" simultaneous contrast " with the artificial reds and yellows prevailing around. At any 

 rate the effect is to remove my neutral point in the green, or the junction of the blue and 

 yellow colours in the spectrum, to a considerably longer wave-length. 



But the most important change is in the reds, gas or candle light increasing their 

 brilliancy and force in a very remarkable degree. Dalton was much struck by this, and 

 describes it strongly again and again. He says, speaking of the full red : — " The red 

 extremity of the image with a candle appears much more vivid than that of the solar 

 image Red and scarlet appear much more vivid and flaming than by day, form- 

 ing a superb colour Crimson loses its dark blue or dark brown appearance, and 



becomes yellowish-red." The change in the lighter shades of pink impressed him 

 still more, as in the case of the geranium flower ; and he adds : — " Pink is by far the 

 most changed ; indeed it forms an excellent contrast to what it is by day. No blue now 

 appears, yellow has taken its place ; it seems a reddish-yellow." I can fully confirm all 

 this. The change in the lighter reds is very astonishing ; and the full red by gas-light 

 is to me a most " superb " colour. Dalton could never bring himself to believe it was 

 really a yellow sensation. I have heard other colour-blind persons declare it to be a red 

 which only appears in that way ; and although I myself cannot detect any new colour- 

 sensation in it, I am obliged to admit that, as yellow, it is extraordinarily saturated and 

 powerful. # There seems something about this change that deserves further inquiry. 



The change of the red also considerably alters the position of our extra-spectral red 

 neutral, driving it further towards the violet end of the spectrum. 



A practical result of these changes is to check the confusion between red and green. 

 For if two objects of these colours appear alike by daylight (say the two outside figures 

 on the diagram, fig. 3), I have only to take them into candle-light, and they will become 

 well contrasted as yellow and blue. 



Another method of distinction, somewhat analogous, was contrived by Professor Clerk 

 Maxwell, by a pair of spectacles, one glass being red, excluding green rays, the other 

 green, excluding red rays, by using which the difference between red and green can be 

 instantly perceived. An account of this is given in Maxwell's Edinburgh paper of 

 1855, and an interesting letter which he wrote to me, when he was kind enough to give 

 me the spectacles, will be found in " Data," AE. 



Having now described the general colour-impressions of dichromic vision, it is necessary 

 to explain that there are some variations in different cases in matters of detail, — such, for 

 example, as in the length of spectrum visible; the exact position of the two dividing points 

 between the two colours ; the nature and arrangement of the various modifications of the 

 colours along the line of the spectrum ; and, probably to some extent, in the exact sensa- 

 tions corresponding to the colours themselves. This leads us to consider the difficulties 

 of the subject, and there are two questions in regard to which there has been great con- 

 troversy. They are — 



* Hering has noticed this, and has offered an explanation of it. See Phil. Mag., August 1893. 



