446 DR WILLIAM POLE ON THE PRESENT STATE OF 



infinite varieties of light and shade. And the consequence of this is that the ' dichromic 

 patient, having his attention naturally directed to this kind of variation, becomes 

 exceedingly sensitive to it ; so that differences of luminosity, or of saturation, or of both, 

 become as strongly marked to him, and as easily discriminated by him, as differences of 

 hue are by the normal-eyed.* 



I think I ought to add a few remarks on a peculiar feature of red-green blindness, 

 which Dalton laid great stress on, though it has seldom been noticed — I mean the re- 

 markable difference, much greater than normal, produced in the appearance of the colours 

 by artificial light. It was, indeed, this peculiarity which first called D Alton's attention 

 to his own defect. He says : — 



I was never convinced of a peculiarity in my own vision till I accidentally observed the colour 

 of the flower of the Geranium zonale by candle-light in the autumn of 1792 [he being then twenty- 

 six years old]. The flower was pink, but it appeared to me almost an exact sky-blue by day ; in 

 candle-light, however, it was astonishingly changed, not having then any blue in it, but being what I 

 called red — a colour which forms a striking contrast to blue. Not then doubting but that the change 

 of colour would be equal to all, I requested some of my friends to observe the phenomenon, when I 

 was surprised to find they all agreed that the colour was not materially different from what it was by 

 daylight. This clearly proved that my vision was not like that of othar persons, and at the same time 

 that the difference between daylight and candle-light on some colours was indefinitely more perceptible 

 to me than to others. 



He afterwards gives many examples of the peculiarity, but as my own experience is 

 precisely similar, I may describe the changes which I myself see in candle or gas light.t 



Yellows appear much paler. I believe this is a normal change. The characteristic of 

 candle or gas light is the greater predominance in it of the long- waved warm colours, 

 which imparts a considerable tinge of them to all surrounding objects capable of reflecting 

 them, and by contrast with which the original yellow colour seems to lose in tone. 

 Primrose or straw-coloured gloves pass for white in the evening. 



On the other hand, blues appear darker in shade, partly by contrast, and partly from 

 the light supplying a less proportion of blue rays. Every lady knows that blue is not a 

 " good lighting-up " colour. 



Greens also appear darker, as their rays (except, perhaps, for the very yellow varieties) 

 are also deficient in artificial light. But they also appear, both normally and to us, to 

 change to a bluer hue ; so much so that the normal-eyed often confuse the two colours. 

 Dalton was surprised at this, as he says : — " I do not understand why the greens should 

 assume a bluish appearance to us and to everybody else by candle-light, when it should 

 seem that candle-light is deficient in blue." I suppose it must be from what is called 



* Sec further explanations on this point in the Phil. Mag., July 1892. 



t These particulars were given in my original paper of 1856 (sent to the Royal Society of London), hut were 

 omitted in printing. I made a postscript in 1859 that I had discovered that the colour-top equations varied to soma 

 extent, even in the same individual, at different times. This was afterwards explained by the variations of the light 

 ■which illumined them. 



