330 
ME. W. POLE ON COLOUE-BLINDNESS. 
agent, its action, when added to blue, will not be to alter the hue, but only the shade. 
Thus, in the diagram, beginning from the vi\id blue and going towards the red, I see 
the blue gradually diminish, and the shade gradually increase, till it meets the red-violet, 
as before, at which point all colour is gone. 
15. I have now gone over two-thfrds of the colour-circle, namely, from yellow round 
through red to blue ; the remaining third comprising the hues which to the normal- 
eyed are known as green. The appearances this division presents to the colour-blind 
eye are very simple. 
Starting from the vivid blue, and passing round in the direction of a watch-hand, I 
find the sensation of colour gradually diminish, and an impression of darkness gradually 
increase, until, at about the second or third division beyond “ bleu vert,” the blue has 
entirely disappeared, and nothing is left but a neutral grey, w'hich matches, to me, 
No. 18 on the grey gamme. Beyond this the illumination begins to increase again, and 
at the same time a sensation of yellow begins to enter ; the light and the colour both 
gradually heightening as I advance, until at the division “ jaune” the darkening influence 
has entirely disappeared, and the full normal yellow hue is obtained. 
Thus the appearance, to me, of the green division of the diagram coiTesponds exactly 
with that of the red or opposite one. First, I find one particular hue (full green to the 
normal-eyed), which to me is entirely neutral and colourless, and 'sisible only as a dark 
grey ; secondly, all the greens on the blue side of this neutral are represented to me by 
shades of blue, and can be matched to my eye by blue gjlus black ; and thirdly, all the 
greens on the yellow side are simply shades of yellow, and can be perfectly imitated by 
darkening this colour. I am, in fact, as totally blind to green as to red ; an element of 
the malady which, I think, has not hitherto received the attention it deserves. 
16. But here a curious question arises, whether these two kinds of blindness have any 
connexion with each other 1 We have seen that insensibility to red induces also a want 
of perception of its compounds, orange and violet ; but can this in any way affect the 
vision of green, a colour into which red can scarcely be supposed to enter % 
It is difficult to answer the question without assuming something in regard to the 
nature of the colour green. If, according to the popular theory, it be supposed to be a 
combination of blue and yellow, our white, which must, as Sir John Herschel has 
observed in his letter to Dalton, be produced by the equilibrium of the two coloui’s we 
see, is not in reality white at all, but green\ Now green is only a colour to the normal- 
eyed, inasmuch as it is contrasted with their white light ; but since we know no such 
contrast, we have no perception of green as a distinct colour, and our green, white, and 
grey become synonymous terms. 
It is certainly rather a startling thought that, if this explanation be true, a portion of 
mankind exist all their lives in an atmosphere of green without knowing it ; that their 
sunlight is green, their snow green, their grey green ; everything in short about them 
green which is not blue or yellow ; and the invisibihty of white is no doubt a harder 
lesson to be learned than that of red. But this is no argument against the fact ; we are 
