ME. W. POLE ON COLOUE- BLINDNESS. 
331 
in no more anomalous position than any man would be if shut up from his birth in a 
room with green windows ; he could form of himself no idea that the light outside his 
prison was of a ditferent hue from that within ; he would call it white if he heard others 
do so ; and if it were as practicable to open our eyes as it would be his to the true aspect 
of nature, no doubt we should be quite as much astonished as he could be, at what had 
remained so long unseen. 
17. If, however, we ought to consider, according to a doctrine now strongly supported, 
that blue and yellow combined properly make white, and that green is an independent 
colour, we can only accept the fact as it stands, that the colour-blind must be insensible 
to two primary or independent colour-sensations, red and green, instead of to one only, 
as has been generally supposed. 
But since it is very difficult for me to conceive that my two colours, blue and yellow, 
are not also independent sensations, the idea would seem to suggest itself of a tetra- 
chromatism in the formation of the normal colour-scale ; or at least that four standard 
colours must be selected in order to enable us to describe all other colours as mixtures 
of them. This question, however, it is out of my province to discuss further. 
18. To recapitulate, in a brief form, the appearance to my eyes of Cheveeul’s colour- 
circle, I may describe it as having one point full yellow, another full blue, two points 
colourless grey, and all the rest simply different tones of blue or yellow. The 
following diagiums will explain this even to those who have not the colour-circle itself 
to refer to. (See next page.) 
And thus I come back to the statement, that I have only three sensations of colour, 
namely, yellow, blue, and that produced by their combination. 
19. It may be serviceable to those who may wish to compare my case with others, to 
state still more definitely my perceptions of colour according to the mathematical system 
of Mr. Maxwell, before alluded to. 
Proceeding on the principle that all my colour-sensations are compounded of two 
elements, blue and yellow, I take cards coloured with the most perfect examples of these 
I can find, namely, ultramarine and “ pale or lemon chrome*,” and form with them one 
of the circles of the colour-top, while in the other cu’cle I place the colour to be 
matched. I find in this way I can match every colour on the set of cards, by simply 
vaiying the proportions of blue and yellow ; only, since the intensity of the blue and 
yellow combination is generally different from that of the matched colour, white or 
black must be used in addition, to make the resemblance perfect. The colour-circle I 
suppose to be dmded into 100 parts. 
The neutral combination of blue and yellow is, to my vision, as follows : — 
(I.) 38i Y. + 61i Ult. = 64 Black + 36 White. 
The combinations for other colours are — 
* These experiments were tried in December 1858, with a colour-top and cards prepared by Mr. Betson, 
Optician, Edinburgh. 
