ME. W. POLE ON COLOIJE-BLINDNESS. 
327 
convenient for the purpose I have yet met with are the ‘‘ Cercles Chromatiques ” and 
“ Gammes Chromatiques ” lately brought out by M. Cheveeul*. 
The former consist of ten graduated colour-circles, one representing the full colours, 
the rest the same colours in different darkened shades. We require only the first circle, 
which is divided into seventy-two radial sectors, each coloured with a different graduated 
hue. Three of these sectors, at 120° apart, contain the full primary colours, yellow, 
blue, and red, and half-way between them are the three secondary compounds, green, 
violet, and orange. The intermediate spaces are filled with different gradations, and the 
diagram, therefore, presents a complete re-entrant circle of hues, of great variety, and 
capable of identification with great exactitude. 
For the representation of toned colours, M. Cheyeeul has prepared a series of what 
he calls “ Gammes Chromatiques,” each consisting of twenty- two oblong divisions ranged 
one over the other. One division, about the middle of the row, contains a certain full 
colour, which is toned upwards in various shades to black, and downwards in various 
tints to white. The Gamme thus represents twenty different tones of the given colour, 
from the lightest tint to the darkest shade. 
I shall also have occasion to refer to Mr. Maxwell’s method of representing colour- 
sensations in a mathematical form, by the aid of his ingenious “ Colour-Top,” described 
in the Transactions of the Foyal Society of Edinburgh, vol. xxi. art. xviii. 
9. I have already stated that my vision is perfectly dichromic) i. e. while that of 
normal-eyed persons is compounded of three primary colour-sensations, mine is formed 
of only two. 
This is the usual theoretical manner of defining the defect ; but in a practical point of 
view it may be desirable somewhat to modify the definition. The more important 
colour-compounds may be practically considered as giving distinct impressions, and on 
this principle we may say, that while normal-eyed persons have seven colour-ideas (red, 
blue, yellow, green, orange, violet, and white), the dichromically colour-blind have 
only three, namely, two primary colours, and the secondary one produced by their 
combination. 
The first mention of the dichromic explanation of colour-blindness is in a letter from 
Sir JoHx Heeschel to Dr. Dalton, which, though dated 2nd of May, 1833, was only 
published in Heney’s ‘ Life of Dalton ’ in 1854. I have little more to do than to show 
the application, to my own case, of the theory there so simply and admirably laid down, 
and which I believe has never yet been followed out so completely as is necessary to 
explain the varying phenomena observed. 
10. The first and most positive fact I am cognizant of is, that there are two distinct 
colours which 1 really do see, and which I can at once identify with those called, by the 
normal-eyed, blue and yellow. The pigments ultramarine and chrome-yellow, or the 
parts of the solar spectrum near the lines D and F of Feaunhopee, excite the colour- 
* Paris, chez Digeon, Eue Galande, No. 65, 1855. M. Cheyeeul guarantees the perfectly similar repro- 
duction of the colours in all copies. 
2x2 
