Dr. W. Pole on Colour-Blindness, 61 



He first shows the possibility of this by a simple example. 

 Suppose, he says, that the rays which ordinarily excite the 

 green sensation do not excite the green sensitive nerves, but 

 do continue to excite the red and blue ones. The eye would 

 become dichromic, and according to the old explanation would 

 see only red and blue. But these two sensations will not be 

 the same as the trichromic red and blue, in which the green 

 element forms part ; and further, it is possible that the 

 green nerves, though not excited hy the green rays, might 

 retain their former excitability by the red and blue ones, — in 

 which case the dichromic colours would be a certain kind of 

 yellow and blue. 



The author then explains (pages 458 to 461) his "Allge- 

 meinere For mole r Dichromasie." First, by algebraical reasoning. 

 Tf x, y, and z represent the three fundamental colour-sensations 

 in the normal eye, and X, Y, Z the three in the dichromic 

 eye, then, as all matches for the former are also good for the 

 latter, X, Y, Z must be linear functions of x, y, z, and the 

 following equations are arrived at, namely : — 



= aX + /3Y + yZ. 

 X^p^+pzy+psz. 



Y = q 1 x + q 2 y + q 3 z. 

 Z = aX + bY. 



The result being that the whole colour value of the dichromic 

 eye appears as a mixture, in variable proportion, of two 

 definite compound colours, X and Y. 



Another explanation is given by geometry, in which the 

 author uses a more comprehensive mode than the ordinary 

 plane diagrams. He adopts Lambert's plan of geometry of 

 three dimensions. The values of the three trichromic funda- 

 mentals x, y, z, are used as rectangular coordinates, by which 

 may be formed an imaginary parallelopipedon (like a brick 

 whose three dimensions 9 x 4| X 3 will represent x, y, and z 

 respectively) : then the extreme corner most distant from the 

 origin of the coordinates will represent the place of the com- 

 pounded colour ; the length of the diagonal line drawn be- 

 tween these two points will represent the quantity of the colour, 

 and its direction will represent the hue. Planes may be also 

 drawn intersecting the figure, which will represent the ordi- 

 nary plane colour-diagrams, and will serve as the bases of 

 " Lambert pyramids, " containing all the colours physiologi- 

 cally possible. It is shown that, under the conditions of 

 dichromatism, the resulting planes will each be uniformly 



