112 Dr. W. Pole on some unpublished 



It is easy from these explanations to give the general 

 location of the spectral colours on the diagram fig. 1. From 

 the first visibility at the red end the colour-impressions, 

 beginning at Bk, will lie along the line BkY, till they reach 

 the maximum brilliancy at Y, corresponding to the normal 

 yellow part of the spectrum. From that they will pass along 

 or near the line YU, till at 0, corresponding to w.-l. 500, 

 they reach the neutral point where the yellow hue ends. 

 Then, continuing along the same line, the blue sensation will 

 come in, and will increase to its maximum (between F and 

 G) at U, after which it will return by the line UBk till the 

 visible spectrum ends. In other words, the line of the 

 spectral colour-impressions is simply along the lines of the 

 lower triangle, Bk, Y, 0, U, Bk. The exact gradations and 

 positions along this course will vary with different individuals, 

 and can, by proper observations, be easily plotted for any 

 individual in their true geometrical position. 



I believe this gives what Sir John Herschel recommended 

 me to do, and so completes, according to his expression, the 

 " exhaustive " description of the phenomena of the yellow 

 and blue dichromic vision, characterizing the great mass of 

 the persons said to be " colour-blind." I have, I am afraid, 

 gone into what may seem, to outsiders, a large amount of 

 detail as to the facts of the case ; but, in the present unsatis- 

 factory state of knowledge of the subject, I believe it is only 

 by a more careful study of the actual facts that there can be 

 any hope of a better state of things. 



William Pole. 



Athenaeum Club, S.W. 

 May, 1892. 



APPENDIX. 

 Table of Comparative Colour-Equations of different persons, 

 to illustrate the varieties existing in Yellow and Blue 

 Dichromic Vision. 



These equations were taken by Prof. Clerk Maxwell's 

 Colour-top, using the following colour cards supplied under 

 his direction (1858-9) :— 



Yellow (Pale or Lemon Chrome) Orange Chrome, Ver- 

 milion, Carmine, Ultramarine, Blue Verdi ter, Emerald 

 Green, Brunswick Green, Black, White. 



Mr. Pole. 

 I. 384 Y.+ 611 Ult.=64 Black +36 Wh. 



11.62 Y. + 38 Ult. = 78 Orange chro .+ 22 Wh. 

 III. 51 Y.+49 Ult. = 88 Emerald gr. 4- 12 Wh. 



