EXPERIMENTS ON COLOUR, AS PERCEIVED BY THE EYE. 139 



Hence, if we divide the line BG in the proportion 19 to 5 at the point /8, 

 and join R& the tint at /3 will differ from that at R (to the colour-blind) 

 only in being more brilliant in the proportion of 100 to 24, and all inter- 

 mediate tints on the line R/3 will appear to them of the same hue, but 

 of intermediate intensities. 



Now, if we take a point D, so that RD is to R/3 in the proportion of 

 24 to 100 24, or 76, the tint of D, if producible, should be invisible to 

 the colour-blind. D, therefore, represents the pure sensation which is unknown 

 to the colour-blind, and the addition of this sensation to any others cannot 

 alter it in their estimation. It is for them equivalent to black. 



Hence, if we draw lines through D in different directions, the colours 

 belonging to any line ought to differ only in intensity as seen by them, so 

 that one of them may be reduced to the other by the addition of black 

 only. If we draw DW and produce it, all colours on the upper side of DW 

 will be varieties of blue, and those on the under side varieties of yellow, so 

 that the line DW is a boundary line between their two kinds of colour, blue 

 and yellow being the names by which they call them. 



The accuracy of this theory will be evident from the comparison of the 

 experiments which I had an opportunity of making on Mr N. and Mr X. with 

 each other, and with measurements taken from the diagram No. 2, which was 

 constructed from the observations of ordinary eyes only, the point D alone 

 being ascertained from a series of observations by Mr N. 



Taking the point y, between R and B, it appears, by measurement of the 

 lines Ry and By, that y corresponds to 



07B + -93R. 



By measurement of Wy and Dy, and correction by means of the coefficient 

 of W, and calling D black in the colour-blind language, y corresponds to 



105 W + -895 Bk. 



Therefore 



By measurement -93 R+ '07 B = '105 W + '895 Bk 



(5). 



By observation N. & X. together '94 R+-Q6 B = '10 W + '90 Bk 

 By X. alone '93 R + -07 B = '10 W + -90 Bk 



The agreement here is as near as can be expected. 



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