438 ON THE THEORY OF COMPOUND COLOURS. 



The green at E appears a good "yellow," and the blue at f from F towards 

 G appears a good "blue." I have therefore taken these as standard colours for 

 reducing dichromic observations. The three standard colours will be referred to 

 as (104), (88), and (68), these being the positions of the red, green, and blue on 

 the scale of the new instrument. 



Mr James Simpson, formerly student of Natural Philosophy in my class, has 

 furnished me with thirty-three observations taken in good sunlight. Ten of 

 these were between the two standard colours, and give the following result : 



337 (88) + 33-1 (68) = W (1). 



The mean errors of these observations were as follows : 

 Error of (88) = 2'5 ; of (68) = 2'3 ; of (88) + (68) = 4'8 ; of (88) - (68) = 1 "3. 



The fact that the mean error of the sum was so much greater than the mean 

 error of the difference indicates that in this case, as in all others that I have 

 examined, observations of equality of tint can be depended on much more than 

 observations of equality of illumination or brightness. 



From six observations of my own, made at the same time, I have deduced 

 the " trichromic " equation 



22-6 (104) + 26 (88) + 37-4 (68) = W (2). 



If we suppose that the light which reached the organ of vision was the 

 same in both cases, we may combine these equations by subtraction, and so find 



22-6 (104) -77 (88) + 4'3 (68) = D (3), 



where D is that colour, the absence of the sensation of which constitutes the 

 defect of the dichromic eye. The sensation which I have in addition to 

 those of the dichromic eye is therefore similar to the full red (104), but 

 different from it, in that the red (104) has 77 of green (88) in it which must 

 be removed, and 4'3 of blue (68) substituted. This agrees pretty well with the 

 colour which Mr Pole* describes as neutral to him, though crimson to others. 

 It must be remembered, however, that different persons of ordinary vision require 

 different proportions of the standard colours, probably owing to differences in the 

 absorptive powers of the media of the eye, and that the above equation (2), if 

 observed by K., would have been 



23 (104) + 32 (88) + 31 (68) = W (4). 



* Philosophical Transactions, 1859, Part I. p. 329. 



