442 ON THE THEORY OP COMPOUND COLOURS. 



TABLE b (continued). 

 v. 

 6. 



7. 



8. 



9. Not Observed 

 10. 

 11. 

 12. 

 13. 

 14. 

 15. 



But, according to our theory, colour-blind vision is not only dichromic, but 

 the two elements of colour are identical with two of the three elements of 

 colour as seen by the ordinary eye ; so that it differs from ordinary vision 

 only in not perceiving a particular colour, the relation of which to known colours 

 may be numerically defined. This colour may be expressed under the form 



aV + &U + cG = D (16), 



where V, U, and G are the standard colours used in the experiments, and D is 

 the colour which is visible to the ordinary eye, but invisible to the colour- 

 blind. If we know the value of D, we may always change an ordinary colour- 

 equation into a colour-blind equation by subtracting from it D (n being chosen 

 so that one of the standard colours is eliminated), and adding n of black. 



In September 1856 I deduced, from thirty-six observations of my own, the 

 chromatic relations of the same set of six coloured papers. These observations, 

 with a comparison of them with the trichromic theory of vision, are to be 

 found in the Philosophical Magazine for July 1857. The relations of the 



