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III. Notes on the Construction of a Colour Map. 

 By Walter Baily, M.A.* 



1. TN my paper on this subject, read before the Physical 

 JL Society on April 8 lastf, in discussing the method of 

 plotting out the curve giving the mixtures of two spectrum 

 colours, I stated that points in the curve lying in the ab- 

 normal or imaginary regions could not be determined by 

 experiment. This I now find is a mistake. 



Let Si and S 2 be spectrum colours, of which the mixtures 

 are to be represented by points on the curve. The part of 

 the curve lying in the imaginary and abnormal regions is 

 obtained by subtracting light of one colour from light of the 

 other colour, and equating the resulting colour to the mixture 

 (if I may use the expression) obtained by subtracting white 

 from some other spectrum colour S. 



We have 



WjS l — w 2 S 2 = S — afW, 



This equation does not represent any possible physical ex- 

 periment, but it may be transformed, without altering the 

 value of any of the quantities, into 



S + n 2 S 2 = WiSx + #W. 



This last equation means : — Make a colour-patch of S and 

 S 2 , and a colour-patch of S x and W, and vary the quantities 

 of S 1? S 2 , and W until the two patches are similar in colour, 

 and then measure the quantity of white. A line drawn to 

 the left from the position of S, and proportional in length to 

 the quantity of white, will define the position of the required 

 point in the curve. 



2. In the map the region on the right of the line of spec- 

 trum colours is occupied by all the colours obtained by mixing 

 spectrum colours and white. I suggest that this should be 

 called the Spectral Region. The equation, for determining 

 points on the mixture curves in the Spectral Region is 



^1 Si + r? 2 S 2 = S + aW. 



The region to the left of the abnormal and imaginary 

 regions is occupied by colours complementary to those in the 

 Spectral Region. I suggest that this should be called the 

 Complementary Region. The equation for determining points 



* Communicated by the Physical Society : read October 28, 1892. 

 t Phil. Mag. June 1892, p. 496. 



