248 C. S. Peirce—Nole on the Sensation of Colors. 



I log rx . i+J log ry . j+ K log rz . k= 

 Iloga;.*+Jlogy.^+K log z.k+logr(Ii+Jj-\-Kk). 

 Thus, the result of increasing the brilliancy of any ligl 

 be to add to the sensation a variable amount of a constant sen- 

 sation, li+Jj+Kk And all very bright light will tend toward 

 the same color, which may therefore be called i\\e color oj fa ighb 

 ness. Moreover, if the three primary colors be mixed in the 

 proportions which each by itself is fust perceptible, the sensa 

 tion produced will be 



log r (li+Jj+Kk), 

 and can only differ by more or less. 



Now I find, in fact, that all colors are yellower when 

 brighter. If two contiguous rectangular spaces, illuminated 

 with the same homogeneous light, uniformly over each, but 

 unequally in the two, they will appear of different colors. 

 If both are red the brighter will appear scarlet ; 



" green " " yellowish; 



" blue " " greenish; 



" violet " " blue. 



If we have the means of varying the wave-length of the light 

 which illuminates the fainter rectangle, we can improve the 

 match between the two, by bringing the fainter toward the 

 yellow. Such motions will converge toward a certain point 

 of the spectrum which they will never cross,— a poii 

 more refrangible than D and having a wave length of 582-KT 6 

 mm., according to Angstrom's map. If both rectangles be 

 illuminated with this light, the fainter appears white or even 

 violet, but if it be varied in wave-length with a view of im- 

 proving the match, it will be found to return to the same point 

 with the utmost precision. 



It appears, therefore, that, if our hypotheses are correct, the 

 color log r {li+Jj+Kk) is like that of the spectrum at A =582, 

 it contains less blue or violet and is consequently of 

 greater chromatic intensity. 



It further follows from Fechner's law that, if any light be 

 gradually reduced in brightness, one element of the sensa- 



disappear after another; and that when ve> 

 will exhibit only one primary color, which is the one which 

 it contains m greatest proportion relatively to the propor- 

 tion m the light which has the color of brightness. Now, 

 although this does not seem to be exactly the case, yet we do 

 get some approximation to it It is true' that any light what- 

 ever, when sufficiently faint, appears white, owing to the self- 

 lummosity of the retina. We cannot, therefore, unfortunately, 

 5!!*! 1 ?**? the P rimar y^ olors by reducing the light of three 

 "" Lt we may, as has often been sug- 



