THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XXXIV.— On a Color System; by O. K Rood, 

 Professor of Physics in Columbia College. 



[Read before the National Academy of Sciences, Nov. 12th. 1891.] 



It is possible to combine by rotation a circular disc of card- 

 board painted with vermilion, with a similar disc covered with 

 a bluish-green pigment exactly complementary to the red, and 

 to ascertain whether by mixture a gray is produced when the 

 two colored areas are equal. If such should be the case, in a 

 certain sense we could say that these two colors were equal, in 

 spite of differences of luminosity or amounts of white light 

 mingled with them. If it were found that to make the mixture 

 neutral, more or less red were required, we could assign to the 

 vermilion disc a coefficient of 1, and to the blue-green disc its 

 proper coefficient whatever that might be. Supposing for the 

 sake of simplicity that the coefficient of the blue-green disc 

 was also equal to unity, we then could mark the positions of 

 the two colors at the extremities of a straight line and at its 

 middle point locate white (gray), and we would then have a 

 line containing, so to speak, all possible mixtures of these two 

 colors, and by simple processes could indicate on it the posi- 

 tions, and consequently coefficients, of any red or blue-green 

 surfaces having hues of the same character ; for example, a red 

 surface reflecting half the amount of red light of our standard 

 disc, would be located half way between the positions of the 

 standard red and of white, while another red disc might even 

 find its position on the same line outside of our standard. In 

 all these cases the vermilion disc would be the material stan- 



Am. Jour. Sci.— Third Series, Vol. XLIV, No. 262.— October, 1892. 

 18 



