O. JSf. Rood— Color System. 265 



still more refrangible than its true complement, and locate a 

 disc which is still more orange, advancing thus more rapidly 

 in the work. In the same way this orange-red disc will enable 

 us to add a still bluer green, until in this way all the colors of 

 the chromatic circle have been provided for and all our colored 

 discs have received coefficents, for of course it will rarely hap- 

 pen that the actual positions of the colored discs fall on the 

 circle, mostly they will be well inside of it. The first part of 

 the work then consists in assigning coefficients to a series of 

 painted discs which are arranged so as to have only small pro- 

 gressive differences of color. 



Mode of experimenting. — To compare together colors which 

 are not strictly complementary a number of precautions must 

 be taken. In the first place, the pairs of discs employed should 

 always have as nearly as practicable about the same coefficients ; 

 consequently the selection of the colored papers offers very 

 considerable difficulty, and in the majority of cases it has been 

 necessary for me to prepare the paper specially for each case as 

 it arose. Of course it is desirable to use papers with as high 

 coefficients as possible, but cases occur where one is compelled 

 to use lower coefficients (blues and cyan blues) and in these it 

 is always necessary, so to speak, to take a fresh start, and to 

 descend to the lower coefficient by a line passing through the 

 center of the circle and not outside of it, that is by reaching 

 the other side of the circle by the aid of a complementary disc 

 with a lower coefficient. 



It is absolutely necessary that the eyes should be thoroughly 

 rested after each individual observation, and they should be 

 protected from side lights by a hood. Only the central por- 

 tion of the retina should be used ; that is, the colors should be 

 observed through a small aperture in blackened card-board 

 held at arms length. It is an advantage and a saving of time, 

 to protect the eyes with quite dark neutral spectacles while 

 engaged in the manipulations not involving observation of the 

 colors. 



As a source of illumination, sunlight falling on a stretched 

 .sheet of bleached cotton cloth was employed, and only the 

 hours near noon were utilized. The frame with the white 

 cloth was outside of the window, inclined at a suitable angle, 

 and consequently was behind the observer as well as placed 

 symmetrically to the colored discs. Extraneous light from 

 buildings and blue sky was screened off and the room suitably 

 darkened. If light from a uniformly overcast sky is employed 

 a different set of coefficients will be obtained, and there is 

 reason to believe that they vary somewhat with the nature of 

 the sky on such days. 



