O. N. Rood—ZIndigo in the Spectrum. 135 
Art. XIX.—On Newiton’s use of the term Indigo with reference 
to a Color of the Spectrum; by Professor O. N. Roop, of 
Columbia College. 
by the following considerations. ; 
_ Experiments were first made with three different samples of 
indigo in order to see whether important differences in hue 
existed when the substance was prepared by different persons. 
One of the best methods of studying the hue of a colored sur- 
ce is to ascertain the nature and amount of the colored light 
which is complementary to it. Discs of card board were 
accordingly painted with indigo as a water-color pigment and 
ese were combined by Maxwell’s method with two discs 
painted with chrome yellow and vermilion, and neutralization 
ected by rapid rotation. 
Indigo as a water-color pigment (prepared by Winsor and 
Newton). 
Ratio of red and yellow necessary to neutralize it. 
Chrome-yellow, 67. Vermilion, 33. 
Indigo as a water-color pigment (prepared by Barnard). 
Chrome-yellow, 65. Vermilion, 35. 
Dry commercial indigo was then rubbed on white drawing 
paper, and gave a result similar to those just detailed; the 
Chrome-yellow, 62. Vermilion, 38. 
