AMERICAN 
JOURNAL OF SCIENCE AND ARTS. 
[THIRD SERIES] 
Art. VIIL—On the Photometric comparison of Light of different 
colors ; by Professor O. N, Roop, of Columbia College. 
[Read before the National Academy of Sciences, Oct. 24th, 1877.] 
THe comparison of the intensities of light of different colors 
has long been considered one of the most difficult o 
may not be without interest to those whose studies lie in this 
direction. The luminosity of card-board painted with vermilion, 
was, for example, measured as follows: a circular dise of the 
vermilion card-board was attached to the axis of a rotation- 
brightness till the observer again became doubtful, when a 
directions, but remaining in ignorance of the results to the end. 
