[From tiie American Journal of Science and Arts, Yol. XY, Feb., 1878.] 
ON THE PHOTOMETRIC COMPARISON OP EIGHT OF 
DIFFERENT COLORS. 
By Professor 0. N. Rood, 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 of photo- 
metric problems, but by the use of very simple means, 1 have 
recently made a series of measurements of this character, which 
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 disc of the 
vermilion card-board was attached to the axis of a rotation- 
apparatus, smaller circular discs of black and white card-board 
being simultaneously fastened on the same axis, so that by 
varying the relative proportions of the latter, a series of grays 
could be produced at will. 
The compound black and white disc was now arranged so as 
to furnish by rotation a gray which was decidedly darker than 
the vermilion; this gray tint was then gradually lightened, till 
the observer became doubtful as to the relative luminosities of 
the red and gray discs; the angle occupied by the white sector 
was then measured. Next, a gray decidedly more luminous 
than the vermilion was compared with it, and diminished in 
brightness till the observer again became doubtful, when a 
second measurement was taken. All this time the manipula- 
tion was performed by an assistant, the experimenter giving 
directions, but remaining in ignorance of the results to the end. 
The mean of ten such experiments assigned to the vermilion 
disc a luminosity of 23'8, that of white card-board being taken as 100. In this experiment and in all those that follow, proper 
corrections were made for the amount of white light reflected 
by the black disc, this having been previously ascertained, in a 
manner which will be described in a future communication. 
In order to test the correctness of the final result, the lumin- 
osity of a blue-green disc, correctly complementary in color to 
the vermilion, was next measured in the same vfay; it proved 
to be 26‘56. The vermilion and blue-green discs were then 
combined, according to Maxwell’s method, so as to obtain a 
pure gray by rotation, and the angular proportions of these 
colored surfaces, and the value of the gray in terms of white 
and black card-board, measured. The gray thus obtained had 
a luminosity of 24’54, that of white card-board being 100. 
Next, the value of this same gray was calculated from the 
measured luminosities of the two colored discs, and the propor- 
tions of these colors required to produce a pure gray by mix- 
ture on the rotation apparatus; the calculated value was 25’47. 
This agreement proves the correctness of the photometric 
comparison, and also of Grassman’s assumption, that the total 
intensity of the mixture of masses of differently polored light 
is equal to the sum of the intensities of the separate compo- 
nents, which, so far as I know, has not before received an ex- 
perimental confirmation. 
Corresponding measurements were made with a green, and 
its complementary purple disc; also with a blue, and its com- 
plementary yellow disc ; the results are given below. 
Luminosity. 
Gray (observed.) 
Gray (calculated.) 
Vermilion, 23"8 
Blue-green, ... 26-56 
2454 
25-47 
Chrome yellow, 80-3 
Cobalt blue, ..35-38 
54-51 
53'92 
Green, 41-19 
Purple, 14-83 
24-94 
26*56