Construction of Spectrum Scale. 
7 
according to the author’s conception of them.* These 
six disks were then placed against a suitable background 
(a neutral gray), in spectrum sequence, with wide inter¬ 
vals for the accommodation of connecting series of disks, 
which were then colored so as to represent an apparently 
even transition from one to the other. When this very 
difficult task had been done as well as the eye alone 
could judge, each intermediate was then measured on 
the color-wheel and the relative proportions (in per¬ 
centages) of its two component colors recorded. After 
this had been done for all the intermedite hues each series 
(the red-orange, orange-yellow, yellow-green, green- 
blue, blue-violet, and violet-red) was taken separately 
and a curve constructed on cross-section paper from the 
recorded ratios. These curves were found to be in all 
cases more or less irregular or unsymmetrical, but never¬ 
theless were sufficiently near correct to serve as a basis 
for a symmetrical curve; and after the points out of 
*In fixing the exact position or wave-length of the spectrum colors considerable 
latitude is allowable, the element of “personal equation”—that is, difference in the 
conception of different persons as to just where the reddest red, greenest green, etc., 
are located, accounting for the considerable disagreement among chromatologists as 
to the wave-lengths. The following table, showing the average, mean, and extreme 
wave-length of each of the spectrum colors as given by nine or more authorities 
together with those of the present work (as determined by Dr. P. G. Nutting, Asso¬ 
ciate Physicist of the U. S. Bureau of Standards) is of interest in this connection : 
This work. 
Average of 
9-12 
authorities. 
Extremes of 
9-12 
authorities. 
Mean of 
9-12 
authorities. 
Red. 
644 
6770 
6440-7028 
6734 (io) 
Orange .. 
598 ± 2 
6074 
5892-6300 
6096 (9) 
Yellow... 
577 ± 1 
5786 
5640-5850 
5745 (io) 
Green. 
520 ± 10 
.5235 
5050-5335 
5193 (ii) 
Blue.. 
473 ± 3 
4738 
4520-4861 
4680 ( 12 ) 
Violet......... 
410 
4176 
4050-4330 
4190 (io) 
From this table it will be seen that the red of this work is appreciably more- 
orange than that of others, the orange slightly more yellowish, and the violet a little 
less bluish than the average; but the author is assured by Dr. Nutting that these 
standards are exceptionally accurate. 
