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 



598 ± 2 

 577 + 1 

 520 ± 10 

 473 ± 3 

 410 



6770 

 6074 

 5786 

 5235 

 4738 

 4176 



6440-7028 

 5892-6300 

 5640-5850 

 5050-5335 

 4520-4861 

 4050-4330 



6734 (10) 

 6096 (9) 

 5745 (10) 

 5193 (11) 

 4680 (12) 

 4190 (10) 



Orange 



Yellow 



Green 



Blue ... 



Violet 





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. 



