£.. L. Nichols—A Study of Pigments. 343 
presented by deeply tinted pigments cannot but be regarded 
with doubt. ; 
It may be urged in favor of the use of these three constants 
of color that the methods just alluded to have led to fairly 
accordant results, have increased very much our knowledge of 
the properties of pigments, and have given us a means of 
describing colors and distinguishing between them which how- 
ever artificial and faulty cannot well be dispensed with. If the 
subject were such as to preclude the adoption of a more scien- 
tific and complete method of determining the character of the 
light reflected by non-luminous bodies, there would be less 
force to the objections just offered to the present system. The 
question of a better method is however not so complex as it 
may seem at first sight. It resolves itself from the nature of 
the case into a perfectly definite problem; to determine what 
Wwave-lengths are present in the spectrum of the light reflected 
y the object in question, and to measure the intensity of each 
wave-length. 
The use of the spectroscope in mapping spectra leaves 
nothing to be desired in the determination of the wave-length, 
the methods in vogue in the study of absorption pak being 
equally adapted to the investigation of the reflected rays. The 
addition to the spectroscope of parts which shall make it pos- 
sible to measure the intensity of all portions of a spectrum as 
readily as we determine the wave-length, will enable us to sub- 
stitute for the present system of questionable color-constants 
4 perfectly definite and complete statement of the wave-length 
and intensity of the rays emanating from any object whether 
viewed by transmitted or by reflected light. ; : 
n instrument which fulfills these requirements is a modifi- 
Cation of the spectro-photometer used by the writer in 1878 in 
the study of the spectrum of glowing platinum.’ Like the 
Spectro-photometers of Vierordt? and Glan’ it depends upon the 
Sensitiveness of the eye to small differences in the brightness of 
neighboring surfaces of identical color ; a property which enables 
the observer to decide with great certainty when two portions 
of a field of view, which is of one tint throughout, but the two 
halves of which can be varied in intensity at will, are equally 
ght. 
In front of the slit of an ordinary one-prism spectroscope 
(see fig. 1) a right angled prism (P) is placed, with its edge 
perpendicular to the slit (S) and bisecting it, and its longest 
face extending downward and outward at an angle of 45. 
' Ueber das von gliithendem Platin ausgestrahite Licht, Géttingen 1879. Also 
this Journal, Dec. 1879 and Jan., 1880. 
; Vierordt, Poggendorff’s Annalen, yol. cxl. 
* Glan, Wiedemann’s Annalen, vol. i. 
