Ordinates 
E. L. Nichols—A Study of Pigments. 
345 
between the planes of polarization of the two Nicols can then 
be calculated the ratio between the intensities of the two spectral 
regions when the polarized spectrum is at its maximum; and 
a series of measurements of the variation of the intensity with 
the wave-length may be obtained, in which the intensity of 
each region is expressed in terms of the intensity of the corres: 
ponding wave-length in the spectrum of daylight. 
In studying the spectra of a variety of substances it is neces- 
Sary, in order to bring them to a common scale, to adopt some 
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urves showing the intensity of the s 
Tomate of lead: 
gths. 
pectra of four pigments (I, red lead ; II, 
III, chromic oxide; IV, ultramarine). Abscisse are wave- 
are intensities. 
Standard of color and of luminosity; for which purpose a pig- 
mparing t trum of thi 
Spectr 
ment presenting a neutral white surface is most suitable. B 
, the spec 
arized 
white surface with the pol- 
um under precisely the same conditions as those 
