344 E.. L. Nichols—A Study of Pigments. 
Rays coming vertically from below this prism are totally re- 
flected by it and pass through the lower half of the slit, the 
The 
Diagram showing the eye-piece and slit of the spectro-photometer, with the 
adjoining parts. 
diagonals of its faces vertical, the optical axis being parallel to 
the axis of the collimator tube. Light passing through this 
Nicol and through the upper half of the slit, the collimator and 
dispersing prism forms a spectrum just below the first one (in 
the field of view). The boundary between the two spectra 18 
the sharply defined image of the edge of the reflecting prism 
where it bisects the slit. The second spectrum is polarized in 
a vertical plane: the first one is unpolarized. A diaphragm (D) 
in the eye-piece of the spectroscope, by means of which vision 
may be confined to the region under observation, and a secon 
Nicol (O), next the eye, complete the spectro-photometer. | The 
second or ‘ocular Nicol” is free to revolve, and its position 18 
indicated by a pointer (I) moving upon a graduated circle (C). 
n order to compare the spectrum of the light reflected by 
any object with the spectrum of daylight, the object is placed 
below the reflecting prism and illuminated either by the direct 
rays of the sun or by diffuse daylight. Before the polarizing 
Nicol a white sheet of paper or a plane mirror reflecting lig t 
from the sky, furnishes the polarized spectrum, It is possible 
by rotating the ocular Nicol to give this spectrum any intensity 
between that which it possesses when the polarizing planes ° 
the two Nicols are parallel and that at which it becomes too 
faint to be visible. Whatever be the character of the light 
reflected by the object to be studied, it is therefore possible to 
find a position of the ocular Nicol for which the region of the 
spectrum under observation and the corresponding wave-length 
of the polarized spectrum are equally bright. From the angle 
