1914-15.] Reflective Power of Pigments in the Ultraviolet. 149' 
mounted in the focal plane CD between the comparison piece of cardboard 
and a similar piece on which a reference scale had been inscribed in Indian 
ink. The spectrum of the slit was sufficiently high to fall partly on all 
three, and was photographed by the quartz lens camera E at a distance of 
35 cms. ; the camera was so placed as to receive only diffusely reflected 
light. With an aperture of F 16 two exposures of ten minutes each were 
made for each pigment, one giving the near half of the ultraviolet in focus, 
the other the further half. During the latter exposure the visible and 
-S 
near ultraviolet regions were screened off'. Two pictures were taken on 
each plate, the unexposed half of the plate being protected from stray 
light by a screen placed within the dark slide. The exposures were made 
in groups of four, the two exposures for each pigment being made on 
different plates. Thus each plate contained two images in focus for the 
same part of the spectrum but for different pigments. 
The relative densities of the images produced by the light reflected 
from the pigments and from the cardboard respectively were estimated 
visually at selected points of the spectrum. As the method of visual 
observation is liable to considerable error, the determinations were 
standardised at first by means of blackened comparison strips of different 
densities. These were prepared by fogging photographic plates, and the 
fraction of light transmitted by each was carefully measured. They were 
