1908-9.] Negative Attempt to detect Fluorescence Absorption. 403 
fluorescent substance kept before the lower aperture throughout the course 
of the experiment. An incandescent gas-lamp G was used for exciting the 
fluorescence. It was placed in a box to cut off all stray light, and its rays 
were focussed by a lens L on the fluorescing body U. The exciting rays 
not required were absorbed by a black screen not shown. The fluorescent 
light from the side of the body entered the instrument. As a comparison 
source an electric glow lamp was used. This was also placed in a box 
(not shown), and its intensity reduced to the right strength by placing 
fogged photographic plates of graded density in front of the opening in 
the box by which the light issued. S and K are two screens. When K 
is in position the exciting rays do not reach the substance, and it does not 
fluoresce. When S is in position the rays from the comparison source do 
not enter the substance and the lower aperture of the instrument. The 
passage from the comparison source to the upper aperture was always 
clear, and the upper half of the field was always illuminated by the 
comparison source alone. 
The first measurement made with this arrangement was to obtain 
the variation of the intensity of fluorescence with the wave-length for 
the substance. To do this, the screen S was placed in position, the 
screen K removed, and the comparison source adjusted to a suitable 
intensity. The lower half of the field was then illuminated solely by 
fluorescent light. The two halves were matched for different wave- 
lengths, the relative intensity of the comparison source and the fluorescent 
light obtained, and thus the shape of the fluorescence curve determined. 
This was repeated with a piece of cobalt glass in front of the exciting 
source in the position of K, in order to see whether the shape of the 
fluorescence curve varied with the nature of the exciting light. The 
shape did not, although the intensity did. 
When the transmission curve and the fluorescence curve of the sub- 
stance were known, a wave-length was taken on the fluorescence maximum 
