404 Proceedings of the Royal Society of Edinburgh. [Sess. 
and the experiment carried out as follows : — First of all the screen K 
was placed in position and the intensity T of the light transmitted by 
the substance when not fluorescing obtained. The intensity of the com- 
parison source was made so small that the transmitted light caused no 
appreciable fluorescence. Then the screen S was placed in position, the 
screen K removed, and the intensity F of the fluorescent light determined. 
Both screens were then removed and a determination was made of the 
light received from the substance when fluorescing and transmitting at 
the same time, C. If there is fluorescence absorption T + F > C. 
If a, /3, y be the angles made by the nicol with its zero position, I 
the intensity of the light received from the comparison source at the 
upper aperture of the instrument in the above three readings, and s, s' 
the transmission factors for the instrument for the two halves of the field, 
s, s' including everything but the effect of the eyepiece nicol, we have in 
the usual way 
Ts' = Is tan 2 a, 
Fs = Is tan 2 /3, 
C s' = Is tan 2 y. 
Hence, instead of T, F, C, we may write the square of the tangent of the 
appropriate angle. The angles a, /3, y were determined by taking readings 
in two neighbouring quadrants and halving the difference. 
The result thus depends on three separate photometric measurements 
and is subject to three times the error of a single photometric measure- 
ment, and that not a very easy one. The space between the sources and 
the apertures of the instrument was built in so as to keep the room in 
absolute darkness. Observations were made in the order TFC, CFT 
so as to eliminate any error due to fluctuation in the relative intensity 
of the exciting and comparison sources. Instead of using an independent 
comparison source, light was reflected from the exciting source on to a 
square of white cardboard and the latter used as comparison source. 
The two would then vary together. This method was not found so 
satisfactory on account of greater difficulty in eliminating stray light. 
Scattered light from particles suspended in the glass or solution was 
expected and looked for, but did not prove to be an appreciable source 
of error. 
The measurements will now be described in order. 
Ukanium Glass — First Specimen. 
This was a pale yellow cube, of unknown manufacture, length of 
side 11 '4 mm., which fluoresced green. The following table gives the 
