1908-9.] Negative Attempt to detect Fluorescence Absorption. 405 
fraction of the incident light transmitted by the glass for different wave- 
lengths : — 
A 
A 
598 h/j. 
•906 
467 ,14,11 
•523 
560 
•829 
452 
•383 
529 
•781 
438 
•195 
504 
•630 
425 
•098 
484 
•570 
As about 8 per cent, of the incident light is lost by reflection at the two 
faces, this fraction varying inappreciably with the colour, for A = 598 /ul/ul 
only 1 per cent, of the incident light is absorbed in the glass. 
The following table gives the intensity of the fluorescence for different 
wave-lengths, compared with an electric glow lamp, in arbitrary units, 
(1) when excited by an incandescent gas-mantle, (2) when the same source 
was used with a filter of cobalt glass : — 
A 
( 1 ) 
( 2 ) 
606 up 
•147 
•068 
576 
•599 
•318 
550 
1-88 
1-04 
528 
3-92 
2-07 
509 
4-86 
2-77 
493 
2-13 
1-09 
479 
•455 
... 
This and the preceding table are plotted in the adjoining diagram 2. 
The fluorescence curves are, of course, not corrected for absorption in the 
glass, i.e. the intensity of the actual fluorescent light emitted by the glass 
is given, not the intensity we would obtain if none of it were absorbed 
in the glass in the way out. This latter, or the typical fluorescence curve, 
as Nichols and Merritt call it, would be more symmetrical. 
The fraction of the incident light transmitted by the cobalt glass filter 
for different wave-lengths is given in diagram 3. 
At first, on measuring T, F and C, T + F was always greater than C. 
A careful investigation, however, showed that this was due to diffuse 
light reflected from screen S. In diagram 4, H represents the aperture 
of the instrument. The exciting rays entered in the direction of the 
arrow, the corners at A and B being screened. The side BD of the cube 
had black paper gummed on it, to absorb the exciting rays. The green 
light from the exciting source was roughly one thousand times as strong 
as the fluorescence from the glass. If one-hundredth of the incident 
light were diffusely reflected from BD to S, and one-hundredth of that 
