1908-9.] Negative Attempt to detect Fluorescence Absorption. 411 
The next table gives the fluorescence curves as in the two former cases : — 
A 
(i) 
(2) 
620 
•089 
... 
598 
•356 
•104 
578 
•946 
•310 
560 
2-82 
•875 
544 
4-86 
1-761 
529 
8-24 
2-928 
516 
9-82 
3-202 
504 
613 
2*444 
494 
2-02 
•811 
484 
•194 
... 
The curves are plotted in diagram 6. It will be noticed that although 
the absorption of the two glasses is somewhat different, the fluorescence 
is the same according to the curves. However, when examined with the 
spectroscope and a narrow slit, the fluorescence maximum of the second 
glass is seen to consist of four bands, three of which are situated at 
516, 535, and 563 fxjj., the fourth being in the yellow. The first glass did 
not show bands, although one seemed to be almost visible. 
The results of two sets of readings made on X = 529 /ul/ul are given : — 
T 
F 
C 
T + F-C 
Probable 
Error. 
1821 
1055 
2810 
66 
11 
766 
1291 
2016 
41 
18 
In analogy with the anomalous dispersion produced by an absorption 
band, if fluorescence produces a change in absorption, it should also 
produce a change in the index of refraction. This was sought for with 
a Jamin polarisation interferometer.* The two interfering beams are 
in this instrument about 12 mm. apart. They were passed through the 
same solution or the same piece of uranium glass and the solution or 
glass in the path of the one beam made to fluoresce. No shift of the 
bands was detected. An arc lamp was used as exciting source, but its 
full intensity could not be utilised, as the fluorescence became so strong 
that the interference bands could not be seen. The bands were obtained 
from a sodium or lithium flame. 
According to theory, the change should have been too small to detect. 
For, if we represent the index of refraction in the neighbourhood of an 
absorption band in the usual way by a complex quantity 
n( 1 - zk), 
* Gom'ptes rendus , 67, 1868, p. 814. 
