32 



FLUORESCENCE OF THE URANYL SALTS. 



men tested. The fluorescence spectrum was observed in a spectro- 

 photometer, the specimen being set up at an angle of approximately 

 45 with the path of the exciting light, so that the collimator of the 

 spectrophotometer could be pointed at the illuminated surface without 

 interfering with the exciting light. Enough of the exciting rays were 

 reflected into the spectrophotometer to 

 enable the range of wave-lengths used 

 in each case to be determined. The 

 spectrophotometer was then set at the 

 crest of the principal fluorescence band 

 and the intensity measured by com- 

 parison with an acetylene standard. 

 Observations of this sort were repeated 

 throughout the absorbing region. The 

 results are shown in figure 22. It will be 

 noticed that the regions of strong excita- 

 tion at 4,910 and 4,775 correspond very 

 closely to the two absorption bands at 

 4,920 and 4,766. Some slight indication 

 is also present of the other absorption 

 bands. It is clear, however, that the 

 ability to excite luminescence is not con- 

 fined to rays falling within the narrow 

 absorption bands, but extends to the 

 region of general absorption lying be- 

 tween. It is not possible to determine 

 the specific exciting power of different 

 rays, as has been done in the case of eosin 

 and resorufin, 1 because of our ignorance 

 of the absorbing power of the salt for 

 different wave-lengths. 2 The results indicate, however, that the specific 

 exciting power varies only slightly with the wave-length, as in the case 

 of resorufin and eosin. 



THE RELATION BETWEEN ABSORPTION AND FLUORESCENCE AS 

 IT APPEARS AT ORDINARY TEMPERATURES. 



In 1885 H. Becquerel 3 made measurements of the spectrum of uranyl 

 nitrate from which it would appear that the frequency interval remains 

 constant in passing from the fluorescence to the absorption spectrum 

 and that the suggestion of E. Becquerel in his classical memoir of 1872, 

 that the emission bands and absorption bands belong to the same series, 

 is in accordance with the facts. 



H. Becquerel also showed that two of the bands are reversible, ap- 

 pearing as emission bands when suitably excited, whereas if light free 



1 Physical Review, xxxi, p. 381. 



2 The distribution of energy in the spectrum of the Nernst glower also has not been determined. 



3 H. Becquerel, Comptes Rendus, 101, p. 1252. 



.42 



.46 



FIG. 22. Intensity of fluorescence 

 (ordinates) produced by exciting 

 light of different wave-length (ab- 

 scissa?) . 



