FLUORESCENCE AND ABSORPTION OF THE URANYL SALTS. 31 



band whose crest was at 4,925, while the wave-length of the very faint 

 absorption band was 4,922. Our previous determination of the wave- 

 length of the luminescence band, when looking at the surface exposed 

 to the exciting rays, was 4,920. These results appear to us to warrant 

 the conclusion that if disturbances due to absorption could be entirely 

 eliminated the two bands would be found to have exactly the same 

 wave-length. 



It must not be forgotten, however, that it is nearly impossible to 

 observe the fluorescence spectrum under conditions which entirely 

 eliminate effects due to absorption. The exciting light always pene- 

 trates to some extent beneath the surface, so that some of the emitted 

 light must pass through the fluorescent material before it can reach 

 the eye. It is natural, therefore, to expect a slight displacement in all 

 cases. Although our most reliable measurement of the wave-length 

 of the absorption band, 4,919, and our best determination of the crest 

 of the luminescence band, 4,920, differ by less than the probable errors 

 of measurement, we feel that it is not unlikely that the difference is a 

 real one, due to the cause just cited. 



The absorption band at 4,760 in the double sulphate differs in posi- 

 tion by 5 units from the fluorescence band at 4,765. A portion of this 

 difference may also be explained by absorption. But it is probably 

 chiefly due to the difficulty in accurately locating the crests of these 

 bands. The fluorescence band is extremely faint, while the absorption 

 band is not very sharp, because of the large general absorption ir^ this 

 region. 



Using a thick layer, formed by grinding down a translucent mass of 

 adhering crystals until a piece about 0.5 mm. thick was obtained, a 

 faint absorption band was observed at 5,127. This corresponds to the 

 brilliant fluorescence band at 5,130. In all likelihood the coincidence 

 here is complete, since measurements of the fluorescence band made at 

 the same time and with the same specimen as that used for absorption 

 measurements gave the same wave-length, 5,127, for both bands. 



EXCITATION BY LIGHT CORRESPONDING TO DIFFERENT PARTS 

 OF THE ABSORPTION REGION. 



It seemed a matter of some interest to determine the relative effec- 

 tiveness of light of different wave-lengths in producing fluorescence, 

 and experiments having this end in view have been made in the case of 

 the double sulphate. We were particularly interested in determining 

 whether wave-lengths falling within the sharp absorption bands at 

 4,918, 4,760, 4,615, etc., were especially effective in exciting lumines- 

 cence. 



The source of the exciting light used in these experiments was a Nernst 

 glower which was mounted in place of the slit of a spectrometer. The 

 spectrum was focussed upon an opaque screen containing a narrow slit, 

 and the light passing through this slit was used in exciting the speci- 



