62 



FLUORESCENCE OF THE URANYL SALTS. 



TABLE 18. Intensities 

 of bands in group 6 

 (excited at +20 C). 



1 Visible, but too dim for 

 spectrophotometric 

 measurement. 



position as well as changed in intensity in a manner to be described in 

 a subsequent paragraph. 



To determine as closely as possible the wave-lengths of the bands, 

 photographs of the spectra of the four double chlorides were taken 

 and many visual settings were made. Fluorescence was excited by 

 means of the carbon arc, the light from which passed through a screen 

 opaque to rays of wave-length greater than about 0.45 /* and was 

 focussed upon the crystal. Various exposures were 

 employed on account of the great d fferenees in the 

 intensity of the bands and special plates were used 

 for the red end of the spectrum. The exciting 

 light was excluded from the camera by the use of 

 suitable screens opaque to the blue and violet except 

 where the absorption spectrum was to be recorded. 



The various negatives were measured by mount- 

 ing them on a micrometer stage in the field of the 

 lantern. The micrometer-screw was carefully cali- 

 brated, so that wave-lengths could be determined by 

 measuring the distance of the crests of the bands 

 from certain reference lines of the mercury spectrum, which was photo- 

 graphed on each negative so as to overlap the fluorescence spectrum. 



This method of projection was 

 found better than the use of the 

 comparator commonly employed in 

 the measurement of line spectra, 

 because of the hazy character of the 

 bands and because bands that are 

 so weak and vague as to be invis- 

 ible even under a low-power micro- 

 scope could be seen and ocated by 

 means of the lantern. Many 

 measurements of the stronger bands 

 were made with the comparator as 

 a check on the determinations with 

 the lantern. 



These measurements confirmed 

 to a remarkable degree the apparent 

 symmetry of the spectrum. When 

 all the bands are plotted on a large 

 scale, in a diagram with the recip- 

 rocal of wave-lengths as abscissae, 

 the spectrum is seen to consist of 8 

 groups of 5 bands each, as already described. The nearly uniform 

 arrangement of the bands of each group repeats itself precisely from 

 group to group, so that corresponding members of the groups form an 



.52 



FIG. 52. 



