PHOSPHORESCENCE SPECTRA. 



41 



FIQ. 3 la. 



Fio. 316. 



coinciding throughout as to wave-length, but separated by a dark line 

 formed by the lower edge of the second prism (#') 



To compare fluorescence with phosphorescence, the sectored disk was 

 shifted upon its shaft until the lower end of the tube was viewed during 

 excitation, the upper end immediately after (fig. 31a). To compare 

 the phosphorescence spectrum at an earlier and later stage, the disk 

 was so set that its position at the moment of excitation was as shown 

 in figure 316. By means of the reflecting prisms at the slit of the spec- 

 troscope, already described, the spectrum of the light emitted from 

 region A was compared with 

 that at B in each case. At 

 +20 C. the banded spectra 

 were found to be identical 

 in every respect, except in 

 brightness ; and the same was 

 true at low temperatures, . 

 where it was possible to in- 

 spect each of the numerous 

 line-like bands individually. 



Of the seven homologous series distinguishable in the fluorescence 

 spectrum, all were present in phosphorescent light, unshif ted as to posi- 

 tion and not perceptibly enhanced or diminished in relative brightness. 



The comparison was less satisfactory as regards minor details in the 

 case of the early and late stages of phosphorescence, some of the fainter 

 bands being invisible, but changes such as might be looked for, i. e., 

 those due to the greater persistence of certain series, could scarcely 

 have escaped notice. 



The significance of these observations is two-fold: On the one hand 

 we find that for the only examples of luminescence which admit of such 

 detailed inspection the spectrum of phosphorescence is identical with that 

 of fluorescence, and since there are no indications to the contrary in the 

 case of other classes of substances thus far studied, it is probable that 

 the above statement will apply to all phosphorescent materials. On 

 the other hand, we find that, in spite of its great complexity, the lumi- 

 nescence spectrum of a uranyl salt is to be regarded as a unit, all its 

 components decaying at the same rate after the cessation of excitation. 



Thus this class of substances (i. e., the uranyl salts) not only conform 

 to the first three criteria of homogeneity discussed in Chapter II but 

 likewise to that based upon the phenomena of phosphorescence. 



CURVES OF DECAY. 



To determine the change of intensity of phosphorescence with the 

 time a simple form of photometer previously used in a study of the 

 phosphorescence of kunzite 1 was mounted in front of the sectored disk. 



1 Nichols and Howes, Physical Review (2), iv, p. 19. 1914. 



