THE LUMINESCENCE OF SIDOT BLENDE. 45 



(curve III) the green band, while still observable, is not so strong as in curve 

 IV, while the band at 0.48^ is readily detected. Curves I and 77 (iron 

 spark) show scarcely any trace of the green band, but the bands at 0.48^ and 

 at 0.42 ju are well marked. 



FAILURE OF STOKES'S LAW. 



As already stated, the green band is most brilliantly excited by the 

 violet, although rays from all parts of the ultra-violet spectrum are also 

 capable of producing a considerable effect. We thus see that there is the 

 same general relation between the position of the luminescence spectrum 

 and that of the exciting light as in the case of fluorescence. In all the cases 

 of fluorescence thus far studied we have found that the two spectral regions 

 overlap. It therefore seemed interesting to see whether the same is true in 

 the present case. 



To settle this point it was necessary to use in excitation a far purer 

 spectrum than that employed in our other experiments with Sidot blende. 

 The rather crude dispersing system shown in Fig. 39 was therefore replaced 

 by a large spectrometer. A spectrum of the arc was thrown on the colli- 

 mator slit by the aid of a prism and single lens, and after a second dis- 

 persion in the spectrometer, light reached the phosphorescent screen as a 

 sharply focused band. Using a shutter described below so as to observe 

 the phosphorescence immediately after the exciting light was cut off, and 

 making observations with the unaided eye instead of with the spectrophoto- 

 meter, it was found that phosphorescence was unquestionably present for 

 exciting light lying between 0.470 \x and 0.497 M- Since the phosphores- 

 cence spectrum can readily be followed beyond 0.46 \x there appears to 

 be the same violation of Stokes's law in the phosphorescence of Sidot 

 blende that we have previously found in the case of fluorescence. 



THE PHOSPHORESCENCE SPECTRUM DURING DECAY. 



Although the phosphorescence of Sidot blende can be detected in a dark 

 room for several hours after excitation has ceased, it remains sufficiently 

 bright for spectrophotometric measurement only for a few seconds. In 

 order to determine the law of decay for different wave-lengths and especially 

 to determine the change, if any, in the phosphorescence spectrum during 

 decay, the following procedure was followed : 



A shutter, shown by the broken line C in Fig. 39, and sliding upon vertical 

 guides, was attached to the box containing the phosphorescent screen. 

 When this shutter was raised it permitted the exciting light to enter the 

 box, as in the experiments just described, but at the same time closed the 

 opening in front of the collimator slit. When the shutter was dropped it 

 first cut off the exciting light and then, a moment later, opened the window 

 in front of the collimator. The observer was in this way protected from 

 the brilliant luminescence produced during excitation, but was enabled to 

 observe the phosphorescence immediately after excitation had ceased. The 

 comparison slit being set to some suitable reading, the observer recorded 

 upon a chronograph the instant when the two parts of the spectrophoto- 

 meter field appeared equally bright. By means of a suitable electric contact 

 on the shutter a record was also made at the time when the exciting light 



