42 



STUDIES IN LUMINESCENCE. 



in the phosphorescence spectrum, observed 5 seconds after excitation had 

 ceased. Owing to the weakness of the phosphorescence excited by Roentgen 

 rays, the latter curve was determined with some difficulty, the readings at 

 the red end of the spectrum being especially uncertain. The indications 

 of a second maximum in the phosphorescence spectrum at 0.62 n or beyond 

 are therefore not to be regarded as entirely trustworthy. 



PHOTO-LUMINESCENCE DURING EXCITATION. 



In the experiments upon photo-luminescence during excitation the carbon 

 arc was first used as an exciting source. The arrangement of apparatus 

 is shown in Fig. 39. The zinc sulphide screen was placed in a light-tight 

 box B, and the exciting light from an arc A , after dispersion by the prism P, 

 entered the box through an opening at the left. A second opening in B 

 allowed the fluorescence light to enter the slit of the Lummer-Brodhun 

 spectrophotometer Si. The comparison source was an acetylene flame F, 



Fig- 39- 



.440 .480 .520/1- .560 .600 



Fig. 40. 



Luminescence spectrum of Sidot blende 

 during excitation by the violet bands 

 in the arc. 



whose rays were reflected into the comparison slit by the block of magne- 

 sium carbonate, M. The second spectrophotometer S 2 and the shutter C 

 were used in later experiments on phosphorescence, but not in the experi- 

 ments now considered. 



Luminescence was excited only by the rays at the violet end of the arc 

 spectrum, and especially in the region of the violet bands. It is to be noted 

 that on account of the absorption of the glass prism and lenses the spectrum 

 extended only a short distance into the ultra-violet. 



Observations taken to determine the luminescence spectrum when the 

 zinc sulphide screen was excited by the violet bands of the arc are plotted 

 in Fig. 40. Owing to the impurity of the exciting spectrum a certain 

 amount of blue and green light reached the screen even when the attempt 

 was made to excite by violet only. vSince this stray light was in part 

 reflected into the slit of the spectrophotometer with the luminescence light 

 of the same color, it was necessary to make a correction to remove the error 

 due to this cause. The correction was determined by replacing the zinc 



