THE LUMINESCENCE OF SIDOT BI,ENDE. 



47 



slit was kept at ioo divisions of its micrometer screw, and the time required 

 for the phosphorescence of any given wave-length to fall from its initial 

 value to this intensity was determined as described above. The curve 

 is obtained by plotting wave-lengths as abscissas and times as ordinates. 

 In curve 77 the width of the comparison slit was 60 divisions ; in curve 777, 

 30 divisions; and in curve IV, 10 divisions. Observations made at 5 divi- 

 sions are not included on the plot. Points lying on the horizontal axis in 

 Fig. 42 were located by determining by trial the wave-length for which 

 there was a balance in the spectrophotometer the instant after dropping the 

 shutter. These points are probably somewhat less reliable than the others. 

 The significance of these results is better shown by plotting them in a 

 different way. In Fig. 43 wave-lengths are plotted as abscissas as before, 

 but intensities, instead of times, are plotted as ordinates. Each curve in 

 this figure shows the distribution of intensity in the phosphorescence spec- 

 trum at some definite time after the removal of the exciting light 



.460 .480 



.500 .520 .540 



Fig. 42. 



.560 



.580 -600 



Curves showing the time required for the phosphorescence to fall from 

 its initial intensity to a given final intensity. The final intensity is 

 kept constant throughout, each curve. 



No curves have been plotted for intervals of more than 1.75 seconds 

 after excitation had ceased, since our data furnished so few points for the 

 later curves that their form would be largely a matter of conjecture. 1 As 

 already stated, however, a curve of the type shown in Fig. 42 was deter- 

 mined for an intensity of 5 divisions. The maximum ordinate of this 

 curve was 10. 1 seconds and occurred at 0.506 ju; i. e., at the same wave- 

 length as in the case of the curves shown in 42. Comparison with Fig. 40 

 shows that the maximum of the luminescence spectrum during excitation 

 also occurs at this wave-length. In the case of the green band of Sidot 

 blende we conclude therefore that the maximum of the fluorescence spectrum 

 occurs at the same wave-length as that of the phosphorescence spectrum, 



'In a series of observations of the kind recorded in Fig. 42 the number of points that could be located was 

 limited both by the difficulty of maintaining constant conditions during the three or four hours of observa- 

 tion, and by the endurance of the observer's eye. 



