PHOTOGRAPHIC STUDIES OF LUMINESCENCE. 



K->5 



there is no change in the distribution of intensities that can be detected 

 by this method and no measurable shift of the maximum. This result is 

 in accordance with those recorded in Chapter V and it does not confirm 

 the two-band theory of decay. 



THE EFFECT OF TEMPERATURE ON THE FLUORESCENCE SPECTRUM. 



It has recently been shown 1 that remarkable changes in the apparent 

 complexity of fluorescence spectra are in some cases produced by changes 

 in the temperature of the substance. 



To determine whether such changes 

 as could be detected by the present 

 method take place in the fluorescence 

 band of Sidot blende when that sub- 

 stance is excited at different temper- 

 atures a series of photographs were 

 taken, the results of which are shown 

 in Fig. 143. 



This figure contains four curves 

 corresponding to the fluorescence of 

 vSidot blende at different temperatures. 

 In each case the length of excitation 

 was varied so as to give, approximately, 

 negatives of equal density, hence the 

 dimming of the band at higher tem- 

 peratures is not evident, though it 

 occurred. In fact, the powder almost 

 ceased to exhibit fluorescence when the 

 temperature was raised sufficiently. 



When the temperature was lowered the powder would again show the 

 same spectrum as before, provided a certain critical temperature had not 

 been exceeded. The .v-axis is changed for each curve, so that the points 

 may be entirely distinct from one another. Curves A, B, C, and D were 

 obtained at temperatures 22 , 67 , 88, and 120 respectively. No shifting 

 of the minimum can be seen. 



.50 



.54 



Fig- 143- 



58 



.62/i 



: Xicho1s and Merritt; Physical Review, xxxn, p. 38. 



