204 STUDIES IN LUMINESCENCE. 



minute. 1 The results of Waggoner and Zeller with short-time phosphores- 

 cence, although less accurate, point to the same conclusion. 



In the case of our own observations on Sidot blende we have expressed 

 the opinion that the curves obtained are to be regarded as showing the 

 decay of the green band alone; for although Sidot blende also possesses 

 bands in the blue and violet, these decay so quickly and are of such small 

 luminosity that they can scarcely affect the curves to any appreciable 

 extent. In the later work of Werner 2 special precautions were taken in 

 the choice of a substance and in the use of color screens to make certain 

 that one band only was studied ; yet the curves obtained are of exactly the 

 same type as those found by other observers. 



The work of Werner, even more strongly perhaps than the work of 

 Pierce and our own work with Sidot blende, thus appears to discredit any 

 explanation of the form of the decay curve that is based upon the assump- 

 tion of two bands in the phosphorescence spectrum. It can not be denied, 

 however, that two bands may well be present, even when special precautions 

 of this kind are taken, and even when the spectrophotometer shows no 

 indication of a double band. The bands of a phosphorescence spectrum 

 are ordinarily so broad that two bands lying close together might readily 

 appear as one. If the case is one where the bands actually overlap it 

 would not be possible to make observations on one alone without working 

 at the extreme edge of the double band, where the intensity w T ould prob- 

 ably be too small to permit of accurate observations. An extremely small 

 shift in the maximum might also fail of detection even by the photographic 

 method of Pierce. It does not at present appear possible to devise a method 

 of obtaining absolutely conclusive proof that Becquerel's explanation is not 

 correct. Since the assumption of two nearly coincident bands offers so direct 

 an explanation of the form of the decay curve, and since it seems certain 

 that curves may be plotted on the basis of this assumption which deviate 

 from the observed curves by less than the experimental errors, it must be 

 admitted that the hypothesis has much in its favor. 



There are several reasons, however, for looking upon this explanation 

 of the form of the curve with suspicion. In the first place, it does not 

 account either for the remarkable changes produced in the decay curves 

 by varying the intensity and duration of excitation, or for the phenomena 

 of hysteresis that have been observed in almost all cases of long-time phos- 

 phorescence. It also leaves untouched the question of the effects pro- 

 duced by exposure to the longer waves. An even more serious objection 

 is the fact that it is necessary to assume the existence of two nearly coin- 

 cident bands in all cases of long-time phosphorescence ; for upon replotting 

 the decay curves obtained by different observers with / _i and / as co- 

 ordinates it is found that the curves are of the same type for all substances 

 thus far tested. It is hardly credible that this is accidental. Only two 

 explanations appear to be possible : either the existence of two nearly coin- 

 cident bands is an essential characteristic of substances showing long-time 

 phosphorescence, or else the peculiarities exhibited in the form of the decay 



'This result has recently been confirmed by H. E. Ives and M. Euckiesh (Astrophysical Journal, vol 

 xxxiv. p. 17.}, iqii). who find that the phosphorescence spectrum remains the same for at least fifteen 

 minutes after the beginning of decay. 



-A. Werner, Ann der Phys., 24, p. 164. 1907. 



