134 



STUDIES IN LUMINESCENCE. 



The powder from which Balmain's paint is made, as will be seen by refer- 

 ence to Chapter IV, shows a more pronounced bend in the decay curve than 

 is the case with Sidot blende. Two determinations were therefore made 

 using this substance and showing the character of the spectrum before and 

 after the bend. For each curve (see Fig. 140) the excitation was 5 minutes. 

 For curve A, the plate was exposed 20 seconds immediately after excitation. 

 For curve B, the phosphorescence was allowed to decay 1.5 minutes, then 

 the plate was exposed for 1 minute. No shift in the minima of the curves 

 can be detected, while a decided change in the shape of the curves would 

 be expected if the two-band theory is correct. 



A PHOTOGRAPHIC TEST OF THE EFFECT OF INFRA-RED RAYS. 



The effect of infra-red radiations in suppressing phosphorescence and 

 fluorescence has been considered at length in Chapter V, in which exper- 

 iments were described indicating that all parts of a fluorescence band are 

 suppressed in the same ratio. As a check on that work and because, on 

 the assumption of the existence of two bands, one band would in all prob- 

 ability be suppressed more than the other, several runs were made in which 



.46 .50 5 A .58 62,/i. 



Fig. 141. 



the fluorescence spectrum of Sidot blende under the influence of infra-red 

 rays was photographed. These rays were obtained from a 16 c.p. lamp 

 held 5 cm. from the powder. The visible rays were screened off by means 

 of thin rubber. 



In Fig. 141 curve A corresponds to the fluorescence spectrum without 

 infra-red excitation, and curve B with infra-red. The exposure of the plate 

 in the latter was twice as long as in the former case. It will be seen that the 

 minimum is not changed appreciably from 11 0.555 by the exposure to infra- 

 red and that the curves agree very well in form, except at short wave-lengths. 



In Fig. 142, curve A corresponds to the fluorescence spectrum without 

 infra-red; curve B to the same with infra-red; and curve C corresponds 

 to the phosphorescence spectrum just after excitation and is added for ease 

 of comparison. In each case the minimum of the curve occurs at n = 0.555 

 approximately. 



The effect of infra-red excitation on the fluorescence spectrum, according 

 to all of the curves obtained, is to decrease the intensity of the band ; but 



