62 SCIENCE PROGRESS 



tary and permanent bands. These regions of exciting wave- 

 length for the two phases largely overlap, so that in general a 

 given wave-length may induce both processes simultaneously ; 

 but some of the shorter wave-lengths of the exciting light induce 

 only the momentary, some of the longer only the permanent 

 process. In fig. i the exciting spectrum corresponding to the 

 momentary process is indicated by the broken line. Thus in 

 the permanent state the energy is at the same time in part stored 

 and in part used for the immediate emission of transformed 

 radiation ; but, while that of some wave-lengths is used for both 

 processes, certain small spectral regions are only available for the 

 one process, certain other regions only for the other. By going 

 to a low enough temperature, the three states have been observed 

 in all phosphoroids. Each band stores its own energy, as can be 

 found by observing the exciting spectrum in the lower state. 



As regards the exciting spectrum, the character of this for 

 the momentary is somewhat different from that for the per- 

 manent state, as can be seen in the figure. The distribution, in 

 the case of the latter, consists of well-defined bands, there being 

 in general more than one exciting band corresponding to each 

 emission band. The most frequent case is that of three sharp, 

 nearly equal maxima of exciting intensity separated by regions 

 in which the light produces no permanent glow. The distribu- 

 tion in the case of the momentary bands is not nearly so sharp ; 

 there is only one band of exciting light corresponding to each 

 emission band, and this is ill-defined and lies largely in the 

 ultra-violet : the position of the less refrangible edge of the 

 band is characteristic of that band, however. 



The theory which Lenard has developed to explain the 

 properties of the bands just described— for the bands are the 

 fundamental things — attributes the phenomena to a photo- 

 electric action * of the light, which liberates electrons from the 

 metallic atoms in the " centres " from which the emission of 

 light proceeds present in all phosphorescent substances. These 

 centres are complex molecules having as essential components 

 an atom of the active metal, together with the alkali metal and 

 sulphur, and they are distributed singly and separately through- 

 out the mass of inactive material which forms the bulk of the 



1 The liberation of negative electricity — electrons — which takes place when 

 light of short wave-length falls upon metals and many other substances is called 

 the photo-electric effect. 



