60 SCIENCE PROGRESS 



wave-length in exciting the particular band of phosphorescent 

 light in question — that is to say, the intensity of the incited 

 light. The distribution of the intensity of the incited light 

 according to wave-length is represented by the shaded curves. 

 The first phosphoroid gives three bands of emitted light; these are 

 represented separately, as there is a different exciting spectrum 

 corresponding to each band ; the three spectra are denoted 

 by a, /8, 7. The second has two bands, a and #, represented in 

 the same manner. It will be observed that the bands are best 

 excited by very narrow groups of wave-lengths and that in 

 general more than one exciting band — usually three — corre- 

 spond to each band of emitted light. The dotted curve gives 

 the distribution of exciting light corresponding to the momentary 

 process, to be referred to subsequently. The intensities of the 

 different bands in the diagram are not drawn to scale, but they 

 are all represented as having the same maximum intensity ; this 

 is done because, though all the bands have perfectly definite 

 spectral positions, their relative intensities vary with the fusible 

 component, the temperature and the manner in which the 

 phosphoroid is prepared. Hence such a diagram can only give 

 the general course, the position of the maximum intensity, and 

 the range of each band. 



The behaviour of a band with regard to temperature is such 

 that it is possible to discriminate between three different states 

 of the phosphoroid. In the coldest state, which Lenard calls 

 the lower momentary state, each particular band rapidly reaches 

 its maximum intensity when incited, and on the cessation of the 

 exciting light as rapidly decays — it being a general rule that 

 a band which is easily incited dies out quickly, and that one 

 which is slowly incited dies out slowly. The light emitted 

 at this stage is often very feeble, sometimes not noticeable ; as 

 the temperature of the phosphoroid is raised, the second or 

 " resting" state is reached, in which light energy is both emitted 

 and at the same time stored up ; when the exciting illumination 

 is cut off, the stored-up energy is liberated as the after-glow, the 

 intensity of the bands gradually diminishing with time. On 

 raising the temperature still further the third temperature state, 

 the upper momentary state, is reached, in which, as in the lower 

 state, there is no after-glow, but a rapid excitation followed by 

 a rapid emission of light. It is necessary, however, to dis- 

 tinguish clearly between the upper and the lower momentary 



