5 8 SCIENCE PROGRESS 



mass together ; it has also an influence on the intensity of 

 the emitted light which will be further referred to. 



The specific character of the phosphorescent light is de- 

 pendent on the presence and nature of the traces of foreign metal. 

 Lenard and Klatt were able to attribute the phosphorescence of 

 calcium sulphide previously investigated by Lommel definitely 

 to traces of particular metals. To each metal corresponds a 

 series of emission bands, the phosphorescent light being always 

 resolved by the spectroscope into bands having a maximum of 

 intensit}' at a given wave-length fading off into darkness on 

 both sides of this maximum. The bands are referred to by the 

 wave-length at which they have their maximum intensity ; and 

 uncertainty as to the identity of a given band, which might arise 

 in the discussion of the displacement of a band by influences to 

 be mentioned later, is avoided by the definition of a band as a 

 complex of emitted wave-lengths which possess common 

 properties in respect of temperature, excitation by light of a 

 particular wave-length, and rate of decay after the exciting light 

 has been cut off. These tests also serve to separate superposed 

 bands. The spectral position of the bands is peculiar to the 

 given active metal, but their intensity and period of decay 

 depend to some extent on the fusible component. A pure 

 phosphoroid is defined as consisting of one alkaline sulphide 

 together with traces of an active foreign metal and a flux. The 

 pure sulphides do not phosphoresce, but an addition of o'oo2 per 

 cent, of bismuth will render barium sulphide strongly phos- 

 phorescent. The colour of the phosphorescent light varies 

 markedly with the temperature of the phosphoroid, the shade 

 obvious to the naked eye being made up of different bands 

 which all vary in intensity independently of one another with 

 temperature. In the case of each phosphoroid, there is a 

 temperature above which it cannot be excited, but there seems 

 to be no lower limit in this respect. 



The investigation of phosphorescence has been greatly 

 facilitated by Lenard's method of plotting the distribution of 

 the exciting and excited light in the spectrum. As long as the 

 phosphorescent glow was treated as a whole, the complexity 

 of the observed phenomena baffled interpretation, but the 

 behaviour of the individual bands is not so incomprehensible. 

 To observe the distribution of excitation, in other words, the 

 relation between the wave-length of the exciting light and the 



