LENARD'S RESEARCHES ON PHOSPHORESCENCE 59 



intensity of the incited light, a spectrum is allowed to fall upon 

 a screen covered with the given phosphoroid, the exciting light 

 from a Nernst lamp or mercury vapour lamp being passed 

 through a quartz prism in order to obtain the ultra-violet 

 portion strong and well dispersed. The parts of the spectrum 

 which are most effective in exciting the phosphorescent light 

 were then at once observable. On examining the incited light 

 through a prism, using the method of crossed spectra, it is 

 resolved into its component bands, the relative intensity of the 

 different parts of which can be estimated. Stokes's law, that 

 the incited light is of longer wave-length than the exciting light, 

 is always obeyed by phosphoroids. As a first result of this 



Co. L-u ci 



p 



Y 



\A— 



^wSk^ 



Or Lu 



P 



100 



300 



(.00 



Fig. 1 



500 



6oojiji 



method, it appeared that to each band ol emitted light 

 correspond definite ranges of wave-lengths which are capable 

 of exciting it ; these selective groups of wave-lengths will be 

 referred to as the exciting spectrum. The composition of this 

 spectrum depends only on the nature of the active metal and of 

 the alkaline sulphide. Further, there are no bands common to 

 different metals, either of excitation or emission. In fig. 1 the 

 spectral distribution of the exciting and incited light is set out 

 according to Lenard's method in the case^ of the two phos- 

 phoroids calcium sulphide containing copper as the active 

 metal denoted by CaCu and strontium sulphide containing 

 copper denoted by SrCu. The sharp unshaded curves indicate 

 the distribution of the exciting light, the abscissae representing 

 the wave-length of the light, the ordinates the efficiency of each 



