28 THE NATUREIOF ANBIAL LIGHT 



plior. " A '^phosphor" is in reality an example of a solid 

 solution and is the basis of some kinds of luminous paints. 



The intensity and duration of a phosphorescent light 

 depend chiefly on the nature of the exciting rays, the color 

 chiefly on the impurity present but the all^alin-e earth metal 

 also exerts an influence. Rise in temperature increases 

 the intensity but diminishes the duration, so that the 

 total amount of light emitted is about constant at differ- 

 ent temperatures. 



The spectrum of most phosphorescent substances is 

 made up of one or more continuous bands having maxima 

 at different wave-lengths. In the light incident on a phos- 

 jDhorescent substance are also bands of light rays which 

 are absorbed and whose wave-lengths are more ejficient 

 than others in stimulating phosphorescence. These bands 

 in the phosphorescent light are usually of longer wave- 

 length than those in the light which excites the phos- 

 phorescence. Tliis fact is known as Stokes' Law, but it 

 has been found not to be universally true. Curiously 

 enough, red and infra-red rays have the power of annulling 

 phosphorescence after a momentary increase in brightness 

 and phosphorescing materials have been used to deter- 

 mine if infra-red rays are given off in the light of ihe 

 firefly. Ives (1910) showed that infra-red radiation had 

 no power of quenching the light of the firefly as it does 

 the phosphorescent light of Sidot blende (ZnS), one fact 

 tending to show that the firefly's light is not due to 

 phosphorescence. Fig. 3 is a reproduction of a photo- 

 graph of the phosphorescence spectrum of ZnS. 



Other facts show that the light of luminous animals is 

 in no sense a phosphorescence and is quite independent 

 of previous illumination of the animal. Luminous bac- 



