30 THE NATURE OF ANIMAL LIGHT 



used at the present time is ZnS containing a trace of 

 radium salt. The rays of the radium continually emitted 

 cause a steady fluorescence of the ZnS. Indeed, if one 

 examines the paint on the hands of a watch with a lens the 

 flash of light from the impact of alpha particles on the 

 ZnS can be distinctly seen, as in the spinthariscope. 



Some animal tissues and fluids, especially the lens of 

 the eye, will luminesce in the path of radium rays, as shown 

 by the experiments of Exner (1903), but there is no evi- 

 dence that luminous animals are especially active in 

 this respect. Ultra-violet rays have the same action. 



The luminous material of practically all luminous 

 forms, if dessicated sufficiently rapidly, can be obtained in 

 the form of a dry powder which will give off light when 

 moistened with water. Coblentz (1912) has exposed this 

 dry material to light, to the ultra-violet spark, and to 

 X-rays and in no case has a phosphorescence or fluores- 

 cence ever been observed. I have examined the action 

 of radium upon Gypridina light. There was no intensi- 

 fying or diminishing effect of twenty milligrams of radium 

 (probably the bromide) on a luminous solution of Cypri- 

 dina material, nor was phosphorescence or fluorescence 

 excited in a non-luminous extract of the animal. We 

 must conclude that animal light is not a fluorescence of 

 any substance due to radiation produced by the ani- 

 mals themselves. 



Many solutions show fluorescence in strong lights. 

 This is especially marked in quinine sulphate, mineral oils, 

 eosin, fluorescein, esculin, rhodamin, chlorophyll, etc. The 

 fluorescence of eosin in 10"^ grams per cubic centimetre is 

 visible in daylight and 10"^^ grams per cubic centimetre in 

 the beam from an arc lamp. It is difficult to realize that the 



