chap, x xvi. ] PHOSPHORESCENCE. 339 



they have been exposed to the light. The sulphides 

 of calcium and strontium remain luminous in the 

 dark for several hours after exposure to a strong 

 light, diamonds, chloride of calcium, some barium 

 compounds, magnesium and other substances also. E. 

 Becquerel has shown that there are few substances 

 not phosphorescent, though in many the luminosity 

 lasts for so short a time that it is made apparent only 

 by special contrivances, such as Becquerel's PHOS- 

 PHOROSCOPE. Phosphorescence is explained in a way 

 similar to fluorescence. The phosphorescent body 

 absorbs certain rays, transforms them, and emits 

 them changed. Becquerel holds the two phenomena 

 to be of a similar character, the fluorescent body occu- 

 pying much less time than the phosphorescent in the 

 process, the former effecting the transformation while 

 the light is upon it. 



Some animals are luminous in the dark ; the 

 glowworm, the lampyre, and certain marine animals 

 whose light produces the phosphorescence of the sea. 

 That this property in animals is due to the same 

 causes as the phosphorescence of sulphide of calcium, 

 for instance, is not certain. The phosphorescent 

 material of the animals is probably a secretion. 

 Both fluorescence and phosphorescence can be produced 

 by passing an electric discharge from an induction coil 

 through a Geissler's tube containing the body to be 

 observed. 



Colour. If in front of the slit of a lantern, 

 whose beams afford a spectrum, a sheet of red glass be 

 placed, nearly all the spectrum will be cut off but the 

 red ; .the red will pass through and appear on the 

 screen ; the yellow will pass with difficulty, and the 

 colours beyond the yellow with increasing difficulty. 

 Similarly, yellow glass will permit yellow rays to pass; 

 all others will be enfeebled , notably the blue and violet. 

 If in the red part of the spectrum some red ribbon be 



