AFTER INSOLATION. 15 



the phosphorescence excited by the other rays to 

 cease ! Bitter was also aware of this, and about 

 the same time Beccaria found that " the violet rays 

 of the spectrum are the most apt, the red rays the 

 least apt, to develope phosphorescence in solar 

 phosphori." Becquerel affirms also, from his own 

 experiments, that the property possessed by light 

 of rendering certain bodies luminous in the dark, 

 appears to reside if not entirely at least to a 

 great extent in the violet rays, whilst the red 

 rays are completely deprived of this property, a 

 fact noted also by Heinrich. 



Biot, Arago, Daguerre, and others, have made 

 many researches on this subject. They have 

 shown, among other curious facts, that with the 

 invisible rays sometimes termed chemical or ac- 

 tinic rays situated underneath the luminous part 

 of the solar spectrum, it is possible to render a 

 phosphorescent body luminous, or at least visible; 

 whilst, when plunged in the visible rays, red, yel- 

 low, orange, green, etc., not only this same body 

 is not lighted up, but its light previously excited 

 by the invisible rays is extinguished. 



This curious phenomenon has been admirably 

 investigated in England by Professor Stokes, who 

 has denominated it Fluorescence, and who has 

 shown that a considerable number of substances, 

 besides those known as solar phosphori, act upon 

 these invisible rays of the spectrum and render 

 them visible. 



