AFTER INSOLATION. 15 
the phosphorescence excited by the other rays to 
cease! Ritter 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 hght 
of rendering certain bodies luminous in the dark, 
appears to reside—if not entirely—at least to a 
ereat 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. 
