Eugene Lommel

“ In order to understand more precisely the circumstances under which fluorescence occurs, the solution of esculine must again be referred to. The light, before it reaches the lens, must be allowed to pass through just such another solution of esculine contained in a glass cell with parallel walls. The cone of light proceeding from the lens, as long as it passes through the air, does not appear to have undergone any material change, it is just as bright and just as white as before. In the interior of the fluid, however, it no longer presents a blue shimmer, but becomes scarcely perceptible. ”

“ Fluorescent light is only perceived so long as the fluorescent substance is illuminated by the exciting rays. As soon as the light falling on it is obstructed, the colored shimmer vanishes. It is only in the case of some fluorescing solid substances, as, for example, fluorspar and uranium glass, that, with the aid of appropriate apparatus (Becquerel's phosphoriscope) , a very short continuance of the fluorescence may be observed to take place in the dark. ”

“ The electric light passing between carbon-points is rich in rays of high refrangibility, indeed the ultra-violet end of its spectrum reaches even farther than that of the solar spectrum. In the light of the magnesium-lamp the ultra-violet rays are also abundant, and both sources of light are therefore particularly well adapted to produce fluorescence, while gas and candle light are nearly inoperative on account of the small amount of the more refrangible rays they contain. ”