light produced is the brighter, the more completely the ray is absorbed.
A second example of the excitation of fluorescence by rays of small refrangibility is exhibited by a solution of chlorophyll. The spectrum projected upon this green fluid fluoresces of a dark-red color, from B to a point within the ultra-violet, exhibiting at the same time bright bands which correspond with the dark bands in the absorption spectrum. Between B and C, where the greatest amount of absorption occurs, the fluorescence is also the most marked. But it is the middle red rays which here act so powerfully as excitants. It is remarkable that the red fluorescent light which the chlorophyll solution emits likewise lies, in regard to its refrangibility, between B and C. Chlorophyll solution affords a proof that all rays of the spectrum, with the exception of the extreme red in front of B, are capable of calling forth fluorescence. Their capacity for doing so depends simply on the power of absorption of the fluorescing substance. The most refrangible violet and ultra-violet rays are, however, characterized by the circumstance that they are capable of exciting all known fluorescing bodies.
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.
There are, however, a number of bodies which, after being excited to self-luminosity by a brilliant light, continue to shine for a certain time in the dark. A series of pulverulent white substances, namely, the sulphur compounds of calcium, strontium, and barium (which should be kept in hermetically-sealed glass tubes), do not exhibit the faintest light in a dark room. Moreover, if they be covered with a yellow glass and illuminated with the light of a magnesium-lamp, they remain as dark as before. But if the yellow be exchanged for the blue glass, and the magnesium-light be allowed to play upon them for a few seconds only, they emit in the dark a soft light, each powder having its own proper tint of color. This power of shining in the dark after having been exposed to the light is called phosphorescence. The property is possessed in a high degree not only by the above-named artificially-prepared substances, but by various minerals, as the diamond, fluor-spar, and a variety of fluor-spar called chlorophane.
