476 THE POPULAR SCIENCE MONTHLY. 



light produced is the brighter, the more completely the ray is ab- 

 sorbed. 



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 JS 

 to a point within the ultra-violet, exhibiting at the same time bright 

 bands which correspond with the dark bands in the absorption spec- 

 trum. Between JB 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 _S and C. Chlo- 

 rophyll solution aftbrds a proof that all rays of the spectrum, with the 

 exception of the extreme red in front of ^, are capable of calling forth 

 fluorescence. Their capacity for doing so depends simply on the 

 power of absorption of the fluorescing substance. The most refrangi- 

 ble violet and ultra-violet rays are, however, characterized by the cir- 

 cumstance that they are capable of exciting all known fluorescing 

 bodies. 



Fluorescent light is only perceived so long as the fluorescent sub- 

 stance is illuminated by the exciting rays. As soon as the light fall- 

 ing on it is obstructed, the colored shimmer vanishes. It is only in 

 the case of some fluorescing solid substances, as, for example, fluor- 

 spar and uraniiim glass, that, with the aid of appropriate apparatus 

 (Becquerel's phosphoriscope), a very short continuance of the fluores- 

 cence 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 hav- 

 ing 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 dia- 

 mond, fluor-spar, and a variety of fluor-spar called chlorojjhane. 



