THE NATURE OF FLUORESCENCE. 



475 



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 i-efrangible rays they contain. 



It would nevertheless be incorrect to infer from the above facts 

 that the more refrangible rays are exclusively capable of exciting 

 fluorescence. Aredfluid whicli is an alcoholic solution of naphthaline 

 red, and which in ordinary daylight fluoresces with orange-yellow 

 tints of unusual brilliancy, will serve to demonstrate that even the 

 less refrangible rays are capable of producing this efiect. In fact, if 

 the sjDCCtruni be projected upon the glass cell containing the fluid 

 (Fig. 5), the yellow fluorescent light will be seen to commence at a 

 point intermediate to G and D^ and therefore still in the red, and to 



FiQ. 5. Absorption and Fluorescing Spectrum of Naphthaline Ked. 



extend over the whole remaining spectrum as far as the ultra-violet. 

 The strongest fluorescence by far is shown behind the line JD in the 

 greenish-yellow rays. It then again diminishes, and becomes a sec- 

 ond time more marked between JE and b ; thence onward the fluo- 

 rescence becomes fainter, then increases again in the violet, and 

 gradually vanishes in the ultra-violet. In naphthaline red, therefore, 

 there are rays of low refrangibility, namely, the green-yellow rays be- 

 hind X>, by which its fluorescence is most powerfully excited. 



The fluorescence spectrum received upon the fluid shows, as we 

 have already mentioned, three regions of stronger fluorescence, and 

 the absorption spectrum of naphthaline, which, by placing a small cell 

 filled with the solution in front of the slit, may be obtained upon a paper 

 screen, gives a key to the cause of this phenomenon. In this spectrum 

 Fig. 5 (1), a completely black band is visible in the green-yellow be- 

 hind Z), a dark band between E and J, while the violet end appears 

 shaded. On employing a very strong solution of the naphthaline col- 

 oring material, the whole spectrum vanishes with the exception of the 

 red end, which remains apparent to a point behind G. If now the 

 absorption spectrum be compared with that thrown upon the fluid, 

 the intimate relation between absorption and fluorescence that has 

 ah'eady been pointed out in the esculine solution is corroborated in 

 the minutest particulars. For every dark hand in the absorption spec- 

 trum coi'responds to a bright band in the ^fluorescing spectrum. Every 

 ray absorbed by the fluid occasions fluorescence, and the fluorescent 



