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.
It would nevertheless be incorrect to infer from the above facts that the more refrangible rays are exclusively capable of exciting fluorescence. A red fluid which 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 effect. In fact, if the spectrum 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 C and D and therefore still in the red, and to
Fig. 5.—Absorption and Fluorescing Spectrum of Naphthaline Red.
extend over the whole remaining spectrum as far as the ultra-violet. The strongest fluorescence by far is shown behind the line D in the greenish-yellow rays. It then again diminishes, and becomes a second time more marked between E and b; thence onward the fluorescence 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 behind D, 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 behind D, a dark band between E and b, while the violet end appears shaded. On employing a very strong solution of the naphthaline coloring material, the whole spectrum vanishes with the exception of the red end, which remains apparent to a point behind C. If now the absorption spectrum be compared with that thrown upon the fluid, the intimate relation between absorption and fluorescence that has already been pointed out in the esculine solution is corroborated in the minutest particulars. For every dark hand in the absorption spectrum corresponds to a bright band in the fluorescing spectrum. Every ray absorbed by the fluid occasions fluorescence, and the fluorescent
