Page:Popular Science Monthly Volume 8.djvu/489

Rh evident from the circumstance that with the electric light they are no more apparent in the ultra-violet than in the other colors, and further, because the same lines are seen in the solar spectrum, whatever may be the fluorescing substance under examination.

Quartz has the power of transmitting the ultra-violet rays far more completely than glass. If, therefore, the glass lens and prism hitherto used for projecting the spectrum be replaced by a quartz lens and prism, the ultra-violet part of the spectrum is rendered much brighter and is extended still farther than before.

The ultra-violet rays of the spectrum can, moreover, be seen, without the intervention of any fluorescing substance, through a glass, or, still better, through a quartz prism, if the bright part of the spectrum between B and H (Fig. 2) be carefully shut off. With feeble illumination its color appears indigo-blue, but with light of greater intensity it is of a bluish-gray tint (lavender). The ultra-violet rays thus ordinarily escape observation, because they produce a much feebler impression on the human eye than the less refrangible rays between B and H.

An explanation is thus afforded why the solution of esculine, apart from its absorption, is colorless when seen by transmitted light; for, since it absorbs only the feebly luminous violet and the entirely imperceptible ultra-violet rays, the mixed light that has passed through it still appears white, and is not rendered materially fainter.

If the solar spectrum be thrown in the above-mentioned manner upon the fluid, its fluorescing part everywhere exhibits the same bluish shimmer; and spectroscopic examination shows that this bluish light has always the same composition, whether it is excited by the G rays, or by the H rays, or by the ultra-violet rays, and that it is formed of a mixture of red, orange, yellow, green, and blue. It is thus seen that the different kinds of homogeneous light, as far as they are generally effective, produce compound fluorescent light of identical composition, the constituents of which, nevertheless, are collectively less refrangible than, or are at most equally refrangible with, the exciting rays.

Among other fluorescing bodies may be mentioned the solution of quinine, which is as clear as water, and has a bright-blue fluorescence; the slightly yellow petroleum, with blue fluorescence; the yellow solution of turmeric, with green; and the bright-yellow glass containing uranium, which fluoresces with beautiful bright-green fluorescence. It admits of easy demonstration that in these bodies also it is