FLUORESCENCE. This term has been introduced by Professor Stokes to denote a remarkable property possessed by certain sub stances with respect to light, and it has been adopted from the fact that fluor spar exhibits the phenomenon in a marked degree.
Suppose a trough, the sides and ends of which are made of plate glass, to be filled with a solution of sulphate of quinine. A ray of sunshine is admitted through a small hole in a shutter, and passed through a prism so as to be decomposed into rays of the prismatic colours. The trough, with its solution, is then placed so as to receive and transmit the solar spectrum. On looking through the ends of the trough, the luminous and least refrangible rays are seen to be transmitted, while the extreme violet rays are absorbed, and seen to penetrate only to a certain depth in the liquid ; and in addition to this, rays beyond the violet, which were before invisible, are now rendered visible, and appear of a celestial blue colour, penetrating to a certain depth, and then disappearing.
If a piece of sensitive photographic paper be placed so as to receive the spectrum transmitted through the fluid, it is found that the usual darkening at and beyond the violet end of it is wanting.
A block of yellow uranium glass possesses a similar property to the solution of sulphate of quinine. So do Eesculine, and other substances.
For further particulars of this curious phenomenon, the reader is referred to Mr. Stokes's original paper in.the "Philosophical Tran sactions" for the year 1852.
It has been said that fluorescent bodies have the property not only of rendering the invisible rays visible, but of imparting chemical action to the luminous rays. This assertion must, however, be received with caution, and will probably turn out to be erroneous.
Mr. Stokes is of opinion that " the phenomena of internal dis persion oppose fresh difficulties to the supposition of a difference of nature in luminous, chemical, and phosphorogenic rays, but are per fectly conformable to the supposition that the production of light, of chemical changes, and of phosphoric excitement, are merely different effects of the same cause." FLtroxixz. F= 7. This is a hypothetic,a1 elementary body, which has not yet been isolated.