similarly self-luminous under the influence of light. This peculiarity was first observed in a kind of spar occurring at Alston Moor, in England, which, itself of a clear green color, appears by transmitted solar light of a very beautiful indigo-violet color. From its occurrence in calcium fluoride the phenomenon has been named florescence.
In order to understand more precisely the circumstances under which fluorescence occurs, the solution of esculine must again be referred to. The light, before it reaches the lens, must be allowed to pass through just such another solution of esculine contained in a glass cell with parallel walls. The cone of light proceeding from the lens, as long as it passes through the air, does not appear to have undergone any material change, it is just as bright and just as white as before. In the interior of the fluid, however, it no longer presents a blue shimmer, but becomes scarcely perceptible.
Thus it is seen that light which has traversed a solution of esculine is no longer capable of exciting fluorescence in another solution of esculine. Those rays consequently which possess this property must be arrested by the first solution of esculine. Similar results are obtained in the case of every other fluorescent substance.
The general proposition can therefore be laid down, that a body capable of exhibiting fluorescence fluoresces by virtue of those rays which it absorbs.
In order to determine what rays in particular cause the fluorescence of esculine, the spectrum must be projected in the usual way; but, instead of its being received upon a paper screen, it must be allowed to fall upon the wall of a glass cell containing a solution of esculine, that is to say, upon the solution itself, and it must then be observed in what parts of the spectrum the blue shimmer appears. The red and all the other colors consecutively down to indigo appear to be absolutely without effect. The bluish shimmer first commences in the neighborhood of the line G (Fig. 2), and covers not only the violet part of the spectrum, but stretches far beyond the group of lines H to a distance which is about equal to the length of the spectrum visible under ordinary circumstances.
From this the conclusion must be drawn that there are rays which are still more refrangible than the violet, but which in the ordinary mode of projecting the spectrum are invisible; these are termed the ultra-violet rays. They become apparent in the esculine solution because they are capable of exciting the bluish fluorescent shimmer in it. If sunlight have been used in the above experiments, the well-known Fraunhofer's lines appear upon the bluish ground of the fluorescing spectrum, not only from G to H, but the ultra-violet part also appears filled with numerous lines, the most conspicuous of which are indicated by the several letters L to S (Fig. 2). That these lines, like the ordinary Fraunhofer's lines, belong properly to solar light, and do not depend upon any action of the fluorescing substance, is
