472 THE POPULAR SCIENCE MONTHLY. 



similai-ly self-luminous under the influence of light. This peculiarity- 

 was first observed in a kind of spar occurring at Alston Moor, in Eng- 

 land, 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 ^?<orescewce. 



In order to understand more precisely the circumstances under 

 which fluorescence occurs, the solution of esculine must again be re- 

 ferred 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 under- 

 gone 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 ob- 

 tained 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 he 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 efiect. 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 Hto^L distance which is about equal to the length of the spec- 

 trum 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 

 xdtra-violet rays. They become apparent in the esculine solution be- 

 cause 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 fluo- 

 rescing spectrum, not only from G to //, but the ultra-violet part also 

 appears filled with numerous lines, the most conspicuous of which are 

 indicated by the several letters L %o 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 



