THE NATURE OF FLUORESCENCE. 



473 



evident from tbe 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, there- 

 fore, the glass lens and prism hitherto used for pro- 

 jecting the spectrum be replaced by a quartz lens 

 and prism, the ultra-violet part of the spectrum is ren- 

 dered much brighter and is extended still farther than 

 before. 



The ultra-violet rays of the spectrum can, more- 

 over, be seen, without the intervention of any fluor- 

 escing substance, through a glass, or, still better, 

 through a quartz prism, if the bright part of the 

 spectrum between.^ and ^ (Fig. 2) be carefully shut 

 ofi". With feeble illumination its color appears indigo- 

 blue, but wnth light of greater intensity it is of a 

 bluisli-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 and H. 



An explanation is thus aflbrded 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 impercep- 

 tible ultra-violet rays, the mixed light that has passed 

 through it still appears white, and is not rendered ma- 

 terially 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 dift'erent 

 kinds of homogeneous light, as far as they are generally effective, pro- 

 duce compound fluorescent light of identical composition, the con- 

 stituents of which, nevertheless, are collectively less refrangible thaiiy 

 or are at most equally refrangible toith, the exciting rays. 



Among other fluorescing bodies may be mentioned the solution 

 of quinine, which is as clear as water, and has a bright-bhie fluores- 

 cence; 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 fluor- 

 escence. It admits of easy demonstration that in these bodies also it is 





