Intelligence and Miscellaneous Articles. 159 



CONTRIBUTIONS TOWARDS THE MORE ACCURATE KNOWLEDGE OF 

 THE PHENOMENA OF FLUORESCENCE. BY DR. VICTOR PIERRE 

 OF PRAG. 



The results of this investigation are as follows : — 



(1) That the property of exciting fluorescence is not confined to 

 the most refrangible rays of the spectrum, but that rays of any wave- 

 length can in general excite fluorescence. 



(2) There is for each substance a definite prismatic colour in 

 which fluorescence first occurs, so that all colours less refrangible 

 than this produce no fluorescence. 



(3) It is seldom that this colour is the one which produces the 

 most intense fluorescence ; generally it is the next more refrangible 

 rays, but always definite rays for a definite substance. 



(4) If rays of a definite colour, that is, of definite wave-length 

 and time of vibration, evoke fluorescence in a substance, not only 

 are rays produced of greater time of vibration than those of the ex- 

 citing rays, but the rays produced by fluorescence are, for each sub- 

 stance, always the same, whatever be the duration of vibration of 

 the producing ray. 



(5) The wave-lengths of the rays produced by fluorescence do 

 not always gradually shade into one another, but there are occasion- 

 ally jumps, so that rays of a certain length are not developed, in 

 which case the spectrum of the fluorescence-colour is traversed by 

 dark lines ; this phenomenon also is independent of the wave-length 

 (direction of vibration) of the exciting rays. 



(6) Among the new rays resulting from fluorescence, those are 

 always the most intense whose wave-length is either equal, or very 

 nearly equal, to that of the rays in which fluorescence first occurs ; 

 in the latter case, however, it is always larger than that correspond- 

 ing to the beginning of the fluorescence. 



(7) In substances which fluoresce in solution, in case they are 

 soluble in different agents, the solvent occasionally influences the 

 character of the fluorescence, so that, dissolved in different solvents, 

 the same substance fluoresces differently. In one and the same 

 solvent the concentration of the solution only affects the intensity 

 of the fluorescence, but leaves its character unaltered. Above and 

 below that degree of concentration which makes the phenomenon of 

 fluorescence most intense, the intensity of the fluorescence in all 

 parts of the spectrum in which it is at all developed appears to de- 

 crease in almost the same ratio ; so that with the feeblest develop- 

 ment it is distinctly perceptible only in the position of the maximum. 



(8) The occurrence of one fluorescing substance with other fluo- 

 rescing or non-fluorescing substances exercises very different effects 

 on the character of the fluorescence ; in many cases it undergoes no 

 change, but in others it is entirely altered. If many fluorescent 

 substances are mixed together, a compound fluorescence is produced, 

 the colour of which, in diffused day- or in direct sunlight, may be 

 very different although the same substances are in both cases mixed 

 together. If the various fluorescent substances do not act on each 

 other so as to alter their fluorescences, such a compound fluores- 

 cence may always be resolved into the. simple fluorescences of those 

 substances which are contained in the mixture; and so far the 



