474: B. Walter on the Relation between 



has already done, the ratios of the amounts of light emitted 

 by a given quantity of fluorescent substance to that absorbed 

 by it ; since it is obviously upon these that the fluorescibility 

 depends. From theoretical considerations Stokes* showed 

 that in such solutions this quantity first increases with 

 the dilution and then finally becomes constant. Lommel, 

 on the contrary, in one of his numerous works on the theory 

 of fluorescence, has maintained that the fluorescibility, his 

 factor «, continually increases with increasing dilution f. 

 A decisive answer to this question can only be obtained by 

 actual measurement of the quantities involved in it. 



My first observations of this kind, which I began in the 

 winter 1887-8, and in which, for want of sunlight, the multi- 

 coloured light of a petroleum lamp served to excite the 

 fluorescence, could yield no decisive result on account of the 

 theoretical difficulty in making a comparative estimate of the 

 separate energies of fluorescence of different wave-lengths % . 

 On account of this I determined to repeat the experiments 

 with homogeneous sunlight, for which the bright spring of 

 1888 offered a favourable opportunity. Experiment now 

 decided undeniably in favour of Stokes. At the same time, on 

 considering the special results of my measurements in con- 

 nexion with some phenomena previously only slightly noticed, 

 the cause of that remarkable behaviour of fluorescing solutions, 

 which Stokes has not explained, became evident. 



The following notice is an abstract of my complete paper § : — 



I. Measurements of the Fluorescibility. 



As fluorescing substance ammonium fluorescein (more 

 shortly fluorescein) was taken in aqueous solution; and 23 

 different solutions of it were experimented upon, whose 

 degrees of concentration varied from O000001 to 40 per cent, 

 of salt. The exciting pencil of monochromatic sunlight was 

 obtained by throwing a spectrum on the screen A B by 

 means of a slit S 1? the prism P l7 and the lens L x ; then by 

 means of a second slit S 2 in the screen A B the desired rays 

 could be sifted out and rendered parallel again by a cylindrical 

 lens L 2 . But before this pencil of rays fell upon the solution 

 of fluorescein under investigation, and contained in the cell 

 G 2 , it had to pass through a fairly dilute solution of the same 



* Stokes, Phil. Trans. 1852, p. 535. 



t Lommel, Pogg. Ann. clx. p. 76 (1877). 



% Walter, Wied. Ann, xxxiv. p. 316 (1888). 



§ Walter, Wied. Ann. xxxvi. p. 502 et seq. (1889). 



