on fluorescence: absorption. 



169 



slit, is common to all instruments of this type. It is clear that if the effect 

 is disregarded, indications of fluorescence absorption may be obtained even 

 if no such effect exists. The method used by Miss Wick and by ourselves 

 involved three readings, namely, the intensity of the fluorescent light alone, 

 the intensity of the light transmitted by the solution when not excited, 

 and the combined intensity when light was transmitted through the solu- 

 tion at the same time that the latter was excited to fluorescence. The sum 

 of the first two readings being found greater than the third, it was assumed 

 that the transmission in the latter case was less than in the first. If, how- 

 ever, each of these readings of intensity had been too great by 2 divisions, 

 as indicated by Fig. 164, the result of such procedure would be to give an 

 apparent fluorescence absorption measured by 2 divisions. 



While it is difficult to see how this source of error alone could account for 

 results of the character obtained by Miss Wick and ourselves, the detection 

 of one source of systematic error made it appear possible that other similar 

 errors might be present, and led us to take up the study of this question 

 anew. The result of the numerous experiments which will be briefly 

 described in this chapter has been to convince us that the phenomenon of 



5 



F, 

 ) Exciting source 



1 F 2 I ^-Plane glass 



""Ground glass 







A 



!ICE3 o 



^Ground glass 



Fig. 165. 



Fig. 166. 



fluorescence absorption either does not exist, or that the effect is so small 

 that the methods thus far used for its detection are inadequate. 



In the first method used an attempt was made to obtain a photographic 

 record of the effect. The arrangement is shown in diagram in Fig. 165. 



The large square cell F h containing a solution of fluorescein, was excited 

 from above by a narrow beam of light, so that a central layer ab was excited 

 while the rest of the solution was not. Directly back of this was another 

 cell F 2 , also illuminated from above., so that the narrow vertical strip cd was 

 excited. The photographic plate P, suitably screened from all sources of 

 light except the fluorescence in Fi and F 2 , would be fogged nearly uniformly 

 over its surface by the light from ab if this alone was excited, and the fog- 

 ging would also be practically uniform if cd was excited. If, however, that 

 part of the liquid in Fi which is excited to fluorescence acquires the power 

 of absorbing the fluorescent light more strongly than before, then we should 

 expect ab to cast a shadow upon the photographic plate. While this 

 shadow could not be expected to be very sharp or very dense, the results 

 obtained by ourselves and Miss Wick indicated that under suitable con- 

 ditions it ought to be clearly visible in the negative. 



