170 STUDIES IN LUMINESCENCE. 



Extended efforts were made to obtain such conditions of concentration, 

 thickness of layer, intensity of excitation, etc., as would bring out the 

 expected shadow on the plate P. Over 100 negatives were made and many 

 of these were under conditions which appeared to us to correspond to those 

 under which the spectrophotometer had indicated a large fluorescence 

 absorption; but in no case was a definite shadow observable. 



The next method of testing the matter is shown in diagram in Fig. 166. 

 Three cells containing a solution of fluorescein, or in some cases resorufm, 

 were used as shown in Fig. 166. i*\ and F 3 were excited by the same source 

 so as to eliminate errors due to variations in excitation; the source used 

 was sometimes a quartz mercury lamp and in other cases the tungsten lamp. 

 Since the solution was exactly the same in all three cells, and since Fi and 

 F 3 were at nearly the same distance from the exciting source, the two colli- 

 mator slits were illuminated with almost equal brightness. Any slight 

 inequality was balanced by opening or closing one of the slits. This adjust- 

 ment being made, the exciting source was extinguished and light was sent 

 through the cell F 3 from a small tungsten lamp T 2 . To balance this illumi- 

 nation, light from another tungsten lamp, after passing through the cell F->, 

 was reflected by a piece of plane glass into the second collimator slit. The 

 balance was obtained by adjusting the distance of T 2 , which slid upon a 

 graduated photometer track. When this balance was obtained the exciting 

 source was again started and the field of the spectrophotometer observed. 

 If the effect of fluorescence is to increase the absorbing power of F 3 we 

 should expect that while fluorescence alone and transmission alone give a 

 perfect balance, there would be a lack of balance when excitation and trans- 

 mission occur simultaneously. When satisfactory conditions of steadiness 

 were obtained no such disturbance of balance could be observed. A sample 

 set of readings is given in Table 20. The small positive result obtained in 

 this case is smaller than the errors of observation. In other cases the results 

 indicated a small decrease in absorbing power during fluorescence. The 

 experiment was tried with solutions of different concentration and different 

 intensities of excitation. But when satisfactory conditions as regards stead- 

 iness were obtained no disturbance of balance could be detected which was 

 greater than the errors of observation. 



Table 20. 



Resorufin. Excited by Mercury Arc. 



Slit in front of F 3 set for equality of fluorescence alone. Readings 51.6, 49.7, 50.7, 



50.7. Slit set at the average 50.7. 

 Lamp T 2 set for F+T, i. c, fluorescence and transmission together. Distances 



= 233,245. Average 239. 

 Lamp T 2 set for transmission alone (7") 233, 233. Average 233. 



F-\-T, 226, 228. Average 227. 

 Slit set again for equality of fluorescence alone. Readings 50.7, 50.4, 50.3, 50.7. 

 Slit set at the average 50.3. 



F+T: 235, 239. Average 237. 



T: 243, 245. Average 244. 

 F-\-T: 226, 237. Average 232. 

 Slit set for F alone: 50.8, 50.8, 50.7, 50.6. Average 50.8. 



F+T: 238, 243. Average 240.5. 



T: 240, 236. Average 238. 

 F+T: 243, 237. Average 240. 

 Average of all: Distance of lamp for T alone =238.3; for T+F = 2$6.o. 

 If this difference is real it indicates that the absorption of the solution during fluo- 

 rescence exceeds its absorption when unexcited by 1.7 per cent. 



