310 Mr. S. I. Vavilov on Dependence of the Intensity of 



Experiments. 



As has been already mentioned, Nichols and Merritt 

 found that the specific fluorescence of Resorufin and Eosin 

 increases towards long wave-lengths. The scope of the 

 present investigation is to state how general is this result. 

 It is of some interest to note that when this increase is a 

 general rule we can give a simple explanation to the law of 

 Stokes. From this standpoint, the fluorescence is excited 

 chiefly or exclusively by the absorption in a small band 

 situated towards long wave-lengths relatively to the resulting 

 maximum of a complex band. 



The appearance of the curve for Resorufin suggests that 

 the band is complex, while in addition to the principal 

 maximum, there are three secondary maxima from the side 

 of the short waves *. The band of absorption of Eosin 

 investigated by Nichols and Merritt differs considerably 

 from that of Eosin studied by us (fig. 3). Unfortunately, 

 among six Eosins at our disposal we did not find a dye very 

 closely resembling the Eosin of Nichols and Merritt. These 

 authors excited fluorescence by a Nernst glower which took 

 the place of the slit of a large spectrometer. The narrow 

 regions in the spectrum thus formed were used in exciting 

 the solution studied. The intensity of fluorescence so excited 

 was evidently very feeble ; this can explain the very con- 

 siderable deviations of computed points, especially in the 

 case of Eosin. 



The diminution of errors of observation had a considerable 

 significance for us when proposing to test the equation of 

 Einstein (4), where the systematical deviations do not exceed 

 15-19 percent, in the conditions of the experiment. There- 

 fore it was necessary to increase the intensity of fluorescence 

 and to avoid the errors in determining the wave-lengths of 

 the exciting light which are almost inevitable in a prismatical 

 resolving of light, especially in the yellow-red part of the 

 spectrum. Therefore we applied, instead of monochromatic 

 light, the light transmitted through the light-filters quanti- 

 tively measured. 



Let us presume that the energy of the exciting source of 

 light in the interval of wave-lengths \ . . . .\ + dX shall 

 be I\. The measurement of intensity of fluorescence is made 

 in that place in the vessel with the fluorescing solution 

 where light has already passed the layer of thickness d. 

 The coefficient of absorption of solutions studied in the given 



* E. Nichols and E. Merritt, loc. cit. 



