CHAPTER II. 



ON THE ABSORBING POWER AND THE FLUORESCENCE OF 



RESORUFIN. 



The experiments described in this chapter were undertaken at the 

 suggestion of the authors by Miss Frances G. Wick. An account of them 

 was first given in the Physical Review. 1 The chief object was to determine 

 whether the typical fluorescence spectrum changes with the concentration 

 of the solution or whether the shift of the band referred to in Chapter I is 

 caused by absorption only. The substance studied was resorufin, whose 

 strong fluorescence in the yellow-red is conveniently excited and readily 

 observed. 



It has frequently been observed that the color of the light emitted by a 

 fluorescent solution is altered by a change in the concentration of the 

 solution. A dilute solution of fluorescein, as indicated in Chapter I, gives 

 a green fluorescence, while the light emitted by a concentrated solution of 

 the same substance shows a distinctly yellow tinge. In other words, an 

 increase in concentration causes the maximum of the fluorescence spectrum 

 to shift toward the longer wave-lengths. 



Such a shift might result from a real change in the period of vibration of 

 the fluorescent molecules, i. e., the form of the typical fluorescence spectrum 

 might depend upon the concentration of the solution. But the observed 

 change in the fluorescence spectrum might equally well result from a change 

 in the absorbing power of the solution. The light emitted by portions of 

 the active substance in the interior of the solution must pass through the 

 solution before emerging, and is therefore weakened by absorption. 

 Since the maximum of the fluorescence spectrum does not occur at the 

 same wave-length as the maximum absorption, being always slightly dis- 

 placed in the direction of the longer waves, it is clear that different portions 

 of the fluorescence spectrum will be affected by absorption in varying 

 degree; the shorter waves will always be absorbed most strongly. The 

 maximum of the fluorescence spectrum is therefore always shifted toward 

 the red to some extent, and the increased absorbing power of concentrated 

 solutions causes the shift due to this cause to increase with the concentration. 



The experimental work naturally falls under three heads, as follows: 



i. The relation between absorption and thickness, to test whether or 

 not the absorption of a fluorescent solution obeys the exponential law of 

 optically perfect substances. 



2. The relation between absorption and concentration, to test the 

 application of Beer's 2 law, i. e., that an increase in the concentration of a 

 solution is equivalent to an increase in thickness. 



3. The study of fluorescence spectra of solutions of resorufin of different 

 concentrations to determine whether or not there is any shift in the maxi- 

 mum of fluorescence as concentration changes. 



'Frances G. Wick, Physical Review, xxiv, p. 356. 

 2 Beer, PoggendorfF's Ann., 86, p. 78. 



