A SPECTROPHOTOMETRY STUDY OF FLUORESCENCE- 



are plotted in Fig. 3, together with the curve of transmission for the solu- 

 tion. It will be seen from this figure that the maximum of intensity of the 

 fluorescence spectrum in these three cases lies in the same region, at 0.5 1 7 /x, 

 and that there is no evidence of any shifting of the fluorescence spectrum 

 with the wave-length of the exciting light. It is obvious, moreover, that 

 not only is it possible in the case of this solution to obtain fluorescence of 

 refrangibility greater than that of the exciting light, but that in the case 

 of the curve marked A the maximum of the fluorescence spectrum is of 

 shorter wave-length than the shortest wave-length used in excitation. 

 These curves likewise agree fully in character, and as regards the position 

 of their maximum, with that for the fluorescence spectrum of the same 

 solution when excited by the un- 

 dispersed light of the acetylene 

 flame. (Not shown in the figure.) 

 These curves for the fluorescence 

 spectrum do not correspond pre- 

 cisely with the typical curve, mean- 

 ing by that term the curve repre- 

 senting the distribution of intensi- 

 ties in the fluorescence spectrum 

 of the surface layer of the fluores- 

 cing liquid. It is possible, however, 

 in the case of a non-turbid medium, 

 to compute from the observed 

 curve the approximate form of the 

 typical curve. Fig. 4 shows graph- 

 ically the result of such a compu- 

 tation. 1 Curve A gives the trans- 

 mission of a glass cell containing 

 a layer 1.1 cm. in thickness of the 

 fluorescein solution. The dotted 

 curve of similar form gives the 

 transmission corrected for losses 

 in the cell when filled with distilled 

 water. From this, by the well- 

 known law of variation of absorption 



with the thickness, the curve D is found for a layer 0.055 cm - m thickness, 

 which is the estimated mean distance through the solution which the 

 fluorescent light passes before entering the slit of the spectrophotometer. 

 The curve B is the observed curve of the distribution of the intensities in 

 the fluorescence spectrum, and from this was computed the curve E, which 

 represents, as nearly as the accuracy of the data will allow, the typical curve 

 for this substance, corrected for absorption. It will be seen that in the 

 case of this solution, under the conditions of the measurements, the absorp- 

 tion of the fluorescent light by the solution produces only a slight shift of 

 the maximum toward the longer wave-lengths. When the fluorescent light 

 passes through a considerable layer of the solution the effect of absorption 

 is much more marked and there is a decided change of color. When a thin 



'In this figure and in Figs. 5. 6, 7, and 8 the scale of wave lengths has been doubled. 



.5/t I 



Fig. 3. Fluorescein. 



.^i 



Fluorescence spectra obtained when the exciting light 

 lies in different regions of the spectrum. For 

 curve .1, the exciting light was confined to the 

 region marked .1 on the horizontal axis, etc. 

 Vertical scales arbitrary. 



