A SPECTROPHOTOMETRY STUDY OF FLUORIC SCENCE. 



13 



Neither of these solutions conforms even approximately with the law of 

 Stokes. Curve A of Fig. 12 (resorcin-blau) was obtained by means of light 

 none of which was of shorter wave-length than 0.642 fx, whereas the fluo- 

 rescence is of measurable intensity to 600 n. The rhodamin solution affords 

 a striking example of the non-validity of the law; for although the maximum 

 of the fluorescent spectrum lies at 0.554 /x, an observable fluorescence was 

 produced by means of very nearly monochromatic light, obtained by the 

 method of double dispersion, using two spectrometers in series, the shortest 

 wave-length of which was 0.602 yu. This limiting wave-length is indicated 

 by the line marked / in Fig. 11. This substance, like eosin, which has its 

 maximum of fluorescence at 0.580 /jl, will respond to the excitation of light 

 of the D lines of sodium or to even longer waves if of sufficient intensity. 



The other substances to be considered in this chapter are more difficult 

 subjects for quantitative work because, being less strongly fluorescent, it 



100 



PO 



60 



40 



20 



.6/i 



.6/2 B A ,7 /A 



Fig. 12. Resorcin-blau in methyl alcohol. 



Curve A. Fluorescence spectrum when excited by red light lying in region marked A. 

 Curve B. Fluorescence spectrum when excited by orange light in region marked B. 

 Curve C. Transmission spectrum of layer i.i cm. thick. 



is necessary to have recourse to powerful illumination, and because in 

 several instances the fluorescence band lies near the limits of the visible 

 spectrum either in the violet or in the extreme red. 



QUININE SULPHATE. 



The quinine sulphate subjected to measurement w y as prepared by making 

 a saturated solution in water containing a trace of sulphuric acid. The 

 solution was exposed to diffuse daylight in a cubical cell of glass, the faces 

 of which measured 8 cm., and the fluorescence was observed through a slit 

 in one of the faces at right angles to that exposed to the light. This slit was 

 placed as near the corner of the cell as possible, so as to bring the layer of 

 liquid lying next the glass on the exposed side into the field of view. The 



