A SPECTROPHOTOMETRY STUDY OP FLUORESCENCE. 



17 



tinctly in favor of the view that it, like other fluorescent materials, is 

 capable of being excited by all wave-lengths of light lying in or on the 

 infra edge of its absorption band ; and that the longest wave-length capable 

 of producing an observable fluorescence is appreciably less refrangible than 

 the shorter wave-lengths of its fluorescence spectrum. 



CHLOROPHYLL. 



The fluorescence spectrum of chlorophyll was studied by Stokes, 1 who 

 found, in addition to the usual red band, a fainter excitation in the green 

 of the spectrum, Hagenbach 2 subsequently made an exhaustive study of 

 this substance. It was his opinion that it did not conform to the law of 

 Stokes. Lommel 3 placed chlorophyll in his first class, to which belong all 

 substances whose fluorescence is independent of the wave-length of the 

 exciting light. 



The solution of chlorophyll used in our measurements was made by 

 digesting green leaves in absolute alcohol and filtering. The transmission 

 curve (Fig. 15) shows four well-defined bands, of which the one to which 



80 



60 



40 



20 



.5/X 



Fig- 15- 



.6/J .7/1 



-Chlorophyll (fresh alcoholic solution from green leaves). 



Curve A . Fluorescence spectrum when excited by Hg arc. 

 Curve T. Transmission spectrum of a layer i.i cm. thick. 



fluorescence is due is more intense and broader than the others. Measure- 

 ments made in the extreme red appeared to indicate the presence of still 

 another region of diminished transparency which it was not possible to map 

 with the spectrophotometer. In the hope of determining more definitely 

 the character of this band and of ascertaining whether the series of striking 

 absorption bands in the visible spectrum extends into the infra-red, we 

 requested Mr. W. W. Coblentz, who was engaged in the study of the infra- 

 red by means of a radiometer and a mirror spectrometer with rock-salt 



'Stokes, Philos. Trans., 1853. 



2 Hagenbach, Poggendorff's Ann., 141, p. 245. 



3 I,ommel, /. c. 



