CHAPTER XIII. 



THE SPECIFIC EXCITING POWER OF THE DIFFERENT WAVE- 

 LENGTHS OF THE VISIBLE SPECTRUM IN THE CASE OF THE 

 FLUORESCENCE OF EOSIN AND RESORUFIN. 



In the case of either a solid, like anthracene, or of a liquid such as one 

 of the fluorescent dyes, fluorescence may usually be excited by light of a 

 great variety of different wave-lengths. In fluorescent solutions there is 

 commonly a well-marked absorption band lying close to the fluorescence 

 band on the side toward the violet, and light of any wave-length within the 

 limits of this absorption band will excite fluorescence. In fact fluorescence 

 may usually be excited by light of much shorter wave-length, so that the 

 solution will be lighted up when exposed to an ultra-violet spectrum. Simple 

 inspection, however, is sufficient to show that the intensity of the fluores- 

 cence excited by various portions of a given exciting spectrum is widely 

 different. It is not clear whether this variation is due to the fact that 

 certain wave-lengths are particularly effective in exciting fluorescence, 

 or whether it results merely from the fact that the absorbing power of the 

 material varies for different wave-lengths. It is clear that light can not 

 produce excitation unless it is absorbed by the fluorescent solution. Dif- 

 ferences in absorbing power might therefore produce wide variations in the 

 apparent effectiveness of different spectral regions in producing fluorescence, 

 even if the specific exciting power, i. e., the fluorescence excited per unit of 

 absorbed energy, were in reality constant for all wave-lengths. 



The determination of the relation between the specific exciting power and 

 the wave-length of the exciting light is a problem of some interest, whose 

 results possess also considerable significance on account of their bearing 

 upon the theory of fluorescence. The present chapter deals with the deter- 

 mination of this relation for eosin and resorufin. 1 



The exciting light was furnished by a Nernst glower which took the place 

 of the slit of a large spectrometer. A narrow region in the spectrum thus 

 formed was used in exciting the solution studied, and the intensity of 

 fluorescence produced was measured by a spectrophotometer as the wave- 

 length of the exciting band was varied. 



The arrangement for exciting and observing fluorescence will be made 

 more clear by inspection of Fig. 181. The light of the Nernst glower, 

 after passing through the large spectrometer before mentioned, was reflected 

 directly upward by a total reflecting prism as shown in the figure, and passed 

 through a horizontal slit S into the cubical glass vessel which contained 

 the solution to be studied. The spectrometer was adjusted so as to bring 

 the spectrum in focus at the slit S. One vertical face of the cubical cell 

 was covered with a sheet of metal containing a slit S' as shown in the figure. 

 The exciting light passing through the slit S excited a narrow vertical 



An account of the experiments described in this chapter was given in the Physical Review, xxxi, p. 376, 

 and p. 381, 1910. 



187 



