CHAPTER XII. 



THE DISTRIBUTION OF ENERGY IN FLUORESCENCE SPECTRA.* 



The energy of most continuous spectra is too feeble to permit of accurate 

 measurements excepting in the infra-red and the longer wave-lengths of 

 the visible spectrum, although we have a few determinations of the energy 

 of the visible spectrum of the acetylene flame by G.W. Stewart 2 and by Cob- 

 lentz 3 that extend beyond the green. The direct measurement of the energy 

 of even the brightest of fluorescence spectra, which are very small in intensity 

 as compared with those of our ordinary artificial light sources, is therefore 

 impracticable. We have shown, however, in previous chapters of this 

 memoir that it is possible to make quantitative spectrophotometric com- 

 parisons between fluorescence spectra and the spectrum of a standard such 

 as the acetylene flame. If the distribution of energy of the source used for 

 comparison be known, it is therefore easy to compute that of the fluorescence 

 spectrum. We have adopted this method in determining the energy distri- 

 bution in the fluorescence spectra of fluorescein, eosin, and resorufin, with 

 the results recorded below. The experimental work naturally falls under 

 three heads : ( i ) The determination of the energy distribution in the stand- 

 ard source; (2) the spectrophotometric comparison of the fluorescence 

 spectrum with the standard; (3) the measurement of the absorption of the 

 fluorescent liquid, in order that the observed curve of fluorescence may 

 be used to compute the typical curve. 



DETERMINATION OF THE DISTRIBUTION OF ENERGY IN THE SPECTRUM OF 



THE COMPARISON FEAME. 



The comparison source used in the experiments to be described in this 

 paper was an ordinary flat flame from an acetylene burner, in front of which, 

 at a distance of 1.4 cm., was mounted a metal screen having a circular hole 

 0.6 cm. in diameter, so as to cut off the light from all but the brighter 

 central portions of the flame. This flame with its diaphragm was mounted 

 in a metal box having a circular window opposite the diaphragm. The 

 box was fastened to a base fitted to slide along a straight metal track. This 

 track was mounted horizontally in the same vertical plane as the axis of one 

 of the collimators of a IyUmmer-Brodhun spectrophotometer, and at such a 

 height that the axis of the collimator extended would pass through the win- 

 dow in the box and through the center of the diaphragm to the flame itself. 



In front of the slit of the collimator was mounted a sheet of clear white 

 glass, the surface of which had been sufficiently roughened by grinding 

 with powered carborundum, so that at whatever distance the flame might 

 be placed the contrast field of the spectrophotometer would be of uniform 

 brightness throughout. The loss of light by the interposition of the ground 



'The contents of this chapter first appeared in the Physical Review, xxx, p. 328. 



J G. W. Stewart, Physical Review, xvi, p. 123. 



a W. W. Coblentz, Bulletin of the Bureau of Standards, vn, p. 243. 



175 



