i88 



STUDIES IN LUMINESCENCE. 



strip in the liquid to fluorescence, and this was observed through the slit S', 

 in front of which the collimator slit of the spectrophotometer was placed. 

 The comparison source for the spectrophotometer, as in various experi- 

 ments described in previous chapters, was an acetylene flame sliding upon 

 a photometer track so that it could be set at different distances from the 

 ground glass in front of the slit. This procedure makes it possible to keep 

 the slit width constant and therefore eliminates the errors which might be 

 introduced in measurements where the variation of intensity is great. 

 The collimator slit of the spectrophotometer was broad and the instrument 

 was set so as to measure the central part of the fluorescence band. 



The Nernst glower which furnished the exciting light was attached 

 permanently to the collimator tube of the large spectrometer. The wave- 

 length of the light used for excitation was varied by swinging the arm 

 carrying the collimator and glower, the other portions of the spectrometer 

 remaining fixed in position. At frequent intervals, corresponding to wave- 

 lengths of the exciting light which differed only slightly from one another, 



To collimator 



> 



of spectrophotometer 



Exciting - > 

 light 5 -> 



Fig. 181. 



the intensity of the fluorescence excited was measured by the spectropho- 

 tometer. The cell containing the fluorescent solution was then emptied 

 and carefully cleaned and a piece of magnesium carbonate was mounted 

 obliquely in the cell so as to reflect light from the slit S into the spectro- 

 photometer. The intensity and range of wave-lengths of the light thus 

 reflected were measured for each of the spectrometer settings previously 

 used. Finally the absorption of a layer of the solution of known thickness 

 was measured for some definite wave-length in the spectrum. By a separate 

 series of measurements the relative reflecting power of the magnesium 

 carbonate was determined throughout the region used. 



The procedure in computing results was as follows: To correct for the 

 unequal reflecting power of magnesium carbonate at different wave-lengths 

 the observed intensity, I , of the exciting light was divided by the reflecting 

 power of magnesium carbonate for that particular wave-length. This 

 correction reached approximately 10 per cent as its highest value. Using 

 the values contained in Chapter XII for the distribution of energy in the 

 spectrum of the acetylene flame, we next determined the relative energy 

 of the light used for excitation as a function of the wave-length. It will 

 be noticed that this procedure not only takes account of the difference 



