4 STUDIES IN LUMINESCENCE. 



of the fluorescent band in the middle of the spectrum, in the regions of the 

 highest luminosity, was selected for detailed study for the purpose of test- 

 ing the conclusions reached by Lommel and the other investigators men- 

 tioned in the opening paragraphs of this chapter. 



Ten cubic centimeters of alcohol were saturated with fluorescein at room 

 temperature and the solution was filtered. To 40 c. c. of distilled water 

 one drop of a normal solution of sodium carbonate (Na 2 C0 3 ) was added. 

 Two parts of the concentrated alcoholic solution were then mixed with 100 

 parts of the water thus rendered alkaline. The fluorescence spectrum of 

 this solution, excited by the undispersed rays of the acetylene flame, was 

 first measured, the distribution of intensities being compared, as in nearly 

 all our subsequent experiments, with the spectrum of the light diffusely 

 reflected from the surface of the block of magnesium carbonate shown in 

 Fig. 1. The absorption spectrum of the solution was then taken, the trans- 

 mission through the cell, which had a thickness of 1.1 cm., being meas- 

 ured by means of the spectrophotometer. The source of the transmitted 

 light was a second similar block of magnesium carbonate illuminated by 

 the same acetylene flame that served for the comparison spectrum. 



Much stress having been laid by some of the previous observers upon 

 the influence of stray light, the following measurements were made. The 

 cell was filled with distilled water, set up precisely in the position in which 

 it had been placed in the study of the fluorescence spectrum, and similarly 

 illuminated by means of the acetylene flame. No measurable stray light 

 was found, but an exceedingly weak fluorescence spectrum due to the glass 

 walls of the cell was detected. Since the maximum of the fluorescence 

 spectrum of the glass was found to lie further to the violet than the fluores- 

 cence spectrum of the fluorescein, and since moreover it was of scarcely 

 measurable intensity, it was not deemed necessary to take further cog- 

 nizance of these sources of error. 



To determine the fluorescence spectrum of the solution when excited 

 by monochromatic light, a mercury arc-light of the Lummer pattern was 

 used for excitation. It was found that the violet lines from the spectrum of 

 this arc produced fluorescence corresponding, as regards the position of the 

 maximum and the general form, with the curve previously obtained by 

 means of the light of the acetylene flame, but that the green line (X = 0.575) 

 excited no fluorescence. This latter result was to be expected, since this 

 line lies altogether outside of the absorption band of the solution in a region 

 for which almost complete transparency exists. The spectrum of the acety- 

 lene flame was subsequently tried as an exciting source, but it was too weak 

 to give easily measurable intensities of fluorescence. The slit of the spec- 

 trometer was then removed and a Nernst filament, .V, was mounted in the 

 axis of the collimator tube as shown in Fig. 1 . This filament was found to 

 be of abundant brilliancy. It was maintained at constant brightness by 

 means of a variable resistance, which was adjusted whenever the fluctua- 

 tions of an ammeter placed in the electric circuit indicated it to be necessary. 



Measurements of the fluorescence spectrum of the solution were made 

 by means of the spectrum of the Nernst filament, using as exciting light 

 three nearly monochromatic regions of wave-length, X= 0.5 18 /1 to 0.536 n, 

 X= 0.487 ix to 0.502,11 and X= 0.460^ to 0.471 /i. The curves thus obtained 



