CHAPTER VIII. 



PHOTOGRAPHIC STUDIES OF LUMINESCENCE. 



Dr. C. A. Pierce, whose investigations of thermo-luminescence have 

 been described in Chapter VI, has attacked by a photographic method 

 some of the problems which arise in the study of the relations of fluorescence 

 and phosphorescence and in the consideration of the form of the curve of 

 decay. An account of his method and a summary of his results, which 

 have an important bearing upon these questions, are given in the present 

 chapter. 1 



THE DISTRIBUTION OF ENERGY IN THE FLUORESCENCE SPECTRUM AND THE 

 PHOSPHORESCENCE SPECTRUM OF SI DOT BLENDE. 



The method used in the study of the distribution of energy in fluorescence 

 spectra consisted of photographing on the same plate the spectrum of the 

 excited substance 2 and four spectra of an acetylene flame. Different in- 

 tensities of the light from this flame were used for the four spectra, but the 

 times of exposure of all four and of the fluorescence spectrum were the same. 

 Knowing the distribution of energy in the acetylene flame, :! the distribution 

 in the fluorescence light was obtained by photometric comparisons of the 

 spectra on the plate. 



During the set of experiments the fluorescent powder was contained in a 

 square dish made of platinum foil. The foil was connected to and sup- 

 ported by copper leads, so that the temperature of the powder could be raised 

 and controlled by passing electric current through the foil. The temper- 

 ature was measured, if different from room temperature, by means of a 

 copper-constantan thermo-couple placed in the midst of the powder. The 

 blue lines of a mercury-arc lamp were used to excite the powder. The 

 various intensities of acetylene light were obtained by screening off all but 

 the center of an acetylene flame and moving the flame with the screen to 

 different distances from the slab of magnesia which reflected the light into 

 the slit of the spectrum camera. The camera consisted of a direct- vision 

 spectroscope set so that the spectrum could be focused on a sensitive plate 

 held in a plate holder. 



The plates were handled and developed in complete darkness. The 

 developer was freshly mixed from a stock solution and of a standard 

 strength, and the length of development was timed, being for most of the 

 plates 8 minutes. The temperature was brought to 20 at the beginning 

 of development and the plates were rocked mechanically during the de- 

 velopment. The plates were fixed, at first for 30 minutes, after washing 

 in three separate waters, and the temperature of the fixing bath was brought 



] See C. A. Pierce. Physical Review, xxx, p. 663; xxxn, p. 115. 



2 The substance employed was the " Emanations-pulver" used in the experiments described in Chapter IV. 

 3 On the distribution of energy in the visible spectrum, by E. L. Nichols; Physical Review, xxi, p. 147. 

 Sept. 1905. 



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