132 



STUDIES IN LUMINESCENCE. 



EXPERIMENTAL. 



The method employed to photograph the decaying band of phospho- 

 rescence was based on the assumption that the conditions and phenomena 

 could be reproduced indefinitely, a fact already established for the substance 

 under investigation. The phosphorescent powder, except in the case of Fig. 

 140, which gives the results obtained with Balmain's paint, was zinc sul- 

 phide, i. c, the sample of " Emanations-pulver" used in the investigations 

 described in Chapter VI. 



The apparatus was that described on pages 130 and 131, but electri- 

 cally operated shutters were used to excite the powder, to expose the plate, 

 and to kill off the remaining phosphorescence with infra radiations before 

 repeating the excitation. With this apparatus the decay curve could be 

 photographed between any two points, time and time again, until an im- 

 pression had been made on the photographic plate. Since, at best, the 

 necessary exposure was very long, varying from a few hours to a much 

 longer time, no attempt was made to deduce the energy distribution, but 



the photographic spectra 

 were compared with each 

 other, the principal weight 

 being attached to the posi- 

 tion of the maximum of 

 the band. Care was taken 

 to obtain negatives of 

 about the same average 

 density, and of not too 

 great density, so that no 

 complications could result 

 from widely different or 

 complete chemical change 

 of the films at any wave- 

 length. With these precautions, negatives were obtained which showed 

 definite maxima, and the results could be repeated as many times as desired. 

 The distribution of denseness on the photographic film was measured, 

 at first, with a photometer, as in the previous experiments; but this method 

 was soon abandoned because of the eye strain induced by comparing very 

 faint fields of light. The apparatus which was substituted consisted of a 

 brightly illuminated slit placed before a very sensitive system of thermo- 

 couples. The photographic plate was pushed past the slit by means of a 

 screw calibrated in wave-lengths. Since the deflection of the galvanometer, 

 which measured the current from the thermo-couples, depended upon the 

 length of time that light was allowed to pass through the slit, a pendulum 

 was made to light and extinguish the electric lamp which illuminated the 

 slit, at predetermined intervals. With this apparatus results were obtained 

 which were consistent with those obtained with the photometer, and the 

 results could be repeated. 



Fig. 138 shows the data obtained by measuring a film with the pho- 

 tometer (curve .4)andwith the thermo-couple (curve B). In thisand in all 



.^6 



.50 



.54- 



.58 



.62 



.66/i 



Fig. 138. 



