13S STUDIES IN LUMINESCENCE. 



cence, even without any corresponding change in current or voltage, due 

 apparently to some erratic cause producing either a deflection of the kathode 

 rays or a change in the intensity of these rays. The most serious of these 

 disturbances were traced to irregular leakage from the wires leading to the 

 Holtz machine. This trouble was made more difficult to control by un- 

 favorable atmospheric conditions, the work being carried on in the spring ; 

 but the disturbances due to leakage were largely removed by using large 

 rods in place of wires, and by avoiding sharp corners. It proved advan- 

 tageous, also, to run the Holtz machine at full speed and to cut down the 

 potential difference between the terminals of the tube by means of resistance 

 in series. The necessary resistance was furnished by a tube containing 

 absolute alcohol. Flickering persisted to some extent even with these 

 precautions, and it was frequently necessary to discard the results of several 

 hours work. The results given here were, however, taken under favorable 

 conditions. 1 



It did not prove practicable to maintain the pressure in the tube constant 

 during the time necessary for spectrophotometric measurements through- 

 out the spectrum. Owing to a small leak, or to the development of gas 

 by the discharge, the pressure gradually increased during the course of a 

 series of readings, so that at the end of about an hour it was necessary to 

 reexhaust. In order to determine the luminescence spectrum corresponding 

 to constant pressure conditions the following procedure was adopted: 



While one observer recorded the readings of the voltmeter and galva- 

 nometer and, when necessary, operated the pump, the other made settings 

 of the spectrophotometer as rapidly as possible, running back and forth 

 through the spectrum until the whole region had been covered a number 

 of times. Curves were then drawn for each wave-length at which settings 

 had been made by plotting intensities, as measured by the spectrophoto- 

 meter, against voltmeter readings. A series of such curves is shown at the 

 left in Fig. 145, which refers to a cadmium sulphate preparation. These 

 curves all have the same general shape, and in most cases each curve 

 contains enough points to determine its form with considerable deiiniteness. 

 After the curves had been drawn as accurately as possible the intensity 

 corresponding to any particular voltmeter reading could be determined 

 by graphical interpolation. The points which form the two curves to the 

 right in Fig. 145 (marked I. V. M. 30 and II. V. M. 17.5) were deter- 

 mined in this way for the voltmeter readings 30 and 17.5 respectively. 

 For these curves the abscissas are wave-lengths expressed in fractions of // 

 as indicated. 



The procedure in the case of our experiments with willemite (Fig. 146) 

 and Sidot blende (Fig. 147) was exactly the same as in the case of cadmium 

 sulphate. A comparison of these three figures shows, however, that while 

 the voltage intensity curves in Figs. 145 and 146 are similar, being in each 

 case concave toward the horizontal axis, the corresponding curves in Fig. 

 148 are almost exactly straight. The difference results from the fact that in 

 the experiments corresponding to Figs. 145 and 146 the current through the 

 tube diminished rapidly as the voltage increased, whereas in the experi- 



>It is probable that the trouble was due to electrostatic charges on the walls of the tube. A larger tube 

 so constructed as to have the kathode at a considerable distance from the walls would probably have been 

 more satisfactory. 



