158 STUDIES IN LUMINESCENCE. 



The conclusion arrived at from the various tests was that, in the first 

 place, no effect is produced by light upon the conductivity of a fluorescent 

 solution unless a region very near the electrodes is illuminated; and that 

 further, this effect at the electrodes is only produced in a very thin film of 

 the liquid immediately in contact with the surface of the plates. 



It may be well to state that, while no reference has thus far been made 

 to a variation of the effects in different parts of the spectrum, frequent tests 

 were made in all of the experiments described, to see if the effects could be 

 produced by parts of the spectrum which did not produce fluorescence. 

 In all of these tests the effects obtained were either very feeble or entirely 

 lacking. Thus, in the red, where the energy of the arc is the greatest, but 

 where no fluorescence was to be detected, no effects were observed which 

 were as great as 0.2 percent either way. The frequent references to fluores- 

 cence in connection with the effects observed seem therefore to be entirelv 

 justified. 



The next step undertaken in the investigation was a study of the vari- 

 ation of the effects at anode and kathode with varying potential difference 

 at the terminals of the cell. 



For this purpose the original form of cell was adopted, with electrodes 

 0.15 mm. thick and 3 mm. wide, placed next to the front wall of the cell and 

 3 mm. apart. The greater distance between the electrodes was adopted, 

 so that while one of the plates was being illuminated no stray light could 

 reach the other electrode. The greater thickness of the electrodes which, 

 in view of the results of previous work could not materially affect the 

 sensibility of the cell, served to keep the resistance of the cell as low as 

 possible. 



The screen in the front of the light-tight box D was so arranged that the 

 cell could be moved into a position to expose either electrode to the light 

 without having to raise the slide in the front of the box to make the adjust- 

 ment. 



As a source of E. M. F. a set of four sal-ammoniac cells was first used, but 

 as these were found to drop slightly in voltage during a run, a pair of storage 

 cells was substituted for them. The cells were connected to the ends of a 

 rheocord, consisting of 10 meters of manganin wire, and sliding contacts 

 were arranged so that any desired potential difference could be obtained 

 at the bridge terminals, up to the limit of the cells. 



To the bridge terminals were also attached leads from a large Weston 

 milli- voltmeter, a resistance being placed in series with the meter, and the 

 whole calibrated to a total range of 15 volts. With this arrangement it 

 was found easy to obtain the potential difference between the terminals of 

 the bridge with an accuracy of 0.0 1 of a volt, and any given value of the 

 potential difference could be maintained constant throughout the time 

 needed for the readings. 



For greater simplicity in computation, and to make it possible to main- 

 tain the potential difference between the terminals of the cell at a constant 

 ratio to the voltmeter reading, the bridge arms were arranged as follows 

 (see Fig. 151): R 3 62,500 ohms, R^ 10,000 ohms, and Ri a 100,000-ohm 

 box to be used as the variable resistance. Thus, unless in a case where the 

 resistance of the cell was so great that the 10,000-ohm box had to be varied, 



