148 



STUDIES IN LUMINESCENCE. 



value the absorption of the liquid made it impossible to excite the whole 

 mass to fluorescence. After trying a number of different methods without 

 satisfactory results we abandoned the attempt to accurately measure the 

 total resistance, and directed our attention to the measurement of the 

 change in resistance due to illumination. The most satisfactory method 

 that we have found for this purpose is that of the ordinary Wheatstone 

 bridge, using a direct current and a galvanometer of high resistance. 



The form of the cell used to contain fluorescent solutions is of consider- 

 able importance in its bearing upon the sensitiveness of the method. Among 

 the galvanometers available for use with the bridge, that which seemed 

 best suited for the work was one having a resistance of about 10,000 ohms. 

 It was therefore desirable to have the resistance to be tested as near to 

 this value as other conditions would permit. On the other hand, the layer 

 of liquid tested should be so thin that it can be excited to strong fluores- 

 cence throughout its thickness. Although we did not find it possible to 



satisfy both of these conditions, they were approxi- 

 mately met by the form of cell shown in diagram 

 in Fig. 149. 



The figure shows both a perspective view and a 

 horizontal section of the cell. A thick piece of plate- 

 glass, G, was cut of such a size as nearly to fill the cell 

 and was held in position, tightly pressed against the 

 two electrodes a, b (of platinum foil), by the corks C, C. 

 Although the drawing is in other respects approximately 

 to scale and nearly actual size, the thickness of the 

 electrodes a, b is greatly exaggerated. The thickness 

 of the layer of liquid between G and the walls of the 

 tube w r as determined by the thickness of the electrodes 

 and was about o. 1 mm. In the section shown in Fig. 

 149 the portions occupied by the solution are left un- 

 shaded. The length of the electrodes was about 20 

 mm. and their distance apart 2 mm. 



The fluorescent substances tested were eosin, fluor- 

 escein, rhodamin, napthalin roth, and cyanin. As 

 already stated, the solvent was in all cases absolute alcohol. The solutions 

 were made quite concentrated, so that fluorescence was confined to a thin 

 layer at the surface. The concentration was so adjusted as to make this 

 fluorescent layer approximately 0.1 mm. thick. 



The cell containing the solution to be tested was made one arm (C) of a 

 bridge. The resistance of the arm .1/ in series with C was in all cases 9,000 

 ohms, while the resistance of the third arm, R, was 5,000 ohms. The 

 fourth arm, X, was varied until an approximate balance was obtained 1 

 and the apparent resistance w T as computed by the formula C = MR/N. 

 Current was furnished by two gravity cells in series. 



Polarization in the fluorescent solution made it impossible to obtain the 

 true resistance by this method, and the apparent resistance of the cell 

 (computed as if polarization were absent) is doubtless in error by 50 per 



ZSpZ=ZZZZZZ 



<m ' 



777,: . ; ,, ^ L^!pz 



6 



a 5 



SECTION 



Fig. 149. 



These resistances were chosen not because they gave the highest attainable sensitiveness, but because 

 they were the most suitable ones that were available at the time. 



