1 62 STUDIES IN LUMINESCENCE. 



regard to the outside E.M.F., the E.M.F. due to illumination would act 

 with the original E.M.F., producing a greater current through the cell 

 and making its apparent resistance, as measured on the bridge, less. 



If, on the contrary, the direction of the impressed E.M.F. were reversed, 

 the photo-E.M.F. would oppose the passage of the current and the apparent 

 resistance of the cell be increased. 



The reversal of the effects when the voltage was raised is not easy to 

 account for. From a number of tests of the polarization E.M.F. of eosin 

 cells, made by comparing the E.M.F. with that of a standard cell by means 

 of a condenser, the effect of maximum polarization appears to be a back 

 E-M.F. of over 2 volts. This value determines the point where any con- 

 siderable current begins to traverse the cell, and hence it would be most 

 natural to expect that just here would occur any marked change in the 

 phenomena, such as the reversal of the illumination effect. This, however, 

 does not seem to be the case, for it will be noticed from the curves that the 

 reversal in the case of the kathode occurs at very nearly 1.7 volts, while at 

 the anode the voltage is much lower. That the effects at high voltages are 

 connected in some way with the polarization of the cell seems, nevertheless, 

 most probable, though in what manner has not as yet been determined. 



Assuming that the effect at low voltage is due, as has been suggested, to 

 a photo-E.M.F. similar to those which have frequently been observed in 

 cells containing silver salts and many other electrolytes, attention was next 

 turned to the effect of light on the cell when no external E.M.F. was applied. 



The same type of cell was used as had been employed in all of the experi- 

 ments on the kathode alone. The terminals of the cell were connected 

 directly to the Sullivan galvanometer which had been used with the bridge 

 in all the previous experiments. 



Very considerable effects were produced by the action of the light, ample 

 in magnitude to account for the phenomena observed with the bridge at 

 low voltages. 



A series of experiments was then undertaken to find if in any way this 

 photo-electric effect was intimately connected with fluorescence. 



In the first place tests were made throughout the visible spectrum, and 

 curves plotted showing the variation of the current produced in the gal- 

 vanometer as a function of the wave-length of the exciting light. The effect 

 was found to increase regularly from the edge of the visible spectrum in the 

 violet to a pronounced maximum just before the absorption band was left 

 in going toward the red, namely, just at the infra edge of the green. The 

 decrease of effect from this point on toward the red was very sudden indeed, 

 and the effect was entirely lost before the edge of the visible spectrum was 

 reached. Three other substances, namely, rhodamin, fluorescein, and 

 naphthalin-roth were found to give similar effects, and in each one the 

 maximum results were obtained at the infra edge of the absorption band. 

 In rhodamin the effects were found to be enormously greater than with the 

 eosin, but with the other two substances the results were less satisfactory. 



With rhodamin tests were made by a condenser method to test the mag- 

 nitude of the E.M.F. produced, and it was found to be as high as 0.2 of a 

 volt. In eosin it is probable that the maximum was not over half that 

 amount. 



