152 



STUDIES IN LUMINESCENCE. 



PHOTO-ACTIVE CELLS WITH FLUORESCENT ELECTROLYTES (DR. 



EXPERIMENTS). 1 



HODGES 



^ 



V 



Before going on to any further systematic investigation, Dr. Hodge 

 attempted to repeat as nearly as possible some of the experiments de- 

 scribed in earlier portions of this chapter. 



The substance first chosen for experiment was eosin, and nearly all the 

 results to be described were obtained with this substance. As a solvent 

 absolute alcohol was used, and in most cases the solution was saturated at 

 room temperature. 



The essentials of the apparatus used in these preliminary tests are shown 

 in Fig. 151. The source of light A was a Schuckert arc lamp inclosed in 

 the light-tight cover which is used with the lamp for projection. After 



passing through a pair of condensers the 

 beam of light illuminated the slit S, then 

 went to a water cell II', and thence to a lens 

 and crown-glass prism, finally forming a 

 spectrum on the front of the light-tight box 

 D, containing the cell C. D was provided 

 with a sliding front by which the light could 

 be admitted to the cell C. 



<For measuring the change of resistance 

 ^> an ordinary Wheatstone bridge was used, the 



cell being placed in one arm. The source of 

 current was a battery of gravity 

 cells, the number of which was 

 varied, as will be seen later. A 

 very sensitive Sullivan d' Ar- 

 sonval galvanometer of 1 , 100 ohms 

 was used with the bridge. The 

 galvanometer was fitted with a 

 concave mirror of 4 feet focus, 

 and its constant with the scale 

 at that distance was 90X10"" 

 ampere per millimeter deflection. 

 The cell used to contain the solution was a small rectangular glass cell 

 6 cm. deep and 1 cm. from front to back on the inside. In this were placed 

 side by side, and at a distance of approximately a millimeter apart, two strips 

 of platinum foil o. 1 mm. in thickness, 5 cm. long, and 3 mm. wide. Behind 

 these in the cell was placed a piece of plate glass, holding the electrodes 

 pressed against the front wall of the cell, and itself held in place by brass 

 springs between it and the rear wall of the cell. A section of the cell is 

 shown in Fig. 152, the thickness of the electrodes and other dimensions of 

 the cell being considerably exaggerated. The dispersion of the prism gave 

 a visible spectrum at the point where the cell was placed about 4 cm. wide. 

 Thus the portion of the spectrum illuminating the narrow space 1 mm. wide 

 between the electrodes was quite small, and it was easy to select the part 

 of the spectrum producing maximum fluorescence without including the 



This section is a summary of a paper by Dr. Percy Horlge; see Physical Review, xxvi. p. 540; and 

 x-xviii, p. 25. 



~1 







O 



Fig- 151- 



Fig. 152. 



