220 MM. Elster and Geitel on the Electro-optical 



right angle. The point of the Fig. 1. 



anode s was about 10 millim. 



above M, consisting of a plati- *~'~^a/ s \4'--' 



num wire (shown in the figure 



as a point) whose direction was 



at right angles to the plane 



ABA 7 B'. The surface of 



the alkali -metal must be so 



bright that the place where the light strikes it must appear 



absolutely black to an eye not in the direction of the reflected 



ray. Sharper angles of incidence than could be obtained by 



inclining the sciopticon were obtained by the use of a silver 



mirror (the metallic side of a piece of plate-glass coated with 



silver) , which rotated about a horizontal axis, and from which 



the horizontal ray was reflected downwards. 



In order to determine the angle of incidence of the ray to 

 the horizontal kathode we employed a simple instrument that 

 is also useful in determining the altitude of the sun. From 

 the centre of a quadrant of pasteboard, graduated into half 

 degrees, hangs a plumb-line, the thread of which touches the 

 graduations. In the prolongation of one of the bounding 

 radii of the quadrant across the centre, is placed a pencil at 

 right angles to the plane of the quadrant. The apparatus is 

 so placed that the shadow of the pencil is thrown by the 

 beam of light from the sciopticon in the direction of the radius. 

 The angle of incidence is then equal to the angle between 

 the radius and the thread, and can be read off upon the 

 divided arc. 



As in the previously described photo-electric measurements, 

 the cell was placed with the galvanometer previously described 

 in circuit with a battery of 100 to 400 Leclanche cells, of a 

 total E.M.F. of about 450 volts, so that the surface of the 

 alkali-metal was the kathode. The intensity of the photo- 

 electric current was read off with mirror and scale ; it is 

 scarcely necessary to observe that the cell was shielded from 

 the light of the lamp used to illuminate the scale, and that 

 no stray light was allowed to escape from the sciopticon. 



Immediately in front of the cell was placed a large Nicol's 

 prism, provided with graduated circle and capable of rotation 

 in horizontal and vertical planes. The cross-section of the 

 beam of light was so cut down by the screen that it traversed 

 the prism freely while it was rotated, as one could easily see 

 by observing the path of the light within the calcspar. 



If, then, the beam of light, polarized by passing through the 

 Nicol's prism, fall upon the kathode-surface in the manner 

 described at any incidence other than normal, the galvanometer 

 shows a periodic change of intensity during the rotation of 



