Light Action in Certain Crystals of Metallic Selenium. 499 



crystal by screws, S, as shown in fig. 2. The whole was 

 mounted on a hard rubber plate, R. With the crystal placed 

 in this manner there was not much change in the conductance 

 on altering the pressure at the contacts. In view of the 

 very large change in conductance which was observed when 

 the entire region between the electrodes was placed under a 

 few atmospheres pressure, and in view of the well-known 

 change of conductivity with pressure in ordinary crystal 

 contacts, it is concluded that only a relatively small re- 

 sistance exists at the electrode contacts. The following 

 observation adds further evidence towards this conclusion. 

 With the contacts 2*5 mm. apart on one crystal the resistance 

 was about 10 7 ohms, and when the distance was shortened to 

 about 0*5 mm. the resistance diminished by a factor of 4*8. 

 Obviously this requires that the resistance shall be distributed 

 somewhat uniformly along the crystal. 



The Change of Conductivity acts throughout the Crystal. — 

 In order to show that not only the conductivity, but also the 

 change of conductivity, is not limited to the surface or to 

 the contacts, the selenium was illuminated alternately from 

 the front and from the back. The distance between elec- 

 trodes was about 3 mm. The key to this experiment lay in 

 the fact that the path along the front side of the crystal was 

 shorter than that along the back side. The current in going 

 by the back side had to traverse the thickness of the crystal 

 twice. Thus we may regard the front and back sides of the 

 crystal as parallel conductors, where the highest resistance is 

 found in the back side. Then suppose for the time being 

 that the light-action produces an effect near the surface only, 

 and that this action is constant. Imagine the selenium 

 illuminated with a narrow beam of light which acts on a 

 portion only of one of the parallel conductors. Obviously, 

 with these assumptions, the given illumination should pro- 

 duce a greater total change in the conductance when it falls 

 on the front side of the crystal than when it falls on the back 

 side. This idea was carried out by focussing an illuminated 

 slit on the crystal. This focussed image was 0*2 mm. wide. 

 The image was moved along the crystal 0*5 mm. at a step, 

 and the corresponding conductance was noted. Then the 

 selenium was reversed so as to expose the back side to the 

 illumination, and a similar set of observations was made. 

 A number of such sets were taken, a sample of one of which 

 is given in the accompanying table. 



2 K 2 



