376 Mr. G. W. White on the 



In all these observations the selenium surfaces were care- 

 fully screened from stray light, usually by an ebonite casing, 

 and sometimes by covering the selenium surfaces which 

 were neither used in contact with the electrodes nor 

 Uuminated by soft wax. Selenium " cells " which produced 

 an E.M.F. in the dark or on illumination, were not used in 

 this investigation. 



A comparison o£ the other properties of the two types was 

 made. A selenium cube was taken, and set up as a bridge 

 of the second type. The increase of conductivity during 

 illumination (" Nachwirkung ") , the recovery on darkening 

 the bridge (Tragheit), and the relation between the 

 conductivity change and the intensity of the light were 

 investigated. This was repeated using the same selenium 

 cube as a bridge of the first type. The relations obtained 

 were identical for the two types. The curves are not 

 reproduced here, the "recovery''' and " light-intensity " 

 curves being of the usual form such as those obtained by 

 Sperling*, and the after-effect"" curves of the same form as 

 those obtained with a " Bidwell" bridge by Ries f . 



Discussion of Results, and other Experiments icith 

 Selenium Blocks. 



The above results show that with blocks of selenium, the 

 resistance change on illumination is greater when the light 

 is incident in the direction of the current, than when per- 

 pendicular to this direction. This is opposed to the result, 

 referred to above, obtained theoretically by Sperling. In 

 other words, the unexpected phenomenon revealed in the 

 present investigation is a highly sensitive bridge of the 

 second type, and the quantitative effect of light on bridges 

 of both types cannot be explained by the change in con- 

 ductivity in a very thin layer at the illuminated surface, the 

 selenium block being homogeneous. 



The high sensitiveness of these bridges could be ac- 

 counted for, if light had the effect of decreasing the specific 

 resistance throughout a very deep layer. Marc i. concluded 

 from his investigations of the " inertia " of selenium, that 

 the action of light is far from being confined to the surface 

 and the illuminated portion of the bridge, being transmitted 

 all over the bridge. Brown § suggested that the " effective 

 depth " may be much greater than the greatest depth 



* Loc. cit. 



t Die elehtrischen JEigenschaften des Selens. 

 t Zeit. Anorg. Chem. xxxvii. 3, p. 459. 

 § Loc. cit. 



