410 EXPERIMENTAL INVESTIGATIONS. 



place : After a certain time of connexion with the galvano- 

 meter, the cover being over the apparatus, the sign of polari- 

 sation changed ; i.e. supposing the needle of the galvanometer 

 to have deviated to the left, and indicated that the exposed 

 platinum was positive, the needle would gradually return, pass 

 the zero-point, and be deviated to the right ; when this was the 

 case, on removing the cover, the effect occasioned by the impact 

 of light was a return of the needle towards the zero-point, indi- 

 cating an influence in the direction of the original polarisation. 

 On setting aside the apparatus for twenty-four hours with the 

 plates in metallic connection, and then repeating the experi- 

 ment, the deviation of the galvanometer was in the direction 

 of the final polarisation. This apparent anomaly may have 

 arisen from a conflict of two classes of currents, the one arising 

 from imperfect mixing or want of homogeneity in the liquid, 

 and the other from the state of surface of the platinum ; 

 the latter would most probably be the current affected by light. 



As the general effect of light was to increase the deflections 

 occasioned by polarisation, whatever direction this assumed, 

 it seemed probable that the exclusion of one of the plates from 

 the light, which I had commenced with in the hope of obtain- 

 ing currents initiated by light, was unnecessary, and that the 

 observed effects would be rather increased by exposing both 

 plates to light. I therefore arranged two platinised plates in 

 a cell without a porous diaphragm, inclined to each other like 

 the letter V, but without contact, and allowed the light to im- 

 pinge on the interior surfaces, or those opposed to each other ; 

 but, to my surprise, the effect was very trifling, the needle de- 

 viating only one or two degrees, and that in a sluggish and 

 irregular manner. 



When, however, the two plates were arranged parallel, the 

 one shading the other from the ligh 1 -, as good deflections were 

 produced as with the porous cell the more so if the back or 

 shaded plate was of polished and the front plate of platinised 

 platinum. 



Why light should produce a greater augmentation of the 

 current when impinging on one than on the two plates I can- 

 not explain on the theory of polarized plates, and will leave 

 for farther experiment. 



