298 Royal Society .-—Prof. W. G. Adams and Mr. E. E. Day 



one case we found that the effect of light was not only to bring the 

 deflection of the galvanometer-needle down to zero, but also to send 

 it up considerably on the other side. 



Here there seemed to be a case of light actually producing an 

 electromotive force within the selenium, which in this case was op- 

 posed to and could overbalance the electromotive force due to 

 polarization. 



The question at once presented itself as to whether it would 

 be possible to start a current in the selenium merely by the action 

 of light. Accordingly the same piece of selenium was connected 

 directly with the galvanometer. While unexposed there was no 

 action whatever. On exposing the tube to the light of a candle, 

 there was at once a strong deflection of the galvanometer-needle. 

 On screening off the bight the deflection came back at once to 

 zero. 



This experiment was repeated in various ways and with light 

 from different sources, the results clearly proving that by the 

 action of light alone we could start and maintain an electrical cur- 

 rent in the selenium. 



All the pieces of selenium hitherto used had repeatedly had elec- 

 trical currents passing through them ; and it therefore seemed 

 desirable to examine the effect of exposure to light on pieces of 

 selenium which had never before had an electrical current sent 

 through them. 



Accordingly three pieces were prepared, as nearly alike as 

 possible, and were annealed. Two of them were found on trial 

 to be sensitive to light ; that is to say, light impinging on them 

 produced an electrical current ; the third piece, however, showed 

 no signs of sensitiveness. Hence it appears that three pieces 

 which were made up from the same stick, which are of the same 

 length, and were annealed at the same time, may, owing to some 

 slight difference in their molecular condition, be very different as to 

 their relative sensitiveness to the action of right. 



In the experiments by which the above results were obtained, 

 the piece of selenium under examination had always been exposed 

 as a whole to the influence of the light ; so that it was not possible 

 to tell whether any one part of a piece was more sensitive than 

 any other. 



In order to examine into this point more fully, we used the lime- 

 light, and then by means of a lens the light was brought to a 

 focus on the particular portion of the selenium plate which 

 was to be tested. A glass cell containing water, and having 

 parallel sides, was interposed in the path of the beam, so as to 

 assist in absorbing any obscure heat-rays. 



The results of these experiments proved conclusively the 

 following points : — 



(1) That pieces of annealed selenium are in general sensitive to 

 light, i. e. that under the action of light a difference of potential 

 is developed between the molecules, which under certain conditions 

 can produce an electric current through the substance. 



