116 Prof. W. G. Adams and Mr. R. E. Day on [June 15, 



Accordingly three pieces were prepared, as nearly alike as possible, 

 and were annealed. Two of ttiem. were fonnd on trial to be sensitive 

 to light — that is to say, light impinging on them produced an electri- 

 cal current. The third piece, however, showed no signs of sensitiveness. 

 Hence it appears that three pieces which \^-ere made up from the same 

 stick, which are of the same length and were annealed at the same time, 

 may, o^^ing to some sHght difference in their molecular condition, be very 

 different as to their relative sensitiveness to the action of light. 



In the experiments by ^-^ hich the above residts 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 inter- 

 posed 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 cim-ent through the substance. 



(2) That the sensitiveness is different at different parts of the same 

 piece. 



(3) That in general the dii^ection of the current is from the less 

 towards the more illuminated portion of the selenium, but that owing to 

 accidental differences in molecular arrangement this direction is some- 

 times reversed. 



The ciu'rents produced in the selenium by the action of light do not 

 resemble the thermoelectric currents due to heating of the junctions 

 between the platinum electrode and the selenium ; for in many cases 

 the current produced was most intense when the light was focused on 

 points of the selenium not coinciding ^ith the jimctions ; also the cur- 

 rent was produced suddenly on exposure ; and on shutting off the light 

 the needle at once fell to zero : the gradual action due to gradual cooling 

 was entirely wanting. 



When the light fell upon a junction, the current passed from the 

 selenium to the platinum through the junction, which is not in accord- 

 ance with the place assigned to selenium in the thermoelectric series of 

 metals. 



Experiments were next undertaken in order to examine what effect 

 would be produced on the strength of a current which was passing 



