228 Prof. Minchin's Experiments in Photoelectricity . 



and if \ were the same in the second mode of connexion, the 

 deflexion would be also of this value ; but since the deflexion 

 is very notably less in the second case, it is almost certain 

 that \ is also less. 



Observe that in the first mode of connexion the action, or 

 possibly only the tendency, of the Daniell is to deposit a 

 layer of oxygen on the sensitive plate ; and if light tends to 

 reduce oxygen at the surface, its action would be increased 

 owing to the additional layer due to the Daniell ; while with 

 the reverse connexions, the reducing action of light would 

 obviously be lessened. 



The electrolytic action of a Daniell battery on one of these 

 cells is not permanent ; it disappears rapidly, and, indeed, it 

 is not certain that anything more than a strain tending to 

 electrolysis is produced. 



The current of a Daniell cell, or any battery, if passed 

 through an impulsion-cell, does not influence the state of the 

 cell — i. e., this current does not alter the sensitive to the 

 insensitive state, or vice versa. 



Supposing S, and therefore A, to be such that when (in 

 the first case) light falls on the cell, the spot on the scale is 

 brought back just to the zero from which it started, if the 

 Daniell is suddenly removed the light will then cause a 

 further deflexion — which is the normal amount due to the 

 light. This fact agrees perfectly with the theory of a reducing 

 action, because, in the position of equilibrium of the spot 

 when both light and the opposing Daniell act, the rate at 

 which light tends to reduce is equal to that at which the 

 Daniell tends to deposit oxygen ; and therefore in this 

 particular case the plate is really in its normal state, so that 

 when the Daniell is removed, light finds the plate in the 

 condition in which it would be if no Daniell cell were con- 

 nected with the photoelectric cell. 



Action of Different Colours. — In a sensitive tin-foil cell, the 

 action of the blue part of the spectrum is very much greater 

 than that of any other part ; but measurable results can be 

 obtained all through, as will be seen by the diagram exhibited 

 to the Society. The spectrum was that of lime-light passed 

 through a prism of bisulphide of carbon, a single cell being 

 used in the experiment. This cell has been, from time to 

 time, under experiment for more than four years, and its 

 action is now exhibited to the Society. It has not appre- 

 ciably deteriorated, but the time of any one exposure has 

 never been more than a few minutes. 



