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IX. The Action of Light on Selenium. 
By Professor W. G. Adams, M.A., F.R.S., and R. E. Day, M.A. 
Beceived May 18, — Bead June 15, 1876. 
This paper contains an account of a series of experiments which have been carried on 
by the authors during the last year and a half, and which have had for their object the 
investigation of the electrical behaviour of selenium, especially as regards its sensitiveness 
to light. 
Early in the year 1873 it was discovered by Mr. Willoughby Smith that when an 
electrical current was passing through a bar of crystalline selenium, its resistance was 
less when the bar was exposed to the action of light than it was when the bar was kept 
in the dark. 
Through the kindness of Mr. Willoughby Smith we have been able to follow up his 
remarkable discovery with, among others, the same bar of selenium with which he 
made his original experiments. 
The objects we have had especially in view have been : — 
(1) To determine whether this change in the resistance of selenium is the direct 
result of radiations, and if so, whether the dark-heat rays, the luminous rays, or the 
chemically active rays produce the greatest changes. 
(2) To compare the changes of resistance in the selenium due to exposure to light 
from different sources and also to light which has passed through various absorbing 
media. 
(3) To determine whether the action is instantaneous or gradual, and to measure as 
far as possible the intensity of the action. 
(4) To examine into the character of the electrical conductivity of selenium when 
kept in the dark. 
(5) To determine whether light could actually generate an electrical current in the 
selenium. 
Discussion of Results previously obtained by one of us, and communicated to the 
Royal Society (see Proc. Roy. Soc. nos. 163 & 166). 
In all the earlier experiments we used the selenium plate belonging to Mr. Willoughby 
Smith. It is a plate of the substance 5‘4 centims. long, 1'2 centim. broad, and -08 
centim. thick. It has been very carefully annealed, so that it is in a very fair crystalline 
condition, and, for selenium, conducts electricity very well. Along each end of the 
plate are attached platinum wires forming electrodes, and those wires are connected 
MDCCCLXXVII. 2 Y 
