ACTION OF LIGHT ON SELENIUM. 
343 
platinum wires were melted into and buried in them, it is somewhat difficult on applying 
the heat to the junctions to know on which junction the heat is concentrated. 
We have investigated the thermoelectric properties of sixteen of the twenty-five 
pieces of selenium on which we have hitherto experimented. 
Effect of Exposure to Sunlight. 
Mr. W. Smith’s plate of selenium, enclosed in its box, was joined up as part of a 
Wheatstone’s bridge as in the first experiments in the paper, the battery being 10 
Leclanche’s cells, and was placed in such a position that, on withdrawing the lid of the 
box, the sunlight was brought to a focus by a powerful lens on one of the junctions of 
selenium and platinum (the marked end of the selenium). By altering the position of 
the box, the sunlight could be brought to a focus on any other point of the selenium 
plate. 
First Experiment. — The current entered the selenium plate at the marked end, the 
resistance being balanced by the bridge ; when the sunlight was brought to a focus on 
the marked end, a deflection was produced to the left, showing that the resistance of 
the selenium was diminished, i. e. that the light assists the passage of the battery-current 
from the platinum to the selenium at the marked end. 
Second Experiment. — The current entering the selenium as before at the marked end, 
and therefore passing from selenium to platinum at the unmarked end ; when the sun- 
light was brought to a focus on the unmarked end, a deflection was produced to the 
left, showing that the light assists the passage of the battery-current from selenium to 
platinum at the unmarked end. 
Third Experiment. — The battery-current was reversed, so that it passed from platinum 
to selenium at the unmarked end ; when the sunlight was brought to a focus on the 
unmarked end, a deflection was produced to the right, showing that the light assists 
the passage of the battery-current from platinum to selenium at the unmarked end. 
Fourth Experiment.— With the same arrangement as in the third experiment, when 
the sunlight was brought to a focus on the marked end, a deflection was produced to 
the right, showing that the light assists the passage of the battery-current from selenium 
to platinum at the marked end. 
In all four experiments, the light assists the passage of the battery-current whether 
it is passing from platinum to selenium or from selenium to platinum at the illuminated 
junction. 
Next, to determine the effect produced by sunlight when there is no battery-current : — 
Fifth Experiment. — The plate of selenium was joined up in simple circuit with a 
galvanometer only. When the sunlight was brought to a focus on the marked end, 
there was a deflection of 30 divisions to the left, showing that the light gave rise to 
a current of its own from platinum to selenium at the marked end. This current 
ceased when the light was removed. 
Sixth Experiment. — When the sunlight was brought to a focus on the unmarked end, 
