ACTION OF LIGHT ON SELENIUM. 
337 
The lime-light falling on the marked end gave a current of 9 from the selenium to 
the platinum at the illuminated end. 
The light falling upon the other end gave a current of 16 from the selenium to the 
platinum at the illuminated end. 
Light falling on the central portion gave a current of 22 from the unmarked towards 
the marked end ; but as the incidence of the light was in this case oblique, we may 
further mention that it was from the less towards the more illuminated portion. 
No. 24, also a new piece, gave the following results : — When the lime-light was directed 
upon the marked end, we obtained a current of 12 from selenium to platinum at the 
illuminated end. 
When the light was made to fall upon the central part, it gave rise to a current of 6 
in a direction from the unmarked towards the marked end. 
The electrodes were then reversed in the mercury-cups so as to bring the other side 
of the metal under the action of the lime-light. 
When the light was directed on the end remote from the mark, it produced a 
current of 42 towards the illuminated end. 
In all these experiments we found that on interposing a screen in the track of the 
beam the deflection of the galvanometer-needle was at once reduced to zero. 
In examining No. 21 with the lime-light the following results were obtained : — 
When the light was directed into the marked end it produced a current of 75 from 
the selenium to the platinum at the illuminated end ; and when the poles were reversed 
in the mercury-cups, and the light was directed on to the unmarked end, it produced a 
steady current of 21 from the selenium to the platinum at the illuminated end. When 
the light was directed on the central portion there was no perceptible current. 
No. 14, which has been used both for direction and also for 'polarization experiments, 
was examined in the same way with the lime-light. 
When the light was brought to bear upon one face, there was a current of 30 to the 
left of zero, and when, by means of a reflector and a lens, the light was directed on to the 
rear face of the selenium, there was a current of 100 in the same direction as before. 
On examining No. 15, which is a piece of selenium which shows the phenomena of 
polarization very well, some powerful currents were developed by the action of the 
lime-light. Having noticed in some previous experiments (p. 334) that this piece was 
very sensitive to light, the 10th shunt was put into the galvanometer-circuit. 
When the light was directed on to the unmarked end, a steady deflection of 60 was 
obtained with the shunted galvanometer, indicating a current from selenium to platinum 
at the illuminated end. 
The light was then directed on to the central part, and the resulting current was 9 
in the same direction as before. The poles were then reversed in the mercury-cups so 
as to enable the light to be thrown on to the other end of this piece of selenium. 
When the light was thrown on the central part there was a current of 50 (with the 
shunt) in the same direction with respect to the selenium as in the previous experiments 
MDCCCLXXVII. 3 B 
