ACTION OF LIGHT ON SELENIUM. 
315 
a maximum. After this the heating and other effects of the current overbalance the 
action of the light, and the resistance henceforward increases steadily. 
When first exposed, after being kept in the dark for some days or even hours, the 
selenium is much more sensitive to light than it is after repeated exposures, and hence 
the quantitative result obtained from the first of any series of experiments is generally 
not comparable with the others. 
Exposure to the light of an ordinary wax taper at a distance of 20 centims. dimi- 
nished the resistance of this plate of selenium by about one eighth part of the whole. 
The light of an ordinary lucifer-match was found to have a powerful effect. 
The diminution of resistance, when this plate of selenium was exposed to a powerful 
gas-light, was found in many cases to amount to from 10 to 25 per cent, of the whole 
resistance. 
Effect of Moonlight on Selenium. 
The effect of moonlight on the selenium was tried during the month of January 1875. 
An experiment was made first with the window open, so that the selenium was exposed 
to cold air as well as to moonlight, and the galvanometer-needle was deflected through 
36 divisions of the scale in about two minutes, on the side indicating a diminution in 
the resistance of the selenium, amounting to about 18,000 ohms. 
An experiment was afterwards made with the window shut, and a deflection of about 
20 divisions of the scale was produced by the moonlight alone. 
On another occasion when the moon was nearer full, but shining very obliquely 
through the window, exposure of the selenium to the moonlight produced a deflection 
of 150 divisions in three minutes, which was balanced by diminishing the variable 
resistance in the bridge by 120 ohms, showing that the resistance of the selenium was 
diminished by 60,000 ohms by the action of moonlight. 
On another occasion, a plane mirror was placed inside a window, which was kept 
shut, and the moonlight was reflected upon the selenium or away from it at pleasure 
without approaching or in any way disturbing the selenium or its connexions. On 
reflecting the moonlight on to the selenium, the needle was deflected through 20 divisions 
of the scale very gradually ; as soon as the mirror was turned away, the needle gradually 
fell back again. This experiment was repeated a great many times, and always with 
the same result. The moonlight was not bright enough to see the seconds-hand of 
a watch clearly. As the moonlight fell on the window at a very oblique angle, the 
mirror was now placed on a balcony outside the window so as to throw the reflected 
beam almost perpendicularly on the panes of glass (the window being kept shut during 
the exposures). The deflection of the needle now gradually rose to 40 divisions, and the 
resistance of the selenium was balanced and found to be diminished by 20,000 ohms. 
On opening the window, the deflection rapidly rose on the same side to 110 divisions, 
and the resistance of the selenium bar was further diminished by about 57,500 ohms. 
These experiments show that exposure to cold produces a change in the selenium in 
the same direction as exposure to moonlight or exposure to any other kind of light 
2 T 2 
