ACTION OF LIGHT ON SELENIUM. 
335 
For this purpose six pieces of selenium (Nos. 20-25) inclusive (see page 322) were 
prepared, and carefully annealed in hot sand. 
No. 20 was first connected with the galvanometer, and when screened from all light 
there was no deflection whatever of the needle. On exposing the selenium to the light 
of a candle no effect could be observed ; but exposure to the light of burning magnesium 
ribbon produced a deflection of 3 divisions as long as the light lasted. Hence this 
piece is slightly sensitive to the action of light. 
No. 21 was then examined in the same way, and, to cut off all the obscure heat-rays, 
a glass cell of water was interposed in the track of the light. Exposure to the light of 
a candle produced a sensible effect. The light of a gas-flame at a distance of 8 inches 
produced a steady deflection of 9 divisions, and the same light at 3-^ inches distance a 
deflection of 14. In all cases the deflection came back at once to zero on screening off 
the light. 
In another experiment with No. 21, exposure to the gas-flame at a distance of 6^ 
inches produced a current indicated by a deflection of 6 divisions to the right of zero. 
Exposure to the burning magnesium ribbon produced a strong “ throw ” of 40 divisions 
on the same side, the motion of the needle stopping at once on the light dying out. 
The electrodes were then reversed in the mercury-cups, and exposure to the gas-flame 
produced a deflection of 5 to the left of zero, while the magnesium light produced a 
deflection of 56 to the left of zero. Hence it appears that, in the case of No. 21, on 
whichever side of the selenium the light falls, it causes a current in the same direction 
through the selenium. 
No. 22 was then examined in a similar manner, but it seemed to be quite insensitive 
to the action of light. 
Hence it appears that three pieces of the same length, which were made from the 
same rod of selenium, and which were annealed together, may, owing to some slight 
difference in their molecular condition, be very different as to their relative sensitiveness 
to the action of light. 
Effect of illuminating one end only of a piece of Selenium. 
In the preceding experiments the piece of selenium under examination was always 
exposed as a whole to the influence of the light, so that it was not possible to tell 
whether any one part of a piece was more sensitive than any other. 
In order to examine into this point, the lime-light was used, and, by means of a lens, 
the light was brought to a focus on the particular portion of the selenium to be 
examined. A plate of glass an inch thick was interposed in the path of the beam, so 
as to assist in absorbing any obscure heat-rays. 
Mr. W. Smith’s plate was the first examined in this way. The light was brought to 
bear upon the end near the marked electrode. This gave a steady deflection of 40 
divisions to the right of zero, indicating a current from the marked towards the unmarked 
