308 A. G. Bell—Production of Sound by Light. 
of testing and signaling during the process of submersion. 
Upon experiment the selenium was found to have all the ° 
resistance required; some of the bars employed measuring as 
much as 1400 megohms—a resistance equivalent to that which 
would be offered by a telegraph wire long enough to reach 
from the earth to the sun! But the resistance was found to be 
extremely variable. Efforts were made to ascertain the cause 
of this variability, and it was discovered that the resistance was 
less when the selenium was exposed to light than when tt was in the 
dark ! 
This observation was first made by Mr. May*—(Mr. Wil- 
loughby Smith’s assistant, stationed at Valentia)—was soon 
verified by a careful series of experiments, the results of which 
were communicated by Mr. Willoughby Smith + to the Society 
of Telegraph Engineers, on the 17th 
nium, and the resistance of the substance was measured. 
on opening the lid of the box the resistance instantane- 
ously diminished. When the light of an ordinary gas burner 
(which was placed at a distance of several feet from the bar,) 
was intercepted by shading the selenium with the hand, the 
resistance again increased ; and upon passing the light through 
rock salt, and through glasses of various colors, the resistance 
was found to vary sworling to the amount of light transmitted. 
In order to be certain that temperature had nothing to do with 
the effect, the selenium was placed in a vessel of water so that 
the light had to pass through a considerable depth of water in 
order to reach h i 
same as before. When a strong light from the ignition of a 
the water, the resistance of the selenium immediately fell more 
than two-thirds, returning to the normal condition upon the 
removal of the light. 
h 
* See lecture by Siemens in Proc. Roy. Inst. of Great Britain, vol. viii, p. 68. 
+ Jour. of Soe. of Teleg. Engin., ii, p. 31 (1873); Nature, vii, 303; Teleg. Jour- 
nal, ILI, (1873), v, 301. M 
¢ Nature, vii, 340, March 6th, 1873. § Ibid. 
j Nature, vii, 361, March 13th, 1873. 
