310 A. G. Bell—Production of Sound by Light. 
Later, Prof. W. G. Adams,* of Kings College, took up the 
question, and his se -pienains seemed to prove conclusively 
that the action was due principally, if not entirely, to those 
rays of the spectrum iia were luminous, | that the ultra- 
red or the ultra-violet rays had little or no ‘effec 
This conclusion was supported by the siaried effect pro- 
duced by the light of the moon, and by the apparent insensi- 
tiveness of selenium to rays passed through a solution of iodine 
in bisulphide 6f carbon. He found that the maximum effect 
was produced by the greenish -yellow rays, and showed that the 
intensity of the action depended upon the meni power of the 
laght, ne directly as the square root of that illuminating power. 
Professor Adams and Mr. R. E. Dayt continued these 
rese ition: and among other interesting and suggestive results, 
discovered that light produces in selenium an electromotive 
force without the aid of a batter 
The most sensitive variety of selenium that has yet been 
oe as was obtained in Germany by Dr. Werner Siemens, 
y continued heating for some hours at a temperature of 210° 
C., followed y. dha slow cooling. 
Dr. mens,{ in a lecture delivered before the Royal 
Institution of Seat Britain, on the 18th of February, 1876, 
stated that his brother’s modification of selenium was so sensi- 
tive to light that its conductivity was fifteen times as — in 
rea as tt was in the dark. 
between the wires. Each cell was about the size of a silver 
dime. The cells were shi placed in a paraffine bath and 
annealed. 
Siemens devised other arrangements of apparatus for reducing 
the resistance. In the form known as “ Siemens’ Grating,” the 
y- Soc., June 17th, 1875, xxiii, 535; see, also, Proc. 
6th, 1876, xxiv, 163; Nature, Jan. 2 20th, 1876, xiii, i 298; Nature, aan, 23d, 1876, 
xiii, 419: Scient. Amer. Su; uppleme ent, amt: , 1876, i 
me v, 113 
nst. Gt. Brit., Feb. 18th, 1876, viii, 6 8; see, also, Nature, xiii, 
407; Ba ent. ares Supplement Apr. Ist, 1876, i, 222; Scient. Amer. aiiomonad, 
June 10th, 1876, 
Monats tsbericht i : Kén. preuss. Akad. der Wissenschaften zu Berlin for 1875, 
p. 280; Phil. Mag., Nov. 1875, IV, 1, 416; Nature, Dec. 9th, 1875, xiii, 112; Mon 
atsber. Berl. A Akad., Feb. 17, 1876; Bore.’ kun, lic, 117; Mons tab. Berl. Akad., 
June 7, 1877; Pogg. Ann., 1877, i 
