480 A. G. Bell—Production of Sound by Radiant Energy. 
Distance from Focal Point of Lens at which Sounds became 
Inaudible with different substances. 
Zinc diaphragm, (polished) --.-.----.----------- 151" 
Hard rubber diaphragm .--- .------. -------- i te, 
Tin-foil . Ae ee Se a, Oe; BS 
Telephone " (Japanned iron).-.-- ---- 2°15 
Zinc ve (unpolished) ... 2.1. 2.22% 2°15 
White silk, (In receiver shown in fig. 1)... 3°10 
White worsted ; Bea A fs, a STOR 
Yellow worsted, * Bis oe fs eet ee 
Yellow silk, - OF cae Re Les WA Ae 
White cotton-wool, “ pe Oak pismaatin: G5 
Green silk, “4 heels Dns ee 
Blue worsted, . een Daa Oe 
Purple silk, “ns oye aoe 4 
Brown silk, " ner oo poe 
Black silk, . a es meee ya 
Red sil < 66 6“ e Gi ce ed 
oe “ce “6 6°50 
Black worsted, # en 
Lamp-black. In this case the limit of audibility 
could not be determined on account of want o 
space. Sound perfectly audible at a distance of.10°00 
(1.) A beam of light is received by two similar lenses (A B, 
fiz. 10,) which bring the light to a focus on either side of 
the interrupting disk (C). The two substances, whose sonorous 
powers are to be compared, are placed in the receiving vessels 
D E) (so arranged as to expose equal surfaces to the action 
of the beam) which communicate as flexible tubes (I G) of 
equal length, with the common hearing-tube (H). he re- 
ceivers (DE) are placed upon slides, which can be moved 
along the a awt supports (I K). The beams of light pass- 
ing through the interrupting, disk (C) are alternately cut off by 
the swinging of a pendulum (L). Thus a musical tone is 
produced alternately from the substance in D and from that 
in E. One of the receivers is kept at a constant point upon 
its scale, and the other receiver is moved toward or from the 
focus of its beam until the ear decides that the sounds pro- 
duced from D and E are of equal intensity. The relative posi- 
tions of the receivers are then noted. 
(2.) Another method of investigation is based upon the pro- 
duction of an interference of sound, and the apparatus employed 
is shown in fig. 11. he interrupter consists of a tuning- 
fork (A), which is kept in continuous vibration by means of 
an electro-magnet (B). 
A powerful beam of light is brought to a focus between the 
prongs of the tuning-fork (A), and the passage of the beam is 
more or less obstructed by the vibration of the opaque screens 
(C D) carried by the prongs of the fork. 
