A. G. Bell—Production of Sound by Light. 311 
two wires, instead of being coiled together, were arranged in a 
zig-zag shape, forming a sort of platinum gridiron. 
is was treated in the same way as the spiral arrangement. 
Another form of cell consisted of a sort of lattice-work or 
basket-work of platinum wires arranged upon a perforated mica 
plate, the wires interlacing with one another, and with the mica 
plate so as to make metallic contact only with alternate wires. 
He also found that iron and copper might be employed, instead 
of platinum. 
Without dwelling further upon the researches of others I 
passage of a continuous and steady current. It is only at the 
moment of change from a stronger to a weaker state, or, vice 
versa, that any audible effect is produced; and the amount of 
effect is exactly proportional to the amount of variation in the 
current. 
It was, therefore, evident that the telephone could only 
respond to the effect produced in selenium at the moment of 
change from light towards darkness, or, vice versa, and that it 
would be advisable to intermit the light with great rapidity so 
as to produce a succession of changes in the conductivity of the 
selenium, corresponding in frequency to musical vibrations 
within the limits of the sense of hearing. For I had often 
noticed that currents of electricity, so feeble as hardly to pro- 
duce any audible effects from a telephone when the circuit was 
simply opened’ and closed, caused very perceptible musical 
sounds when the circuit was rapidly interrupted; and that the 
higher the pitch of the sound the more audible was the effect. 
I was much struck by the idea of in this way producing sound 
by the action of light. 
I proposed to pass a bright light through one of the orifices 
in a perforated screen consisting of a circular disc or wheel with 
holes near the circumference. Upon rapidly rotating the dise 
an intermittent beam of light would fall upon the selenium and 
a musical tone should be produced from the op Be the 
the of which would depend upon the rapidity of the rotation 
of the dise. 
Upon further consideration it appeared to me that all the 
audible effects obtained from variations of electricity could also 
