SCIENTIFIC SUMMARY. 
377 
scarcely any audible effects from a telephone when the circuit was simply 
opened or closed, caused very perceptible musical sounds when the circuit 
was rapidly interrupted, and that the higher the pitch of sound the more 
audible was the effect. We were much struck by the idea of producing 
sound by the action of light in this way. Upon further consideration it 
appeared that all the audible effects obtained from variations of electricity 
could also be produced by variations of light acting upon selenium. We saw 
that the effect could be produced at the extreme distance at which selenium 
would respond to the action of a luminous body, but that this distance could 
be indefinitely increased by the use of a parallel beam of light, so that we 
could telephone from one place to another without the necessity of a con- 
ducting wire between the transmitter and receiver. It was evidently neces- 
sary, in order to reduce this idea to practice, to devise an apparatus to be 
operated by the voice of a speaker, by which variations could be produced in 
a parallel beam of light, corresponding to the variations in the air produced 
by the voice. 
1 We proposed to pass light through a small number of small orifices, 
which might be of any convenient shape, but were preferable in the form 
of slits. Two similarly perforated plates were to be employed. One was to 
be fixed and the other attached to the centre of a diaphragm actuated by the 
voice, so that the vibration of the diaphragm would cause the movable plate 
to slide to and fro over the surface of the fixed plate, thus alternately 
enlarging and contracting the free orifices for the passage of light. In this 
way the voice of a speaker could control the amount of light passed through 
the perforated plates without completely obstructing its passage. This 
apparatus was to be placed in the path of a parallel beam of light, and the 
undulatory beam emerging from the apparatus could be received at some 
distant place upon a lens, or other apparatus, by means of which it could be 
condensed upon a sensitive piece of selenium placed in a local circuit with a 
telephone and galvanic battery. The variations in the light produced by the 
voice of the speaker should cause corresponding variations in the electrical 
resistance of the selenium employed : and the telephone in circuit with it 
should reproduce audibly the tones and articulations of the speaker’s voice. 
I obtained some selenium for the purpose of producing the apparatus shown ; 
but found that its resistance was almost infinitely greater than that of any 
telephone that had been constructed, and we were unable to obtain any 
audible effects by the action of light. We believe, however, that the obstacle 
could be overcome by devising mechanical arrangements for reducing the 
resistance of the selenium, and by constructing special telephones for the 
purpose. We felt so much confidence in this that, in a lecture delivered 
before the Royal Institute of Great Britain, upon the 17th of May, 1878, 
was announced the possibility of hearing a shadow by interrupting the 
action of light upon selenium. 
1 The first point to which we devoted our attention was the reduction of 
the resistance of crystalline selenium within manageable limits. The 
resistance of selenium cells employed by former experimenters was measured 
in millions of ohms, and we do not know of any record of a selenium cell 
measuring less than 250,000 ohms in the dark. We have succeeded in pro- 
ducing sensitive selenium cells measuring only 300 ohms in the dark, and 155 
