A Perfected Transmitter.
“We have devised more than fifty forms of photophonic transmitters. In one of them (several others are described by Professor Bell), a beam of light passes through a lens of variable focus formed of two sheets of thin glass or mica containing between them a transparent liquid or gas. When vocal vibrations are communicated to this gas or liquid, they cause a vibratory change in the convexity of the glass surfaces with a corresponding change in the intensity of the light as it falls upon the selenium. We have found the simplest apparatus to consist in a plane mirror of flexible material, such as silvered mica or microscope glass, against the back of which the speaker’s voice is directed.
Selenium cylinder with reflector.
“A large number of trials of this apparatus have been made with the transmitting and receiving instruments so far apart that sounds could not be heard directly through the air. In a recent experiment Mr. Tainter operated the transmitting instrument, placed on the top of the Franklin School House in Washington, D. C.; the receiver being arranged in a window of my laboratory, at a distance of 213 metres. Upon placing the telephone to my ear, I heard distinctly from the illuminated receiver: ‘Mr. Bell, if you hear what I say, come to the window and wave your hat.’
“We have found that articulate speech can be reproduced by the oxyhydrogen light, and even by a beam from a kerosene lamp. The loudest effects follow upon interrupting the light by means of a perforated disk swiftly rotated. Because this apparatus is noiseless it allows a close approach of the receiver while not interfering with its message.
“We have endeavored to ascertain the nature of the rays which affect selenium, placing in the path of an intermittent beam various absorbing substances. In these experiments Professor Cross has rendered us aid. When a solution of alum, or bisulphide of carbon, is employed, there is but slight reduction in loudness, but a solution of iodine in bisulphide of carbon cuts off most of the audible effect. Even an opaque sheet of hard rubber is less obstructive.
A perforated disc rotated yields a succession of sounds from light.