Accordingly I proposed to pass a bright light through one of the orifices in a perforated screen consisting of a circular disk with holes near its circumference. Upon rapidly rotating the disk an intermittent beam of light would fall on the selenium, and from a connected telephone a musical tone would be produced, its pitch depending upon the rapidity with which the disk spun round. Variations of Light Necessary. “Upon further consideration I saw that the effect could not only be produced at the extreme distance at which selenium would normally respond to the action of a luminous body, but that this distance could be indefinitely increased by using a parallel beam of light, so that we might telephone from one place to another with no conducting wire between the transmitter and the receiver. To reduce this idea to practice it was necessary 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 variations in the air produced by the voice. I proposed, therefore, to pass light through two plates perforated by many small orifices. One of these plates was to be fixed, the other was to be attached to the centre of a diaphragm actuated by the voice. In its vibrations the diaphragm would cause the movable plate to slide to and fro over the surface of the fixed plate, by turns enlarging and contracting the free orifices for the passage of light. The parallel beam emerging from this apparatus could be received at some distant place on a lens focussing it upon a sensitive piece of selenium placed in a local circuit, with a telephone and a galvanic battery. The variations in the light produced by a speaker’s voice should cause corresponding variations in the electrical resistance of the selenium at the distant place, and the telephone in circuit with the selenium should reproduce audibly the tones and articulations of the speaker’s voice. It is greatly due to the genius and perseverance of my friend, Mr. Sumner Tainter, that the problem thus entered upon has been successfully solved. Special Treatment of the Selenium.