A totally reflecting prism placed inside the glass cylinder projects the light which penetrates the film upon a selenium cell situated at the end of the cylinder.
To eliminate the sluggish action of the selenium transmitter a selenium cell similar to that at the transmitting station is arranged at the receiving apparatus, and exposed to precisely similar variations of light, the arrangement being such that the lag of this cell counteracts the lag of the transmitting cell.
Such, for instance, were those of Spindler and Wrangell in the Black Sea by sinking an electric lamp, those of Paul Regnard by measuring the change of electric resistance in a selenium cell or the chemical action of the light on a mixture of chlorine and hydrogen, by which he found a very rapid diminution in the intensity of light even in the surface layers of water.
An illumination of variable intensity (according to the deeper or lighter shades of the portion of the picture on which the light falls) thus takes place on the selenium cell.
As the glass cylinder, driven by a motor, revolves upon its axis while also advancing (by means of a screw thread on the axis), all portions of the picture are successively brought under the beam or pencil of light and cause a beam of varying intensity to fall on the selenium cell.
