Another scheme for modulating the light falling on the film on the
cylinder, consists in placing a light wedge against the face of a lens
which images the vibrating mirror (or light source) on the film.
As the light is constantly imaged on the film by the lens its slight
displacement toward the dark end of the light wedge by the vibration of
the mirror decreases the strength of the light falling on the film,
while displacement toward the thin edge of the light wedge gives greater
exposure on the film.
It is obvious, therefore, that the vibration of the mirror determines
the exposure at successive positions on the film; and as these
displacements follow the varying strength of the incoming electric
current, and the latter in turn is determined by the light values of the
picture at the sending station, it naturally follows that when the film
is developed a duplicate of the distant picture results.