Detail view of the optophone. A, switch for the electric lamp; D, perforated disk, placed in front of the lamp and driven by a small electric motor from the commutator M; G, glass plate, on which the image of slit placed above the disk; H, a crank, by which the operator can move the carriage with the lamp and disk from right to left; L, a line changer, which moves the pulpit, on which the glass plate with the book rests.