1. Apparatus for the televisual monitoring of a zone in which an arc welding operation is carried out on metal parts by the continuous deposition of a weld bead, using a welding machine incorporating a movable support (1) and a welding head (4) mounted for oscillation on the support (1) in order to make a transverse sweep during the deposition of the weld bead, this monitoring apparatus incorporating at least one video camera (10) fastened to the support (1) of the welding machine, the video camera (10) incorporating interposed on the path of the light rays coming from the welding zone and upstream of a device (22) for converting these light rays into video signals, a filter composed of a plate (24) and electrodes (26, 27) disposed in contact with one or the other of the two faces of the plate (24) in accordance with a determined network, and the monitoring apparatus also incorporating, associated with the camera, an electronic unit (12) connected through a video cable to a control station for the reconstruction of the image on a television screen, and analyzing the video signal and controlling the potentials of the electrodes (26, 27) in order to regulate the electric fields passing through the plate (24) in the direction of its thickness to values making it possible to obtain a desired density of the filter in different zones defined by the network of electrodes (26, 27), wherein the plate (24) is made of a ceramic material having light rotatory properties, and the electrodes (26, 27) are composed of rectilinear conductive bands in contact with each of the faces of the plate (24) of ceramic material, defining zones (30) on the plate (24) which are disposed in succession in lines (L.sub.1, L.sub.2, L.sub.3, L.sub.4) and columns (C.sub.1, C.sub.2, C.sub.3, C.sub.4) so as to form a square-meshed network.