1374305 Cutting and welding by fusion UNION CARBIDE CORP 8 Nov 1971 [9 Nov 1970] 51752/71 Headings B3V and B3R [Also in Division H1] An optical system for viewing a metallic workpiece being worked in a vacuum chamber C by an energy beam generating gtm 12, includes a mirror 10, which is mounted on the lower end of the gun 12 for reflecting light from the workpiece surface and which is movable for exposing a new mirror surface when the exposed surface becomes contaminated with vapours from the workpiece, and a telescope 26 positioned outside the chamber C for viewing the image reflected by the mirror 10. The mirror 10 may be of metal or may initially comprise a clear glass slide which is transformed into a mirror surface when metallic vapours are deposited on it. The mirror 10 is mounted on an electron beam gun 12 together with an adjustable objective lens 14 for focusing the image reflected by the mirror 10 on to a crosshair reticle 18 and with a collimating lens 20 for directing the image and crosshair into the telescope 26. The workpiece image may also be observed by a device 28 including a prism and eyepiece. The workpiece surface is illuminated by at least one light source (32), (Fig. 2, not shown) which may be reflected onto the workpiece by a rotatable mirror (40) (Fig. 3, not shown) covered by glass (41). The workpiece may be welded or cut.