and c and to the center 26 of the lens element 32 in fact the ground and polished surface 34 lies within a plane which is parallel to and at a known given distance from the plane formed by the points a b and c so as to accurately locate the outer surface 34 with respect to such plane and the center of the lens element 32 the panel is then assembled into a projection tv tube in the normal manner and sent to a set manufacturer the set manufacturer receives the finished tube and then must place an external projection lens system upon the tube in correct orientation with respect to the internal lens element 32 which is under vacuum however with the known locations of outside reference points a b and c formed on the outer rim portion 28 of the skirt portion 32 of panel g and the known distance between the plane of such points and the plane of the ground and polished face 34 relative to the center 26 of the lens element 32 the set manufacturer simply references his external lens system on the flat ground surface 34 on the outside of the panel and aligns corresponding fixture points on his mounting ring with the outside reference points a b and c formed on the outer ring portion 28 accordingly the set manufacturer has then accurately located the inside lens surface 32 from the outside of the tube and has accurately positioned his lens system on the geometric center of the inside lens element and then locks the system in place on the tube by any known mechanical means although we.