can seat itself properly on the surface of the glass. The forces developed in the crossed-beams or x-member are transmitted to the shoes by tension or compression-struts.
In the crossed-beam design, the amount of astigmatism provided by the harness is varied by merely turning the pivot screw at the midpoint of the two beams (see Figure 2). In the x-member design, adjustment is accomplished by merely tightening a single screw against one of the compression struts. (see Figure 3a). In both of these designs the warping harness contacts the glass at eight points and the linkage is such that the applied force is the same at all points, no matter what its magnitude might be.
One of the requirements of the warping harness is that it should hold its adjustment more or less indefinitely. When we say "hold" we mean that flexures should remain constant to within less than one-millionth of an inch. Since the forces involved are considerably more than the weight of the mirror blank, and since one-millionth of an inch is a much closer tolerance for mechanical parts than we are accustomed to dealing with, some new precautions must be observed. Since the pressures in the system will normally take up all play at the joints, the main precautions to be observed are to be sure that all joints are fitted well and that no material is used in the harness which might creep or otherwise yield slowly to the forces. One joint in particular which is important is the joint between the shoe and the glass blank. In order that this joint should fit well, the shoe should be lapped-in against the glass. This can best be done after the mirror is rough-ground or during fine-grinding stages. This should be done without any abrasive between the metal shoe and the glass. If abrasive is used, the contact areas will usually turn out to have turned-down edges.