curvature and the machining accuracy of the rib 54 close tolerances between the mirror blank outside diameter and the cavity 52 or on the thickness of the mirror are not required the mirror is held in place against the mounting rib 54 by a compressible rubber pad or large compressible wave washer 56 which in turn is secured by an end plate 59 and screws 60 to implement the invention as illustrated in fig2 the mounting rib 54 is configured to include two support surfaces 62 and two bending surfaces 64 the surfaces are disposed approximately 90 degrees apart in this illustrative embodiment the two support surfaces are disposed at opposite sides of the mirror perpendicular to the plane of incidence i e the plane containing the entrance and exit slits 16 38 flowcell 40 and grating 26 the two bending surfaces 64 are disposed at opposite sides of the mirror in the plane of incidence the level or height of the support surfaces 62 relative to the surface of the mounting rib 54 slightly lower than the level of the bending surface 64 to effect bending in this illustrative embodiment a height difference of about 0 005 34 is implemented between the bending surfaces 64 and the support surface 62 to provide the necessary bending the surfaces 62 64 are implemented in this.