Image Description -  0717-6

A schematic is captioned “Figure 6. Alternate lens testing configuration
using a flat mirror to retro the beam (from ref. 1)”.  The schematic
shows a test lens between the spherical Fizeau reference surface and an
auxiliary reference sphere.  The spherical Fizeau reference surface is
flat on the side, which attaches to the interferometer and concave on the
side, which faces the test lens.  The auxiliary reference sphere is shown
as an object with both sides flat.  The focus of lens and center of
curvature is between the test lens and the Fizeau reference surface with
the focus located closer to the spherical Fizeau reference surface.