support the prism. Although the lenses were typically assembled indoors
using an optical source projecting an image on a screen, this is not practical
in the field and attempts to center the prism in the frame should yield
satisfactory results.
If the prism falls out of the frame during handling, or if it is found on the floor
intact, it may be reinstalled using the following procedure. Remove all
litharge from the prism and cavity in the frame. Try to locate and save any
wooden wedges, if possible. Apply approximately 1/8" thick layer of EVS to
the ends of the prism and insert into the frame. Use Tyvek tape to
temporarily hold the prism in place. Insert wooden wedges between the
frame and prism, as shown in Figure 7-1.
To increase thickness
Glass Prism
Lens Frame
Wood wedges may be opposed
triangles for adjustability.
Wood Wedges
Use two wedges on the
long side of the prism and
one on each short side
Figure 7-1. Wooden Wedges
Apply EVS to the crevice between the prism and frame. Use nonmetallic or
shaped wooden tools (from tongue depressors or Popsicle sticks) to smooth
the EVS. Remove excess before it dries by using a damp cloth. After the
EVS sets, remove the tape and gently clean the prism, if necessary.
If the wedges are in place, fill the gap with EVS, as described in section 7B,
allow to dry, remove the Tyvek tape and gently clean the prism, if necessary.
Apply paint to the EVS after it dries (may require a later visit).
D. Bull's-eye. A bull's-eye lens panel present different challenges. The
construction of the lens is very precise, and considering the age of these
lenses, they must be handled very delicately. Inspect the bull's-eye rings to
determine whether they are starting to separate from the other rings. Note
how far the ring has dislodged from its supporting members and identify its
ring number. Beware that loss of one loose ring may cause a domino effect
7-2
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