Detail of the observation window.
Mercury Familiarization Manual notes that the observation
windows was located above the main instrument panel and consisted of an inner
and an outer assembly. The inner window assembly was made up of three glass
panes sealed in a titanium frame and attached to the cabin wall. The panes
were independently sealed to provide a pressure seal between the panes. The
outer window assembly consisted of a glass pane sealed in a titanium frame,
attached to the capsule outer skin. The outer window assembly was sealed
separately from the inner window assembly, to provide a complete seal. The
outer window conformed to the curvature of the capsule conical section.