The Periscope was a navigation aid. It was used to determine attitudes in orbit such as normal orbit, retrograde, and earth vertical. It could also be used to compute altitude, drift, and speed. It was very heavy and was deleted in later missions. The astronauts decided the new window and a new smaller, lighter navigational reticle worked well enough. A lighter housing was retained after it was deleted to support the instrument panel.
For further reading, any of the Mercury Familiarization Manuals will provide all the details you could want...
Scott
For further reading, any of the Mercury Familiarization Manuals will provide all the details you could want...
Scott