As kids we used two mirrors spaced apart and inclined at 45 degrees to look around corners and over heads of people in front of us.
The challenge now was to view the outside world above the surface of the ocean and perform high resolution photography from a depth of some 40 feet below the surface while the submarine was tossing about in the waves and its window being covered covered with mist and waves. often shrouded in fog..
Beyond this, the residual aberration, secondary spectrum, stood as a major hurdle to achieving the desired diffraction-limited resolution. It is discussed on the page titled as such.
Not addressed here ia another major hurdle, image motion due ro mast vibration and boat motion. Image motion during the film exposure time could destroy image quality. Ve4y short film exposure times can be an enormous h3lp but such is contrary to the very long focal lengths needed to capture distant fine object detail.