Periscopes for Royal Navy turret hoods had 2 settings, Periscope (field 35 degrees, no magnification) and Telescope (5.6 degrees, 6.5 power varying to field 1.7 degrees and 21 power). Switching between periscope and telescope was by throwing a lever (latter position labelled "TEL"), and adjusting telescope power was by rotating a milled sleeve on the eyepiece. "Considerable trouble has been experienced during recent actions owing to spray getting on the windows of gun sighting periscopes". A pair of hoses could direct a blast of air or water against the window for cleaning. It could also be cleaned by hand from within the turret with some difficulty. The periscopes were originally made by Vickers, and later by Ross.
- The Sight Manual, 1916, p. 17.
- The Sight Manual, 1916, p. 18.