On 20 February 1915 The Illustrated London News referred to a contraption which appears in every film set on the Western Front, 1917 being no exception; the ‘trench-periscope (or, to give it its correct name, a hyposcope)’. The word ‘hyposcope’ was used in 1902 by the Daily Chronicle, which described the apparatus as having ‘the peculiarity … that, by an optical contrivance, the marksman, completely under cover, may fire round a corner, so to speak, at an enemy’. The Illustrated London News described the hyposcope as being ‘on the principle of the camera-obscura’. The Illustrated War News 23 December 1914 clarifies the workings, slightly, perhaps not wanting to be seen giving away any secrets that might be useful to the enemy.
Periscopes and hyposcopes attached to rifles were used during the war, notably the Youlten hyposcope, tested originally in 1903, and the later Beech’s periscope rifle. The word ‘hyposcope’ seems to have disappeared during the conflict; it does not appear in Farrow’s American Dictionary of Military Terms (1918), nor does the OED have any postwar citations. The British Newspaper Archive suggests the word had disappeared by the middle of the war, with only three mentions in 1916, and only three since then.
The image here shows, we hope, a hyposcope being tested.
Languages and the First World War 