5924325 Enemy seen. Fire! Pulling the trigger of a trench mortar - the shell on the deadly flight in mid-air\', World War One, 1915 circa (b/w photo) by Unknown photographer, (20th century); National Army Museum, London; (add.info.: Enemy seen. Fire! Pulling the trigger of a trench mortar - the shell on the deadly flight in mid-air\', 1915 circa. Stereoscopic photograph, World War One, Western Front (1914-1918), 1915 circa. Soldiers of the Seaforth Highlanders (Ross-shire Buffs, The Duke of Albany?s) fire a trench mortar. Periscopes are positioned to view no-man?s-land and enemy positions beyond in relative safety. As well as two trench mortars, rifles with fixed bayonets are close to hand. Stereographs consist of two identical photographs paired in such a way that when seen through a special viewing instrument, a stereoscope, they appear as a three-dimensional images. The photographs were mounted on thick card that was given a slight curvature to increase the illusion of depth.); by National Army Museum ; out of copyright.