The Lifeguard folding periscope invented and patented by Edgar Duerr, a second-generation family member. The life-saving instrument enabled front-line soldiers to observe enemy movements from 50cm (20 inches) below the parapet, providing them with a view of up to 91m (100 yards). Approved by the War Office and sold to individuals, the pocket-sized periscope weighed (20oz) and measured 11.5 x 11.5 x 5 cm (4.5 x 4.5 x 2inches) when closed.