Hevelius saw military applications for his invention. Johannes Hevelius described an early periscope (which he called a 'polemoscope') with lenses in 1647 in his work Selenographia, sive Lunae descriptio. If the two telescopes have different individual magnification, the difference between them causes an overall magnification or reduction. The overall design of the classical submarine periscope is very simple: two telescopes pointed into each other. More complex periscopes using prisms or advanced fiber optics instead of mirrors and providing magnification operate on submarines and in various fields of science. Military personnel also use periscopes in some gun turrets and in armoured vehicles. This form of periscope, with the addition of two simple lenses, served for observation purposes in the trenches during World War I. In its simplest form, it consists of an outer case with mirrors at each end set parallel to each other at a 45° angle. The left one uses an erecting prism whereas the right uses an erecting lens and a second image plane.Ī periscope is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position. The two periscopes differ in the way they erect the image.

Is periscope free to use