In right angle measuring of a point, the vertex of the right angle must be fixed on the measuring line. By using a prism, an incoming beam of light is deflected by 90 degrees or 100 gradians through reflection or refraction on the glass surfaces. The observer, therefore, can look in both directions of the legs of the right angle at the same time.
Fig.: pentagon prism, Hertel & Reuss Optik, Kassel, Museum of Art and Culture Dortmund: Museum handbook part 2, history of surveying
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