Sociedad Ibero-Americana de la Historia de la Fotografia Museo Fotográfico y Archivo Historico "Adolfo Alexander"
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LEICA Oberflächen Messgerät 

Surface roughness measuring instrument after Forster - very early model built from IIIa/IIIb body model. Manual opening and closing curtain through large button at normal speed dial place. Motorized continous film advance synchonized together surface under study. Film is continously wound through the camera frame slit with the opened shutter, while light from the 'Oberflächenmessgerät' projects a magnified image of a mechanically crafted surface in order to show an enlarged "curve" of the surface irregularities horizontally on the film, So, no needle or surface scratching that leads to errors. Crank rewind. Surface variations are registered in deep contrast plotted curve. Production year of this example around 1938.

Together Leitz commercial catalogue around 1955.

see Leica Historica 1985 Page 76 Abb. 28 + Lager Vol. I page 67