Abstract
An optical theodolite has graduated glass circles, which are read through an optical system that projects the images in such a way that they are seen by the observer without need for him to change his standing position. The images of the relevant portions of the horizontal and vertical circles are usually seen, either simultaneously or one after the other, through the eye-piece of a reading microscope situated alongside the telescope. They are read either directly by means of a glass scale or indirectly by means of an optical micrometer, which, in higher order theodolites, allows a coincidence setting to be made of the diametrically opposite graduations, thus giving an immediate mean value of the two diametrical graduations.
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