Early German petrographic microscope, ca. 1875

 

This is an early form of a petrographic microscope. It consists of blackened and nickel-plated brass, steel as well as the horseshoe made of black-lacquered zinc alloy. The upper optical and focusing systems are constructed on a steel prism column, erected from the stand holding both the stage and the horseshoe base. The coarse focusing adjustment can be made by two optional methods: either by sliding the tube into its sleeve, where it can be anchored by a small knurled knob screwing into the sleeve, or with the rack and pinion movement based in the prismatic column. Above all, this large stroke is designed to test unusually thick samples.

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The rotating stage is very unique. It can be turned very smoothly by only one finger due to a kind of rough dentation of the edge. The outer ring of the circular stage is marked with 360°. Against it and for measuring purposes (of the angle of extinction, the angles of carlsbad twinning, etc.), a rectangular engraved division is placed on the stage holder.

The tube diameter for accepting the ocular (and therefore also the ocular diameter) is 30mm, unlike the usual 23mm, the common diameter used by makers in France and the German countries. At the same time, the objective thread is RMS standard. When the microscope was purchased, it came with an early 20th century unsigned objective, which was replaced for the sake of the photos seen here with a contemporaneous A objective by Carl Zeiss, Jena. 

Apart of the original objectives, this microscope is undoubtedly missing some of its other components. A screw connected to the lower side of the stage holder was most likely intended to hold a substage sleeve for a Nicol-prism polarizer. The analyzer, as in other early petrographic microscopes, must have been another Nicol prism set within a goniometer and assembled on top of the ocular eyepiece. Similar attachments can be seen on the early petrographic microscopes by Rudolf Fuess of Berlin, made for Prof. Harry Rosenbusch.

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The identity of the maker of this interesting unsigned microscope is enigmatic. To the best of our knowledge, there are no records for any similar instrument in any other collection. By many features (the stand, the base, and especially the circular dentated stage and its holder), it is similar to the early models made by Fuess and Rosenbuch. However, it is probably not one of their products. It may be a one-off factory prototype, but due to the lack of any written records known to us so far, this assumption remains highly speculative. There is some doubt whether it was designed as a petrographic microscope, as there is no flange above the eyepiece to place and index a graduated cap analyzer, and there is no provision for centration of the objective or stage; nor is there a provision for any kind of waveplate.  Thus, despite some similarities to Fuess instruments, it may also be interpreted as a primarily biological microscope with a rotating stage.  Or, it could have been a special-purpose microscope built on special order.  

The author would like to gratefully acknowledge the kind assistance of Dan Kile, PD dr. Timo Mappes, and Dr. Joe Zeligs for fruitful discussions and their views concerning this microscope.

 

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