Early Petrographic Microscopes
Petrographic microscope by R. Fuess after H. Rosenbusch, Berlin, ca. 1875
In the publication of "New Yearbook for Mineralogy" in February 1876, Rosenbusch made the following demands in his article on that revolutionary instrument entitled "A new microscope for mineralogical and petrographic investigations":
1) With fixed crossed Nicols, one must be able to comfortably rotate the examined object centrically in its own horizontal plane.
2) One must be able to read with desirable accuracy the angle through which an object has been rotated in the horizontal plane.
3) the planes of vibration of the Nicols must have a known position that can easily be restored at any moment after any displacement that has been carried out.
4) Where the adjustment to the maximum of the extinction cannot be carried out with the necessary sharpness due to some circumstances with ordinary white light, one must be able to make use of sharper methods conveniently.
This microscope bears no serial number. According to Prof. Ing. Timo Mappes, it was not until the late 1870s that the company's microscopes were consistently signed and numbered. It was bought from a dealer in California but with no clear provenance. It is very unusual to find such a microscope in the USA, as these early petrographic microscopes were very German and, after their introduction in ~1875, were quickly replaced, by the 1880s, by later models. Unless it was brought to the USA by another collector, the only geologist who performed in the USA any sort of petrographic study (a discipline that was still in its birth) was Ferdinand Zirkel, a professor of Geology from Leipzig whom the US government hired to come to the United States in 1874 to examine the great collections of minerals made during the Geological Exploration of the Fortieth Parallel. However, based on the evidence at hand, we cannot go that far as to suggest that this was his microscope.
Prototype petrographic microscope(?) ca. 1875-1880
This is an early form of a petrographic microscope. It consists of blackened and nickel-plated brass and steel, and the horseshoe is 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. Two optional methods can make the coarse focusing adjustment: 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 on the prismatic column. Above all, this large stroke is designed to test unusually thick samples.
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The rotating stage is 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 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 from 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. As in other early petrographic microscopes, the analyser 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 of 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. There is no provision for the centration of the objective or stage, nor is there a provision for any 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.