At the time this microscope was manufactured, it was the largest model sold by the firm. Interestingly, one of the objectives supplied with this instrument, a 1/8 with correction collar, was made specifically for this microscope by J. Grunow of New York, suggesting that at some point in its history this microscope made its way to America and was used there during the 19th century. As with the Merz objectives, the Grunow objective has engraved on the canister with the magnifications achieved with the four different eyepieces. The Merz objectives are a 1/3, 1/9, and 1/18 (immersion) with correction collar.
On the basis of the 1866 Merz price list shown below, on the number oculars supplied (4), and the fact that it has the rack and pinion coarse adjustment, this instrument most closely conforms to Stativ Nr. I, Microscop Nr. 3. There are unoccupied fittings within the case, which most likely held a bullseye condener.
The Merz firm was founded in 1839 by Georg Merz (1793-1867) after taking over the business started by Joseph Fraunhofer. After a brief partnership with Joseph Mahler, the firm continued on with Georg's sons Ludwig (1817-58) and Sigmund (1824-1908). By 1858 after the death of Ludwig, instruments were signed as G. & S. Merz. In later years the firm was run by Sigmund's cousins Jakob and Matthias Merz. See this short biography of Georg Merz (in German).
The Merz delivery books have an entery for serial number 866 as follows:
September 4. 1868: Röhrer (Ertel & Sohn) hier
1 Microscop Nr. III mit Nr. 866 mit 3 Objectiven
1/3", 1/9". 1/18" Immersion & corr,
4 Ocularen 1, 1 1/2, 2, 3,
micrometer in 1 1/2 für 1/9" I = 0.004 mm Deckgl. 8 für 1/9" und 1/18" ....154 fl.
The firm Ertel & Sohn was a maker and retailer of scientific instruments located in Munich with a workshop near that of G. & S. Merz.