Made by Jos. Zentmayer, Philad No. 14.
The Grand American model microscope, c. 1860
The horizontal movement of the mechanical stage utilizes a worm gear while the vertical movement is uses a fusee chain.
While the original wood case did not survive with this microscope, the accessory box did. Among the remaining accessories is a Zentmayer objective with its canister marked 8/10, two oculars, a camera lucida, a centerable achromatic condenser, a substage polarizing prism in a rotating mount, and the analyzing prism which mounts onto the end of the draw-tube.
Zentmayer's Grand American Microscope
Is eighteen inches high when arranged for use. The instrument is mounted on a broad tripod with revolving platform, beveled, silvered, and graduated in degrees for measuring the angular aperture of Achromatic Objectives. Upon this platform two pillars are planted, which carry the curved bell-metal bar to which the body of the instrument, the stage, the secondary body, and mirrors are attached. The bar supports almost the entire length of the body, giving great steadiness and freedom from tremor. The movement of the body it effected by rack and pinion, connected with two large milled heads, which form the coarse adjustment. It has a graduated draw-tube to receive the eye-piece, erector, and analyzer. A fine micrometer screw with graduated and silvered head, acting on a lever, forms the delicate fine adjustment.
Below the stage is the secondary body, a short tube, perfectly centrical to the main body, and moved by rack and pinion, to receive accessories.
The large plane and concave mirrors are so attached as to facilitate oblique illumination and to swing in one plane to the optical axis of the instrument. To ensure smoothness and durability in the movements, the touching parts are of different metals; one being always of hammered brass, the other of bell metal.
The stage is firm, broad, and steady, and only 3/16 inch thick, giving great facility for extreme oblique illumination. It has rectangular movements of one inch in both direction, operated by milled heads that work upon the same axis, with an additional one on the other side of the stage (not visible in the cut), by which diagonal movements are obtained. Upon the square stage a revolving object-carrier is placed. The beveled and silvered edge of the revolving plate is graduated into degrees, and serves as a goniometer, Graduation are also connected with the rectangular movements of the stage, to indicate the position of an object in view so that, when once recorded, it can be easily refound.
The low serial number (No. 14) of this Grand American model suggests it is among the earliest surviving Zentmayer microscopes. Zentmayer's Grand American microscope was first produced in 1850s and was presented in October 1859 to the Acadamy of Natural Sciences in Phildadelphia. In an article written by Zentmayer entitled "What I Know About Late Improvements of the Microscope" published in the Journal of the Franklin Institute, LXXXIV, 1877, he describes the history of the design of this model and he specifically mentions the microscope with serial number 14.
He has written: "In order to make the investigation a thorough one, it will be necessary for me to call your attention to the so-called Grand American Stand, made for this academy in October, 1859, and which now stands before you. The novel points of this stand, which I claimed at that time, were: 1st. The stage, with graduated revolving plate to serve as goniometer. Although very firm, it is only 3/16" thick, and is, even at the present date, the thinnest mechanical stage made. 2d. The graduated revolving base for measuring the angular apertures of objectives. 3d. The hanging of the mirror to a joint as near as possible to the plane of the stage.
Early in 1860, I made three stands (Nos. 13,14 and 15) precisely like the Grand American, but somewhat lighter. No. 15 was made for a gentleman who was not in favor of mechanical stages, and who desired me to design for him a revolving stage, the object to be moved by hand, and it was for him that I constructed the first of my graduated stages, giving a complete revolution in the optical axis, in a large ring, which is adjustable within another by three screws, in order to have the axis of the stage coincident with the optical axis of the instrument, exactly the same as the one before you, which I made early in 1866. This stage has been for years extensively copied, in France and in England."
As suggested in the above quote, No. 14 is among the first few Grand American microscopes that were produced. This particular example does not have graduations for the rotating stage plate, which is a common feature found on most of the instruments produced subsequently. The Grand American microscope was the top-of-the-line model produce by Zentmayer until 1876 when production of this model ceased and was replaced by the American Centennial model microscope. Elsewhere on this website is an example of a binocular version of the Grand American microscope.
Biographical Sketch of Joseph Zentmayer
See this article from: Proceedings of the American Microscopical Society. Vol. 14, No. 3, 1893.
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