MAKER: JOSEPH ZENTMAYER

MODEL: 'GRAND AMERICAN'

c. 1865

SIGNED:J.Zentmayer, Maker, Philad' ^a  

SERIAL NUMBER: 291

DESCRIPTION

HISTORY

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DESCRIPTION:

Signed in Script on the Foot: J. Zentmayer, Maker, Philad ' ^a  .   This well-built, heavy, Jackson-Lister-limb microscope arises on two pillars from an American style flat tripod foot with calibrated rotation via a silvered scale. The limb is one solid piece of brass which incorporates the support for the main optical tube, the support for the substage, and protruding part into which the swinging tailpiece fits. The limb has a curved triangular cutout distinctive to Zentmayer's Grand American Microscopes from their inception.

The swinging tailpiece carries the gimbaled mirror housing with a plane and concave-sided large mirror. The top of the tailpiece rotates on a screw which penetrates part of the limb which protrudes down for this purpose. The mirror assembly can move up or down the tailpiece, this movement facilitated by two small knobs.

The substage support for the condenser is simply a continuation of the limb beneath the stage incorporating a straight rack and pinion vertical movement via bilateral knurled knobs; i.e. the limb continues beneath the stage to support the substage as a single piece of metal. The tension on the pinions is adjustable via a central screw.

The substage sleeve (right) has a cap which has an RMS thread to accept, for example an objective as a condenser. This cap can be attached to the top or bottom of the sleeve. With the cap removed, or relocated to the bottom of the sleeve, the sleeve can accept other accessories such as a traditional condenser or polarizer on top.

The support for the round stage saddles the limb, held on it by two screws, one screwing into the limb from each side. The stage rotates by use of concentric rings and there are three screws that can be used to center the axis of rotation. The stage rotation is calibrated via a silvered scale; it can rotate 360 degrees. A Zentmayer gliding slide holder runs on top of two slightly raised linear bars. The glider, with a partly glass bottom, is held down by an ivory-tipped screw. The tension can be adjusted to allow smooth movement, yet keep the gliding slide holder from sliding away after the desired position has been reached. This mechanism, invented by Zentmayer, allows very smooth and controlled motion via the two handles on either side. The gliding slide holder has a small knob to register the slide on the left side, and has an adjustable front register to accomodate a variety of slide widths. Two knurled knobs serve as handles to control the movement of the glider, and each has a hole in it to accomodate accessories like a stage forceps.

The nosepiece fine focus wheel is calibrated and silvered. The main optical tube is focused via staight rack and pinion nearly identical to the one on the substage, other than the size of the knobs. There is a draw tube, calibrated in ₁/^8th inch increments, over a four inch range, and labeled every inch. The single ocular has a slot to admit a Jackson type micrometer. The drawtube is threaded at its distal end to accept an analyzer or an erecting lens. There is a single moderate power objective signed Queen.

This stand has great rigidity and the rotational movements of the foot and stage and the inclination of the limb on trunnions are achieved via surprisingly sophisticated designs which prevent rotation from loosening the tension. This instrument has been tastefully restored, and images before and after are available to those interested.

HISTORY OF THE ZENTMAYER MICROSCOPES

Joseph Zentmayer was born in Germany 1826 and emigrated to the U.S.A. after receiving training in Europe. He apparently started his own business in 1853 and built his first big microscope almost identical to this one about 1858 to the design of a Dr Hunt of Philadelphia; it was still advertised in the T. H. McAllister Catalog of 1876.
The Grand American was made from about 1858 through about 1876 when it was replaced by Zentmayer's American Centenial stand. I have seen a quote of about 55 stands of this type produced, but there is no objective evidence to support this claim and the figure would seem low considering how long it was made-18 years. The arrangement of the goniometer stage, and the gliding slide holder on it, as shown here, were both invented by Zentmayer in 1859, according to The American Naturalist of 1873. According to an article by Zentmayer himself, and published in the Journal of the Franklin institute in June of 1877, he invented the completely rotating and calibrated stage of this type, with a gliding holder for the slide, 'early in 1860'. The catalog entry for 1876 of T.H. McAllister also states this type of gliding stage was invented by Zentmayer, but it dates the invention to 1862. Needless to say, it is likely that considering all these discrepancies, it can perhaps best be estimated to about 1860.

The Zentmayer Grand American model originally had a mechanical stage (using a fusee-chain drive for one axis), or as an option the Zentmayer gliding stage shown here.

Although Zentmayer's purported use for the calibrated revolving base was to measure numerical aperture, its purpose as described by Zentmayer's contemporary Bulloch was as a convenience for photography. When he developed his method of taking photomicrographs, Dr Joseph J Woodward, an Army Surgeon, used this model microscope for that purpose.

Fully equipped, this stand was among the most expensive of American microscopes of the time,and would have sold with a full set of accessories for $700 in the 1860's; that would be over $12,000 today. For this reason, few could afford such an expensive instrument!