Model IIIa petrological microscope with mechanical stage, c.1908
This is an example of the Fuess Model IIIa.
It is made entirely of brass with bright lacquered, black enameled, and
nickel plated surface finishes. Coarse-adjustment is by rack and pinion.
The fine-adjustment is by micrometer screw located at the back of the limb.
The end of the body-tube carrying the objective is movable by two screws
for centering. To facilitate the change of objectives, the latter are not
screwed on, but held by a clamp. Immediately above the clamp is a slit for
the introduction of a Klein’s plate, quarter-wave plate, etc.
The stand measures about 15-inches tall inclined as shown in some of the photos.
The rotating stage is divided into 360º, with verniers reading
to 5 minutes. This particular example is furnished with the mechanical
stage that is usually associated with the Model II having calibrated
movements. The sub-stage,
which focuses by rack and pinion, holds a polarizer and a three
element condenser. The top two elements can be moved aside for low
power work using a lever mounted on the side of the stage. The
analyzing prism in mounted on a slide within the tube as is the
Bertrand lens which can be focused manually by moving it within the
tube. The mirror is mounted an arm which is moveable side to side.
The microscope came equipped with one eyepiece. Stored within
a leather covered box are four objectives numbered 0, 2, 4, and 7,
two glass compensator plates, and the quartz wedge mounted on glass.
An earlier version of the Model III from 1893 is also represented
in this collection. The version shown on this page differs from the
earlier version mainly in having a larger size, in the shape of the limb,
and the use of a wider tube taking a 30 mm ocular. The mechanical stage
on the microscope is an additional refinement, not normally supplied with the standard model IIIa.
A trade card
attached to the door of the case indicates that the microscope
was retailed by Charles X. Dalton, Boston.
This microscope was purchase from the great-grandson of the original
owner, Dr. Milton K. Thompson. Dr Thompson's practice was in Muskogee,
Oklahoma area. His speciality was ear, nose, and throat surgery. It would
seem that he may also have had an interest in geology/mineralogy.