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c. 1910


Serial number: 12345

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petro tube This special petrographic tube has a fixed tube length of about 7 1/2 inches (180 mm, one of the German standards). This tube is not listed nor pictured in any Watson catalog that I have seen. It also has a feature different from any other Watson microscope that I know of; this is a wheel of selenites above the nosepiece analyzer.

noseAttached to the end of the petrographic tube is the special fitting for the five centerable objective mounts carrying Oehler objectives; the knob securing the objectives screws tighter in the counterclockwise direction, and features a tapered bottom that fits into the curved tapered side of the objective mounts. Immediately above that is a window slot for wave plates, or the calcite slider, with a shutter controlled by a tiny knob. Above the wave plate slot sits the nosepiece analyzer. The analyzer can be slid out of the optical axis, but its vertical position cannot be varied.

A rotating disk with a blank and three different aperture elements, sits above the nosepiece analyzer, and can be raised or lowered via a small knob on the opposite side; the first element fills the entire ring and is a retardation plate, probably a first order red. The other two elements are low power lenses that can be used to examine a subject without an objective in place. These two elements seem to be identical to each other. This disk can move up or down to match the focus of these low power lenses to the eyepiece one is using.

Next, on the upper part of the tube, is the Bertrand lens which can slide in or out of the axis and also can be focused up or down to properly focus on the rear focal plane of the objective. Above the Bertrand lens and just above a knurled ring, is another window with its shutter controlled by the knurled ring rotating around the tube. This shutter is for admitting an eyepiece micrometer.

petro eyepiece A special 'best large size' crossed web eyepiece fits the top of the petrographic tube. It has a slot to line up with a screw on the tube so it does not rotate. It also has a slot which matches the shutter in the tube which sits above and is controlled by, a knurled ring. The eyepiece micrometer fits into this slot. This eyepiece has a top element that can, via screwing it in or out, focus on the micrometer rulings.

gonio An eyepiece analyzer, fits on top of the outer optical tube and also has a registration slot and also has windows for the waveplate to sit above the eyepiece. This analyzer has a silvered scale engraved in single degrees, numbered every ten, and with click stops every forty five degrees. The wave plate, or calcite slider with this microscope, can also fit below the retractable analyzer in the nosepiece. This analyzer is designed to be used with the larger eyepiece pictured above

polarizer Watson Pol EntryThe petrographic polarizer 'condenser' is also calibrated, can rotate in its housing and has stops every 45 degrees with a pointed indicator. This is the large size sold by Watson.

plates Two sliders came with microscope for use with the petrographic tube. One is an eyepiece micrometer slider, which fits into the slot at the bottom of the cross-webbed eyepiece. The other is a calcite slider plate which could be placed in front of the goniometer prism analyzer or in front of the nosepiece analyzer. The calcite slider is used mainly to confirm exact extinction for crossed polars.