Petrographic Microscope

The petrographic microscope - also called the polarizing light microscope - is the standard instrument for identifying minerals by their optical properties. All designs share the same fundamental layout: from the bottom up, the instrument consists of an illuminator, a substage assembly, a rotating stage, objective lenses, an upper polarizer, a Bertrand lens, and ocular lenses. A slot above the objective lenses accepts optical accessory plates. ^[1]

Illuminator

The illuminator sits in the base of the microscope. It contains an incandescent bulb or LED, controlled by a rheostat for brightness adjustment. Mirrors and lenses direct the light upward and filters adjust the color balance to approximate natural sunlight. ^[1]

Substage Assembly

The substage assembly carries the lower polarizer, the condensing lens, the auxiliary condensing lens, and the aperture diaphragm. It can be raised or lowered on many instruments by a gear mechanism. ^[1]

The lower polarizer is a sheet of optical-quality polarizing film that converts the illuminator’s ordinary light into plane-polarized light before it reaches the sample. On most instruments it is fixed so that the transmitted vibration direction is east-west as seen down the microscope tube, though on older instruments it may be north-south. Polarizers are sometimes called Nicols, after the calcite Nicol prisms used on earlier instruments. ^[1]

The condensing lens focuses the illuminator light onto the area of the sample just beneath the objective, producing orthoscopic illumination - light that arrives at the sample in a nearly parallel beam. The auxiliary condensing lens can be swung into the optical path to produce conoscopic illumination, which is strongly converging light used to examine interference figures in combination with the Bertrand lens and a high-power objective. ^[1]

The aperture diaphragm is an iris diaphragm that controls the size of the cone of light passing up through the microscope. Closing it reduces the cone size and improves image contrast. It is not intended for brightness adjustment - that function belongs to the illuminator rheostat. ^[1]

Stage

The circular stage is mounted on bearings for smooth rotation. Stage rotation angles can be measured with a goniometer and vernier scale on the stage rim. A thumbscrew or lever locks the stage in position when needed.

Objective Lenses

The objective lenses provide the primary magnification of the system. Student-model microscopes typically carry three objectives - approximately 2.5×, 10×, and 40× - on a rotating nosepiece. Lens markings include magnification, numerical aperture, tube length, and coverslip thickness. Oil-immersion objectives are marked “oil” or “oel.” ^[1]

The numerical aperture (NA) is defined as NA = n sin(AA/2), where n is the refractive index of the medium between the lens and the sample, and AA is the angular aperture - the half-angle of the cone of light the lens can accept. A larger NA gives better resolving power but requires a shorter free working distance (the gap between the lens end and the sample surface). Most student high-power lenses have NA = 0.65. The NA of the high-power objective is needed when interpreting interference figures. ^[1]

Upper Polarizer (Analyzer)

The upper polarizer, also called the analyzer, is positioned above the objective lenses on a slide or pivot so it can be inserted into or removed from the optical path. Its vibration direction is at 90° to that of the lower polarizer. When inserted, the polarizers are described as crossed - with no sample on the stage, the field is entirely dark because plane-polarized light from the lower polarizer cannot pass the orthogonally oriented upper one. When the analyzer is removed, the microscope operates in plane light. ^[1]

Bertrand Lens

The Bertrand lens (also called the Amici-Bertrand lens) is a small optical element mounted just below the ocular on a pivot or slide. When inserted, it redirects the observer’s view to the back focal plane of the objective, where interference figures form. It may carry an iris diaphragm or pinhole to restrict the field, and some instruments include a focusing adjustment. ^[1]

Oculars

The oculars (eyepieces) slide into the upper end of the tube and magnify the objective’s image for the human eye. Ocular magnification is typically in the range of 5 to 12×. Total microscope magnification equals objective magnification multiplied by ocular magnification - a 40× objective with a 10× ocular yields a total magnification of 400×. ^[1]

Binocular microscopes have two ocular lenses - one fixed-focus and one adjustable to compensate for differences between the observer’s eyes. Reticle markings, including crosshairs and a micrometer scale, are mounted in the ocular. High-eye-point oculars are available for observers who wear glasses.

Accessory Plates and Direction Conventions

A slot above the objective accepts accessory plates - metal or plastic frames holding an optical element. The two standard plates are the gypsum (full-wave) plate and the mica (quarter-wave) plate, used for measuring optical properties such as fast and slow ray directions and optic sign. ^[1]

Compass directions describe orientations in the microscope image by convention: north is toward the top, south toward the bottom, east to the right, and west to the left. The diagonal positions are NE-SW and NW-SE. ^[1]

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