EVOLUTION OF THE PETROGRAPHIC MICROSCOPE AND ITS IMPACT ON ADVANCES IN THE MINERALOGICAL AND GEOLOGICAL SCIENCES

The study of minerals using polarized light was well established in the early 1800s due to the pioneering studies of Brewster in England and Biot in France. The innovative work by Sorby in England in the mid-1800s subsequently affirmed the utility of the microscope for the geological sciences. The petrographic microscope, a specialized form of polarized light microscope, was designed by Rosenbusch in 1875 to facilitate quantitative measurements of mineral properties.

Evolution of the mechanics and optics of the petrographic microscope in the late 1800s, in conjunction with an increasing body of published data, greatly facilitated geological work; rocks previously deemed intractable to study became identifiable with the microscope. The early emphasis was on descriptive petrology, evidenced by the detailed chromolithographs published in the late 19^th century; such studies were largely superceded by applied and theoretical studies in the 20^th century.

Although European scientists, such as Rosenbusch and Zirkel in Germany, and Michel-Lévy and Lacroix in France, had embraced petrological studies earlier than their American counterparts, by the late 1800s microscopical petrography had taken hold in America, largely as a result of the publication of Zirkel's 1876 treatise on Microscopical Petrography as part of King's Geological Exploration of the 40^th Parallel. In general, European workers focused more on the academic and theoretical aspects of mineral optics, whereas American scientists concentrated on more the practical elements of petrographic studies.

Mechanical improvements in microscopes throughout the 20^th century included a progression toward heavier stands that were designed to accommodate an increasing array of accessories to facilitate the accurate measurement of a variety of optical properties. Optical changes included the replacement of calcite prisms by Polaroid, and achromatic objectives by apochromatic flat field optics.

Throughout most of the 20^th century the petrographic microscope remained a mainstay of geology and mineralogy laboratories. Its decline began in the 1970s with an increasing reliance on microbeam analytical methods. The petrographic microscope remains an effective tool for mineral identification, and it is unfortunate that its utility is often overlooked.