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

The development of the petrographic microscope near the end of the 19th century facilitated rapid advances in mineralogy and geology. Early studies in France and Germany elucidated the identification of important rock-forming minerals, including the feldspar, amphibole, pyroxene, and mica group minerals. Rocks could thereafter be classified based on trace mineral constituents, and fine-grained rocks which were previously intractable could also be classified by microscopical studies.

Improvements in mechanical and optical design led to increasingly accurate measurements of the optical properties of minerals. Microscopes continued to evolve toward heavier, modular units that accommodated an increasing array of sophisticated accessories. The commercial introduction of Polaroid in the mid-1940s facilitated the adaptation of binocular heads, while optics evolved from simple achromats to flat-field, coated lens systems.

Methods in microscopical petrography were advanced in the mid-19th century by Sorby in England, subsequently taken up by Rosenbusch and Zirkel in Germany, and thereafter adopted by King in the United States. By the early 20th century, American workers had made notable contributions, particularly in their introduction of reflected light methods for ore microscopy. The early emphasis on descriptive petrology (characterized by the magnificent chromolithographs of the late 19th century) was supplanted in the 20th century by applied and theoretical studies.

Surprisingly accurate work was done in the late 19th and early 20th centuries with instruments that many would consider primitive by today’s standards. These instruments show an elegance in form and function not seen in modern microscopes. It is important to recognize the historical significance of these microscopes and accessories, and to insure their preservation as a part of our mineralogical heritage.

The decline in the use of the petrographic microscope began in the mid-1970s. However, it remains one of the most effective tools for mineral identification, and it is unfortunate that it is now often overlooked.