Microscopes provide a means of greatly enlarging objects so that we may determine their structure and composition. Microscopes are arranged so that we can observe either reflected light from magnified objects (for example, when looking at sand grains or insects) or transmitted light that has passed through very thin sections of rock or biological material. In reflected-light microscopy we usually look principally at the structure and texture of objects, although quantitative measurements of diagnostic optical and physical properties may be made on polished specimens with specialist high-power microscopes. These features may be diagnostic of composition, of function or of history, for example, the characteristic fracture pattern of quartz, the details of insect mandibles revealing their function and mode of operation, or the imprint of successive modes of abrasion on sand grains showing some of the past processes they have been subjected to. Transmitted-light microscopy may also be used to examine details of internal structure and the relationships between mineral components of rocks. It may show us the inner structure of biological systems (as in the stem of a plant), the form of crystals, inclusions and zoning of minerals, or the nature of grain boundaries to distinguish sedimentary rocks from other types. A more sophisticated type of microscopy is the examination of thin sections of rocks and minerals using polarized light to see both structural features and also effects related to optical properties that allow precise identification of the minerals present. These effects may take the form of spectacular colours and of colour changes of the minerals with orientation to the light.
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I. N. McCave
B. W. D. Yardley
- Springer Netherlands
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