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== Creation of a figure ==
In [[optical mineralogy]], a [[petrographic microscope]] and cross-[[polarised light]] are often used to view the interference pattern. The [[thin section]] containing the mineral to be investigated is placed on the microscope stage, above one linear polariser, but with a second (the analyser) between the objective lens and the eyepiece. The microscope's [[Condenser (optics)|condenser]] is brought up close underneath the specimen to produce a wide divergence of polarised rays through a small point, and light intensity increased as much as possible (e.g., turning up the bulb and opening the diaphragm). A high power objective lens is typically used. This both maximises the solid angle subtended by the lens, and hence the angular variation of the light intercepted, and also increases the likelihood that only a single crystal will be viewed at any given time.
To view the figure, the light rays leaving the microscope must emerge in parallel. This is typically achieved either by pulling out the eyepiece altogether (if possible), or by placing a [[Bertrand lens]] (Emile Bertrand, 1878) between the objective lens and the eyepiece.
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