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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 [[Optical microscope#Stage|stage]], above one [[linear polarizers|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 more or less 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.
Any crystal section can in principle produce an interference pattern. However, in practice, only a few different crystallographic orientations are both 1. convenient to identify, to allow a figure to be produced, and 2. able to produce reliable information about crystal properties. Typically, the most useful and easily obtainable orientation is one looking down the [[Optic axis of a crystal|optic axis]] of a crystal section, which yields a figure referred to as an '''optic axis figure''' (see below). Such crystal orientations are findable in thin section by looking for slices through minerals which are not isotropic but that nevertheless appear uniformly black or very dark grey under normal cross-polarised light at all stage angles (i.e., are "[[Extinction (optical mineralogy)|extinct]]"). If you are far from looking down an optic axis, a ''flash figure'' may be seen - a higher order birefringence colour, interrupted four times as the stage is rotated through 360 degrees by "flashes" of black which sweep across the field of view.▼
▲Any crystal section can in principle produce an interference pattern. However, in practice, only a few different crystallographic orientations are both 1. convenient to identify to allow a figure to be produced, and 2. able to produce reliable information about crystal properties. Typically, the most useful and easily obtainable orientation is one looking down the [[Optic axis of a crystal|optic axis]] of a crystal section, which yields a figure referred to as an ''optic axis figure'' (see below). Such crystal orientations are findable in thin section by looking for slices through minerals which are not isotropic but that nevertheless appear uniformly black or very dark grey under normal cross-polarised light at all stage angles (i.e., are "extinct"). If you are far from looking down an optic axis, a ''flash figure'' may be seen - a higher order birefringence colour, interrupted four times as the stage is rotated through 360 degrees by "flashes" of black which sweep across the field of view.
== Characteristic figures of uniaxial and biaxial minerals ==
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