Lens Bertrand

The Bertrand lens, an auxiliary lens that is focused on the objective back focal plane, is inserted with the sample between fully crossed polarizers, and the sample is oriented to show the lowest retardation colors. This will yield interference figures, which immediately reveal whether the sample is uniaxial (hexagonal or tetragonal) or biaxial (orthorhombic, monoclinic, or triclinic). Addition of the compensator and proper orientation of the rotating stage will further reveal whether the sample is optically positive or negative. 

Bernoulli principle, 11 656-657 Berry pseudo-rotation, 16 62 Bertrandite, 3 638, 640-641 Bertrand lens, 16 470-471 Beryl, 3 638, 640 color, 7 329... 

Orient needle in light path and switch to the back focal plane of the objective (usually labeled B for Bertrand lens). Find the needle and lower it until just above the cells then switch back to the normal focal plane for viewing. 

Extinction straight or oblique if oblique angle Interference colours and order With transmitted cros.sed-polarized light and Bertrand lens ... 

Bertrand lens a removable lens in the tnbe of a petrographic microscope that is nsed to examine interference fignres. 

Bertrand lens n. An auxiliary, low-power lens which maybe inserted in the bodytube of the microscope between the eyepiece and the objective for observing the back focal plane of the objective useful for conoscopic observations in crystal optics and for checking microscope illumination quality. 

Conoscopic observation n. The study of the back focal plane of the objective by removing the eyepiece, by inserting a Bertrand lens, by examining the image at the eye-point above the eyepiece with a magnifier or by using a phase telescope is called conoscopic because the observations are associated with the cone of light furnished by the condenser and viewed by the objective (cf., orthoscopic). 

Bertrand lens A lens that, when inserted into the optical train of a microscope, allows the viewer to see an image of the filament used to establish Koehler illumination. A lens that focuses on the rear apeture of the objective lens assembly. 

To visualize that picture, we simply need to remove the eyepiece, or to introduce a so-called Bertrand Lens above the analyzer. In this case an interference pattern appears which divides the field of view to four quadrants by the so-called Maltese crosses (Figure 5.13). For uniaxial materials the arms of the cross (so-called isogyres) are parallel to the polarizers, and the center of the pattern is dark. 

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