guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Conoscopy Figure 4-47. Light paths in the polarized-light microscope Raith, Raase & Reinhardt – February 2012 A: Orthoscopic illumination mode. In finite tube-length microscopes, the objective produces a real inverted image (intermediate image) of the specimen which then is viewed with further enlargement through the ocular (A-2). In infinity-corrected microscopes, the objective projects the image of the specimen to infinity, and a second lens placed in the tube (tube lens) forms the intermediate image which then is viewed through the ocular (A-1). This imaging design allows to insert accessory components such as analyzer, compensators or beam splitters into the light path of parallel rays between the objective and the tube lens with only minor effects on the image quality. B: Conoscopic illumination mode. Parallel rays of the light cone which illuminates the specimen create an image in the upper focal plane of the objective (B). In the case of anisotropic crystals, an interference image is generated which can be viewed as an enlargement by inserting an auxiliary lens (Amici-Bertrand lens). The interference image can also be directly observed in the tube through a pinhole which replaces the ocular. 113
Guide to Thin Section Microscopy Conoscopy 4.2.5.2 Conoscopy of optically anisotropic crystals In the conoscopic mode, waves deviating from vertical incidence by increasing tilt angles travel increasing distances within the birefringent crystal. According to the relationship Γ = d * (n z ' – n x ') described in chapter 4.2.3, it has to be concluded that the retardation Γ of the waves increases with increasing angles due to the continuous increase of d' (= length of the light path in the crystal plate). Accordingly, the interference colours in a conoscopic interference figure should generally increase outwards. However, the interference figure is to a much larger extent controlled by the orientation of the vibration directions and the birefringence values of the wave couplets within the observed volume of the anisotropic crystal. As described previously, the birefringence for waves parallel to optic axes is zero. It increases as the angle between the optic axis and ray propagation direction (or wave normal, to be precise) increases. Raith, Raase & Reinhardt – February 2012 Figure 4-48: Generation of an interference figure in the upper focal plane of the objective by imaging of parallel sets of rays that pass the crystal plate at different angles. The example shows these relationships for a uniaxial crystal (calcite) cut perpendicular to the optic axis. The geometry of interference figures obtained from anisotropic crystals can be illustrated with the model of the skiodrome sphere developed by Becke (1905). The crystal is considered to occupy the centre of a sphere. Each ray propagation direction of light waves within the crystal has a corresponding point on the spherical surface where the ray pierces that surface. In each of these points, the vibration directions of the related waves can be shown as a tangent (e.g., O and E in case of optically uniaxial minerals). When connecting all the tangents of equal vibration direction, a geometric mesh of vibration directions is generated that depends on the optical symmetry of the crystal (Fig. 4-49,50). 114
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