Polarized Light Microscopy for Determining the Principal Refractive Indices of Components in Polished Thin Sections (Part 2 of 2)
THE MICROSCOPE
2015, Volume 63:4, pp. 173–187
DOI
https://doi.org/10.59082/UZLX7935
AUTHORS
Franz W. Nentwich
ABSTRACT
This is the second part of a two-part article. The first part was published in Vol. 63, No. 3.
In 1767, the Duc de Chaulnes determined the refractive index (RI) n of a glass plate with a compound microscope from the equation, Refractive Index ≈ Real Thickness/Apparent Thickness. This method was adapted by Sorby in 1877 to determine the principal RIs of mineral plates. For thin sections the method was hampered by the difficulty of identifying a section’s underside with transmitted light, but this surface can be identified easily with incident darkfield illumination. Another complicating factor, the effect of permanent coverslips can be avoided with uncovered polished thin sections. Transmitted circularly polarized and circularly analyzed light expedites the selection of suitably oriented anisotropic crystals, and guided by their interference figures, these crystals can be rotated into the orientations needed to measure principal RIs. The ω RI of uniaxial crystals can be determined from crystals in any random orientation, while the ε, α, β and γ indices can be determined with a narrow slit, the long axis of which is oriented perpendicular to the analyzer’s privileged direction. Accuracy is attained by calibration with a RI standard and by accurate measurements of axial translations of the microscope stage relative to the microscope tube.