Polarized Light Microscopy for Determining the Principal Refractive Indices of Components in Polished Thin Sections (Part 2 of 2)

摘要

It is now clear why Sorby's (58) RI determinations with a low power objective of 2/3 inch (16.9 mm focal length, corresponding to a magnification of about 10×), were consistently too high by 0.010 for minerals of RI about 1.5 and too high by 0.018 for minerals with a RI of approximately 2.0. Sorby's discrepancy can be attributed to his use of the original Chaulnes equation, which incorrectly assumes a slope of one for the objective. The difference between a hyperbola and a line is still relatively small for a 0.6 NA objective, e.g., when solved for n, Equation 5 is approximately linear for a 0.6 NA objective in the range of RIs encountered, as shown in Figure 2 for the curve of n versus d/s, based on the data in Lawless and Devries (46).