guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Microscope
The numerical aperture of the objective, and hence its resolution, can be increased by filling
the space between the front lens of the objective and the specimen with an immersion liquid
of suitably high refractive index (oil; n ~1.56). Thereby the refraction of light rays at the
interface between the cover glass and oil is minimised, and a wider cone of light rays enters
the objective (Fig. 1-4B).
For this purpose special objective systems with small focal length and short free working
distance have been designed: oil-immersion objectives. While “dry” objectives operating in
air do not have numerical apertures beyond 0.95 (the theoretical limit being N.A. = 1),
aperture values up to 1.40 can be achieved with oil-immersion objectives, depending on the
refractive index of the appropriate immersion liquid (water = 1.333; glycerine = 1.455;
immersion oil = 1.515; methylene iodide = 1.744).
Specific properties of the objective such as magnification, numerical aperture, optical tube
length, degree of aberration correction, and cover glass thickness are engraved on the outer
objective barrel (Fig. 1-5, Table 1). Objectives designed for polarized-light microscopy
consist of strain-free lens systems and are marked with the inscription P, PO, or Pol. Table 1
also gives the free working distance (FWD) between the specimen and the front lens of the
objective for selected objectives of some major manufacturers.
1.2.2 Ocular (Eyepiece)
Fine structures in the intermediate image are only resolved by the human eye if they are
viewed at visual angles >1'. Commonly, this requires a further magnification of the intermediate
image by the ocular. The optimal resolution is achieved if the total magnification of
the microscope is the numerical aperture of the objective multiplied by 500 to 1000:
M = M O * M L = 500 * N.A. ↔ 1000 * N.A.
If the total magnification lies below this range, finest structures in the intermediate image
remain invisible. If it is higher, the intermediate image is magnified without any further gain
in resolution (= empty magnification).
Raith, Raase & Reinhardt – February 2012
Modern oculars consist of two multi-lens components, the “eye lens” and the “field lens”,
that correct optical aberrations of the ocular itself and eliminate residual aberrations of the
intermediate image. A Periplan ocular, for example, contains seven lenses that are cemented
into a single doublet, a single triplet, and two individual lenses. A fixed internal diaphragm is
positioned in the focal plane of the ocular, between the “eye lens” and “field lens”
components, in focus with the intermediate image, and defines the circular field of view.
For polarized-light microscopy, an ocular with exactly adjusted crosshairs (or a crossed
micrometer disc) mounted on the fixed diaphragm not only provides the ‘N-S’ and ‘E-W’
reference directions for the vibration directions of the polarizer and analyzer, but also serves
to measure angles (Ch. 2.1, 4.2.1). For measuring and counting objects in a thin section, glass
discs with engraved linear and crossed micrometers or grids can be placed in the diaphragm
plane. By adjusting the height of the eye lens, diaphragm and reticule are brought into focus
with the intermediate image.
The specific properties of oculars are inscribed on their casing (Fig. 1-5, Table 1).
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