Page 26 - A Practical Introduction to Optical Mineralogy

THE MICROSCOPIC STUDY OF MINERALS THE REFLECTED-LIGHT MICROSCOPE
(a) The cover glass or coated thin glass plate (Fig. 1.4a). This is a Objectives
simple device, but is relatively inefficient because of light loss both Objectives are magnifiers and are therefore described in terms of their
before and after reflection from the specimen. However, its main magnification power, e.g. x 5. They are also described using numerical
disadvantage when at 45° inclination is the lack of uniform extinc- aperture (Fig. 1.5), the general rule being the higher the numerical
tion of an isotropic field . This is due to rotation of the vibration aperture the larger the possible magnification. It is useful to remember
direction of polarised reflected light which passes asymmetrically that, for objectives described as being of the same magnification, a
through the cover glass on returning towards the eyepiece. This higher numerical aperture leads to finer resolved detail, a smaller depth
disadvantage is overcome by decreasing the angle to about 23° as of focus and a brighter image. Objectives are designed for use with either
on Swift microscopes. air (dry) or immersion oil between the objective lens and the sample.
(b) The mirror plus glass plate or Smith illuminator (Fig. 1.4b ). This is The use of immersion oil between the objective and sample leads to an
slightly less efficient than the cover glass but, because of the low increase in the numerical aperture value (Fig. 1.5). Immersion objec-
angle (approaching perpendicular) of incidence of the returning tives are usually engraved as such.
reflected light on the thin glass plate, extinction is uniform and Low power objectives can usually be used for either transmitted or
polarisation colours are quite bright. This illuminator is used on
reflected light, but at high magnifications(> x 10) good images can only
Vickers microscopes. be obtained with the appropriate type of objective. Reflected-light
(c) The prism or total reflector (Fig. 1.4c). This is more efficient than objectives are also known as metallurgical objectives. Achromatic
the glass plate type of reflector but it is expensive. It would be 100 objectives are corrected for chromatic aberration, which causes colour
per cent efficient, but half of the light flux is lost because only half fringes in the image due to dispersion effects. Planochromats are also
of the aperture of the objective is used. A disadvantage is the lack corrected for spherical aberration, which causes a loss in focus away
of uniform extinction obtained. A special type of prism is the triple from the centre of a lens; apochromats are similarly corrected but suffer
prism or Berek prism, with which very uniform extinction is from chromatic difference of magnification, which must be removed by
obtained because of the nature of the prism (Hallimond 1970,
use of compensating eyepieces.
p. 103). Prism reflectors are usually only available on research
microscopes and are normally interchangeable with glass plate
reflectors. One of the disadvantages of the prisms is that the
objective x20 immersion
incident light is slightly oblique, and this can cause a shadow effect
NA = 0.45
on surfaces with high relief. Colouring of the shadow may also
occur.
Figure 1.4
Incident oil , n = 1.52
to eyepiece
illuminators.
IV

surface in focus
resolution ,
optic axis of ---1 d = 0.61-lm
microscope (A. = 550 nm)
1
Figure 1.5 Numerical aperture and resolution. N.A. = n sin p., where
N.A. = numerical aperture, n = refractive index of immersion medium, and
p. = half the angle of the light cone entering the objective lens (for air, n = 1.0).
d = 0.5 A./N.A. where d = the resolution (the distance between two points that
sample
can be resolved) and A. is in microns (1 micron = 1000 nm). The working distance
(w in the diagram) depends on the construction of the lens; for the same
(a) Cover glass (b) Smith illuminator (c) Prism illuminator magnification, oil immersion lenses usually have a shorter distance than dry
illuminator
objectives.
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