As they say, "The Weakest Link", is certainly true with microscopes. The optical train can be complex, with (possibly) a lamp, filters, iris diaphragms, mirrors, lenses, a condenser, a sample stage, objectives, beam splitters, prisms, eyepieces and cameras each playing a role in the image quality, including both inherent component quality and alignment.
The following sketches attempt to show how basic diascopic (transmitted light) and episcopic (reflected light) systems are generally constructed. They are intentionally vague, as I am not trying to show every detail of any given instrument. In fact, it is very rare for them to be set up horizontally like they are shown! For an upright microscope, rotate them counterclockwise, and for an inverted microscope, rotate them clockwise! Easy!
Not shown are fold mirrors and the effect of the prism set that is present in the binocular head. To include a dedicated camera in the system, often times a "trinocular" head is used, which has a beamsplitter in conjunction with the prism set. Many systems also include control of the polarization state of the incident and observed light, sometimes with dramatic effect.
They may also be outfitted with both types of illumination in one instrument. Often, episcopic systems are designed with objectives that are to be used with no cover slip in the optical path, called NCG (no cover glass), or other designations.
The "tube length" shown is an important parameter for objective selection. As shown, episcopic systems are often designed with a longer tube length than the diascopic counterpart. Standardized numbers for this parameter are 160 mm / 170 mm, depending on manufacturer and vintage, increasing to 210 mm for episcopic designs. While the 160, 170, and 210 mm tube lengths are finite, most modern high-end systems are almost all designed around "infinity corrected" objectives, which require an additional lens, typically of 200 mm focal length, in the path.
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