Parts of a Light Microscope

Eyepiece

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The eyepiece of an optical microscope produces a "real image", meaning that light actually passes through the image - as opposed to simply appearing to have come from the image.
The eyepiece of an optical microscope can usually be removed and dismantled so that an eyepiece graticule can be inserted into the arrangement of lenses that form the eyepiece if/when the microscope is used for measuring distances e.g. sizes of cells, organelles, etc..
Note that the eyepiece magnifies the image produced by the objective lens (typically by x10), but it does not resolve the image.

Barrel

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The barrel is the upper part of the microscope and the part through which rays pass between the eyepiece (above) and the objective lens (below). The barrel can usually be moved, i.e. adjusted upwards or downwards, in order to focus the microscope. That is, the precise position of the barrel may be adjusted to improve the quality of the image seen by the eye by moving the positions of the lenses relative to the position of the specimen on the stage.

"Arm" / "Neck"

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The part of an optical microscope that arches backwards - so on the other side of the microscope from the user when he or she is sliding a specimen forwards into place - is sometimes casually referred to as the "arm" (or occasionally the "neck") of the microscope. This is not part of the optical path of the microscope but is the mechanical support that holds the upper part in place.

Turret

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The turret is the part of the light microscope that holds the objective lenses. Although only one objective lens is used at any one time, microscopes usually have several objective lenses of different magnifications. The turret is rotated by the user when he or she selects which objective lens to use and when the user changes the objective lens in use e.g. to change magnification.

Objective lenses

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Light microscopes are usually fitted with several objective lenses but only one objective is in use (hence in the optical path) at any one time. The objective lens performs both magnification and resolution of the object (specimen).

Specimen (object)

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The specimen (term used in biology) is the object (term used in optics, the area of physics concerned with light and other parts of the electromagnetic spectrum) of the optical system formed by the light microscope when correctly set-up.
The specimen has usually been carefully prepared for viewing using a light microscope and must be sufficiently thin and/or transparent enough to be effectively illuminated from the opposite side from that of the eyepiece, i.e. samples on microscope slides are very thin because they are illuminated from below yet sufficient light to form a good quality image must reach the eyepiece which is above the slide.

Stage

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The microscope stage holds the specimen in position at 90^o to the light path. That means perpendicular to an imaginary line between the centre of the light source (below) and the centre of the eyepiece (above).

Condenser (lens)

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The condenser is a lens that focuses light on to the specimen. Even illumination is essential in order to obtain a clear and meaningful images. This is usually a simple matter of the microscope manufacturer selecting an appropriate condenser lens, which may not be adjustable by the user.

Iris diaphragm

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In the same way as the iris of the eye, the iris diaphragm in the illumination light path of light microscope controls the amount of light available to reach the specimen, and therefore ultimately the eye of the person using the microscope.

Substage illumination (lighting)

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Location of light source: The only illumination of the specimen should be from the substage position. Additional illumination from other, e.g. higher, positions reduce the contrast in the final image which reduces the quality of the image seen by the user.
Colour of light source: "White" light, which is a combination of a wide range of wavelengths, is frequently used but light of predominately shorter wavelengths e.g. blue light enables a higher resolution image to be formed.

Stand / Base

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The "stand" or "base" is not part of (within) the optical path but is an important part of the outer mechanical structure of the microscope. As demonstrated in the following microscope video demo, it is important to support the base of a light microscope when carrying or moving it.