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eyepiece, objective, condenser, nosepiece
Aperture iris diaphragm - This device is part of
the substage condenser. It serves to control the angle of the cone of light emerging from
the top of the condenser. When adjusted so that the rear lens element of the objective, as
seen down the eyepiece tube (open), is just filled with light, the full numerical aperture
(NA) of the objective is being utilised. Under these conditions the objective provides
maximum resolution, but some glare may be present, which reduces image contrast. If the
aperture iris is adjusted to fill about 70% of the objective's rear element with light,
glare is reduced and contrast is improved, without significant lose of image detail.
Closing the iris further will increase contrast but image detail will be lost. A further
problem will be the introduction of details that are not actually present in the specimen.
Therefore, it is most import that this device not be used to control light intensity while
view the specimen. Set it as described, and leave it alone. There is a simple way to avoid
constantly removing the eyepiece to set the aperture each time you switch objectives.
Place a small dot of white paint on the adjustment lever near the point where it
enters the condenser. Adjust the iris for each objective in turn, placing a similar dot on
the condenser body inline with the dot on the lever. From then on simply move the lever
until its dot lines up with the one for the objective in use.
Body tube - This part supports the eyepiece and objective(s). It is critical that the tube be constructed so that these optics share a common axis. Most 20th century scopes with body tubes (i.e., not modular) are designed for a mechanical tube of either 160 mm, or 170 mm. Mechanical tube length is the distance from the top of the eyepiece tube to bottom of the Society threaded objective holder.
Coarse focus knob - As the name
suggests, this control (typically a pair, one on each side) moves either the body tube, or
the stage/substage, up or down in a quick manner. This is accomplished by means of a rack
and pinion assembly. The pinion is a toothed wheel (the knobs are attached to either end
of the axial) that rides along a diagonally grooved bar or "rack", attached to
the stage or body tube. A good coarse focus control will provide smooth, back lash free
movement, often adequate for initial focusing at magnifications as high as 400x. Examine
the workings of this control carefully when evaluating a scope (while observing a
specimen). It should operate smoothly if your viewing experience is to be a pleasurable
one. (Ideally, the rack and pinion surfaces should be completely free of grease when
conducting this test.)
Condenser - Probably the most overlooked component. Very few people recognise the importance of a good quality, properly adjusted condenser, including many professional microscopists. The purpose of the condenser is to illuminate the specimen with even, high intensity light. In the past this was achieved by focusing an image of the light source, often a candle or lamp wick, onto the specimen ("critical illumination"). These were large, even illumination sources so everything worked fine. However, things changed with the advent of the tightly wound, relatively small electric filament. Now you could see the filament superimposed on the specimen, most annoying. Fortunately a German scientist came up with a more appropriate technique. The electric lamp filament is focused onto the aperture iris diaphragm, which for optical reasons, prevents it from being visible during observation. The Koehler technique, as it was called, is used today. However, for practical reasons microscope lamps now have a diffuser filter in front of the filament. In addition, they are equipped with a simple condenser and iris diaphragm, referred to as a field diaphragm. The condenser is used to focus the iris onto the specimen. Opening the field diaphragm until it just fills the field of view eliminates extraneous light, cutting down on glare. If your lamp is not fitted out in this fashion keep the condenser as close to the under surface of the slide as possible (do not move it up and down to control light intensity). To avoid seeing the surface of the bulb while observing insert a diffusing filter into the filter holder below the condenser.
Condenser focus knob - This control moves the condenser unit up and down.
Draw tube - At one time "all good instruments" had a body tube equipped with an inner sliding draw tube. This tube enabled users to the correct mechanical tube length when certain accessories were screwed on between the eyepiece and objective, or when using objectives designed for longer mechanical tube lengths. Today few, if any scopes have such a device (for "infinity corrected" optics tube length is not an issue), in part due to an ever increasing trend to remove all user adjusted features (which also reduces cost). Limiting what users can do prevents a universal tendency among students, and others, to blindly tinker with knobs and other moving parts on a microscope. On the the down side good microscopists now have to settle for "average" quality images. Such is life.
Eyepiece (Ocular) - This optical component magnifies the "primary image" formed by the objective. In addition, because the primary image cannot be seen by the eye (but can be projected onto a sheet of paper), it converts it into one that can, the so called "real image". Eyepieces are found in a number of different designs. For viewing purposes the Kellner design is preferable. The top element is an achromatic doublet, and it provides a larger, flatter, better corrected field of view then the basic Huygenian design (normally furnished with 1950's and earlier stands). An added bonus is the higher eyepoint, which makes viewing more pleasurable (very high eyepoint models are available for spectacle wears). Although some eyepieces are designed to complement a specific series of objectives, for the most part they are interchangeable among manufactures. The vast majority are standardised at either 23 mm, or 30 mm outside diameter. An eyepiece cannot improve resolution of the image formed by the objective, but a poor quality one can degrade this image.
Eyepiece tube - A fixed tube into which the eyepiece is inserted. For mainstream, "professional" scopes, the inside diameter is either 23 mm, or 30 mm.
Fine focus knob - This control allows for precise
focusing of the specimen. Experienced microscopists use this control far more than the
coarse focus control (especially with today's parfocal objectives). It is absolutely
essential that this control work smoothly, with zero rebound effect (test - focus and
leave for five minutes - image should still be razor sharp). Therefore, this control
should be checked carefully under viewing conditions, before purchasing a new or used
microscope. It is VERY expensive to have a fine focus control repaired, especially on
Filter holder (carrier) - A swing-out circular carrier, or C-shaped frame, attached to the under side of the condenser body. Filters for reducing light intensity (neutral density), providing monochromatic light (specific colour - typically "daylight"), polarized light or introducing other special light characteristics are placed here. The diameter of holders has not been standardised. (Most are either 33 mm or 32 mm.) It is possible to ignore the holder altogether and place the desired filter directly over the light port in stands with built-in lamps (or in front of external lamps for that matter). When black and white photography was all the rage (back when the dinosaurs roamed), skill in the use of filters often determined the quality of the image. However, with today's colour and digital imaging, filters are not used that often.
Foot (base) - It rests on the bench top and supports the stage and body of the microscope, and in many cases also houses the lamp. A well designed base will ensure that the image does not dance about during focusing, or while manipulating the specimen. There are a vast number of different base designs.
Limb (arm) - The arm is attached to the foot (in scopes without an inclined viewing head by means of an "inclination joint") and supports the body tube. The shape of the arm, and the way in which the body is attached, are often used to illustrate the history of the microscope's development. Today most student and research stands (older term for a microscope without optics) have very liner, computer designed arms far different in appearance from the one shown in the photograph (which is a classical "Lister limb" - made about 1930). In such modern stands the "body tube" has been replaced by two removal parts, a viewing head and an objective changer, with the top end of the arm forming the middle section. This type of arm is very strong and can better support additional equipment, such as video cameras. (Also, as the stage, not the arm moves during set-up, there is no longer any concern about additional weight causing the body tube to drift downward, and out off focus.)
Mirror (inboard lamp) - At one time all stands came with a mirror, even when a base lamp was supplied. Combined with an outboard light source the mirror serves to direct light into the condenser. Except for specialised mirrors, all are second surface mirrors, in other words the silver coating is applied to the back, rather than front glass surface. In most cases there are two surfaces, a flat, or "plano" surface for directing a parallel light beam into the condenser, and a curved or concave surface for directly focusing light onto the specimen with the condenser removed (use with objectives of 10x or less). Always use the flat surface with a condenser. The silvering must be free of blemishes, if not they can appear as artefacts in the image. To my knowledge it is not economical to have such mirrors resurfaced, and finding replacements may be next to impossible. In other words, avoid a used scope with a damaged or missing mirror.
Nosepiece (objective changer) - A rotating device to which objectives are attached. Although it seems hard to understand today, such a convenience was not common place until the advent of the 20th century. The quality of objective changer is often a good indication of a microscope's overall quality. It should move smoothly, and most important, should have a distinct click or feel when an objective is properly "seated". Most older nosepieces can accommodate four objectives. However, if you have a choice between a four and five place unit, take the latter. As your skill improves the fifth spot will prove useful.
Objective - This, together with the condenser, is the microscope! If you have a poor quality objective nothing you can do will change this. A bit like car horsepower, "what you got is what you got, there ain't no more". It is much better to start with two good quality objectives then four mediocre ones.
While modern, fixed focus (not infinite focus) objectives have a standardised "adjustment length" (so called "DIN standard"), and are Society threaded, the degree of optical correction varies. (For a brief overview visit - http://www.btrip.mednet.ucla.edu/bri/lenses.htm)
Objective lenses are very tiny and as a result great care is need to form and assemble such lens systems. This generally translates into time, which in turn translates to cost. Furthermore, it is of paramount importance that manufactures of such equipment maintain a very high level of quality control. Keep this in mind when shopping for objectives (complete microscopes as well).
An important point to consider when buying older objectives is compatibility. Before DIN was universally adopted by main stream manufactures objectives were generally shorter, typically having an adjustment distance of 37 mm (measured from the shoulder of the attached objective to the plane of focus in the specimen). By contrast DIN objectives are much longer, with an adjustment distance of 45 mm. (The longer barrels provide room for additional of lenses needed to improve field-of-view and field flatness.) So what does all this mean? Combining long and short barrel objectives destroys one of the unique features of 20th century microscopes - parfocality. A revolving nosepiece permits rapid changeover between objectives. In practical terms it is essential that the focus of the image be preserved during the change of objectives. Parfocal objectives allow this to happen (at worse only slight refocusing is required). If you mix DIN and short barrel objectives you will be constantly refocusing - or you will not be able to focus at all if the stand is designed for 37 mm objectives. Ergo, do not mix short barrel and DIN objectives. As the difference in length between the two types of objectives is quite significant they are relatively easy to distiquish (always measure from the top of the objective shoulder to the surface of the front element). As a general rule a "short barrel" objective will always be less than 37 mm in length, a DIN objective will always more than 37 mm (10x and higher).
Stage clips - These are the basic stage slide holders. Supplied in pairs, they are adequate for general slide manipulation up to a maximum of 400x (if properly adjusted, which can be tricky). In the hands of a skilled operator a good pair can serve very well in this magnification range. However, nothing can beat a well made "mechanical stage". (If your scope lack clips elastic bands may do in a pinch.)
Stage - This is the platform or "stage" that supports the specimen (which are typically mounted on glass slides). To do this job properly it must be perfectly perpendicular to the optical axis, dead flat and of adequate size. A microscope with a dinged or out of line stage should be avoid. As mentioned above mechanical stages are often supplied. They may be integrated into the stage itself (with the stage deck moving rather than the slide) or they can be attached. Either way they make the life of a microscopist more enjoyable. However, as movement as well as size are magnified when using a microscope, they must be well made and in top shape. This is another item to be scrutinised under viewing conditions when shopping for a scope. As with the focus controls, movement must be smooth and backlash free.