The Compound Microscope
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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 back lens of the objective, as viewed through the eyepiece tube, is just filled with light, the full numerical aperture (NA) of the objective is being utilized. 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 75% of the objective's the back lens this is reduced and contrast is improved, without significant lose of image detail. Closing the iris further will increase contrast but some image detail will be lost. A further problem will be the introduction of details ("artifacts") that are not actually present in the specimen. Therefore, it is very import that the iris not be used to control light intensity. (You can simply this process by placing a small dot of white paint on the iris adjustment lever near the point where it enters the condenser. Now set the iris for each objective in turn, placing a similar dot on the condenser body inline with the dot on the lever. Lining up the lever dot, with the respective objective mark, will give you the correct iris setting - no need to remove the eyepiece.)
This part supports the eyepiece and objectives. 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 Adjustment 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.)
This probably the most overlooked microscope component. Very few people recognize the importance of a good quality, properly adjusted condenser, including many professional microscopists. It is a vital part of the illumination system, and is designed to collect, control and concentrate light from the lamp onto the specimen. As with objectives, the optical elements can introduce a variety of aberrations which are corrected to varying degrees, depending on the type of condenser one is using. Condensers (as well as filters and objectives) are available to provide specialized, contrast enhancing illumination such as darkfield, Rheinberg, polarization, differential interference contrast, and phase contrast. If you are using a basic bright field condenser, with a simple lamp or daylight (cloudy sky - not direct sunlight) ensure that it is racked all the way up, so that the top element is almost touching the under side of the slide. Do not move the condenser up and down to adjust light intensity. To ensure maximum contrast and resolution remove the eyepiece, look into the tube and open the condenser iris until about two thirds of the objectives rear element is filled with light. As with moving the condenser, the iris diaphragm (aperture iris) should not be used to control field brightness. If the field of view is too bright use a neutral density filter, or lamp rheostat to correct this problem. Microscopes with internal or external lamps that include a field diaphragm and adjustable condensing system can be set up for Köhler illumination. This technique provides the best possible illumination.
Condenser focus knob
This control is used to precisely adjust the vertical height of the condenser. A good quality control mechanism will be appreciated once you get into Koehler illumination, or use different condensers that need carefully focused.
At one time "all good instruments" had a body tube equipped with an inner sliding draw tube. This tube enabled users to the adjust the 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's microscopes no longer have or need such a device.
Eyepieces are produced 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 large, flat, well corrected field of view, compared to the basic Huygenian design (often supplied with 1950's and earlier stands). An additional bonus is the higher eye point, which makes viewing more pleasurable (very high eye point models are available for spectacle wears). Although some eyepieces are designed to complement a specific series of objectives (compensating eyepieces), for the most part, they are interchangeable among manufactures. The vast majority are standardized at either 23 mm or 30 mm outside diameter. An eyepiece cannot improve the inherent resolution of the image formed by an objective, but one of poor design can degrade it.
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 (e.g. - set the focus and leave for five minutes, the image should remain razor sharp). Therefore, this control should be checked carefully under viewing conditions, before investing in a microscope. It is VERY expensive to have a fine focus control repaired, especially on older models.
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 near monochromatic light (colour specific - e.g. "daylight"), polarized light or introducing other special lighting characteristics are placed here. The diameter of holders has not been standardized although most range between 30 and 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 captured image. However, with today's digital colour imaging, coloured filters are not used that often.
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.
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 (or internal 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 specialized 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 artifacts 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.
This, together with the condenser, is the microscope! If you have a poor quality objective nothing you can do will change this. 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 standardized "adjustment length" (so called "DIN standard"), and are Society threaded, the degree of optical correction varies. In many cases the barrel of the objective is engraved with information on its optical characteristics.
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 barrel provides more room for wider lens combinations 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. Therefore best not to 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 distinguish (always measure from the top of the objective shoulder to the surface of the front element). As a rough 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), powers above 40x almost 45 mm.
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.)
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 in one axis rather than the slide holder) 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 be maintained in top shape. This is another item to be scrutinized under viewing conditions when shopping for a scope. As with the focus controls, movement must be smooth and backlash free.