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PLEASE NOTE: This glossary is a work in progress and is being edited and added to all the time. Some links may not yet be active so please be patient.

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Literally 'algae seeker.'   -A small simple microscope of moderate power designed to determine if a given sample of water contains diatoms or similar life forms. The term was first used by Edourd Thum who sold such devices, of cylindrical form and of at least moderate magnifying power.

free of chromatic aberration.

A lens system used above a specimen that has usually had polarized light projected through it. This upper system is usually oriented 90 degrees to the polarizer, thus blocking out all light rays not changed by the specimen itself. Without an anisotropic specimen, the field will appear black when both analyzer and polarizer are present and oriented 90 degrees from each other; this is known as extinction. Analyzers can form part of a special eyepiece (eyepiece analyzers), can take the form a removable extension of the nosepiece (nosepiece analyzers) or be integrated into the main optical tube, often with facility for throwing them out of the optical path.

ANIMALCULE(S): Microscopic life form(s), especially protozoa.


ANISOTROPIC: In microscopy, a material or specimen which changes the orientation of polarized light and is therefore visible between crossed polars. These materials are called birefringent.

APLANATIC: A lens in which spherical aberration is eliminated by the use of compensating optics; should not be confused with flatness of field.

ARM: Part of a compound microscope that supports the main optical tube. This is usually used to refer to the part which is extending forward from the limb to hold the optical tube.

APLANATIC: Free of Spherical Aberration. Not quite the same as planar which refers to flatness of field.

A device for measuring the numerical aperture of an objective. The Abbe apertometer consists of a special calibrated scale with adjustment that sits on the stage, and a special objective that screws on to the inside of the drawtube of the microscope. Note that this type of apertometer requires both parts.


A limb which consists of a straight bar either attached to the stage, or driven by a pinion box attached to the stage, rising above it, with another straight piece attached to the top at right angles, projecting forward to support the optical tube. Bar Limbs were commonly made by both Ross and Powell & Lealand.

BIREFRINGENT: see anisotropic.

BULLSEYE CONDENSER: A thick plano-convex lens used to concentrate light on an opaque specimen. These can be attached to the main optical tube of the microscope on a flexible arm, or attached to the limb supported by an adjustable rod, or attached to a weighted separate support, when it can also be called a 'Bench Condenser'


A drawing aid which, when attached to the eyepiece of the microscope, allows one to see an image of the object being studied superimposed on a drawing surface, thus making it simple to 'trace' the object. An Article Reviewing the Camera Lucida for the Microscope is now on this site.


Distortion of an image seen through an ordinary lens which results in loss of definition because light of each different color is refracted to a different focal point. This can lead to colored fringes or halos around the object. This effect is illustrated by the images to the left (courtesy of Dr Brian Matsumoto). The upper image suffers from chromatic aberration due to use of a non-achromatic condenser. By using an achromatic condenser, the image is markedly improved.

COAL OIL: Kerosene


A solid glass cylindrical but symmetrical biconvex lens with a V-groove cut in the center of the cylinder to exclude part of the aberrant light rays. The image therefore is seen with a lesser degree of spherical aberration, though not as little as the Hastings or Steinheil triplets, which have no stop in the center and are corrected for both chromatic and spherical aberration. The performance of the Coddington lens is generally superior to the Stanhope type.

COMPOUND MICROSCOPE: -A compound microscope is a microscope which uses a lens or group of lenses close to the object being viewed to collect light (called the objective lens) which focuses a real image of the object inside the microscope. That image is then magnified by a second lens or group of lenses (called the eyepiece) that gives the viewer an enlarged inverted virtual image of the object.


A device which, through compressing one glass plate against another provides an adjustable degree of pressure against a subject thus flattening it, or immobilizing, or partially immobilizing the subject, without damaging it. A livebox can be used as a crude compressor, but it is hard to make small adjustments to the degree of compression. Although Varley's original live box has a screw-on top, allowing fine adjustments, this method was abandoned as the twisting motion so applied often distorted the specimen. For this reason, later versions of his design have a press-fitting cap. A heavy compressor was used to flatten teased out fibers of meat to make them translucent, in order to inspect the meat for trichinella larvae. The process of meat inspection by this method was made a bit easier by partial digestion of the meat with enzymes. More recent methods completely digest the muscle and the parasites are found in the sediment, and are examined with a stereomicroscope without the need for a compressor. The article on compressors on this web site illustrates many forms of compressors and how they work.

SUBSTAGE CONDENSER: An optical element or group of elements used to concentrate light from a mirror or other light source so that the light is focused on the specimen. Further improvement in resolution can be obtained by using achromatic and aplanatic optics for the condenser, just as in the objectives.

CONTINENTAL LIMB: This type of limb, originally popularized on the European mainland, was commonly made by the French and the Germans. The main characteristic of this type of limb, is a Pillar serving to support the optical tube. This pillar classically arises from the stage itself and has an inner support (often triangular) over which the outer support(usually cylindrical) rides. A knurled knob at the top of this outer tube is the fine focusing mechanism. The arm attaches to the top of the pillar projecting forward to support the main optical tube. British and American makers eventually added the Continental limb to their range of models. See, for example the Bausch & Lomb CCDS. Since the term 'Continental' refers to the European continent, many simpler pillar-supported microscopes made on the Continent are also appropriately called 'Continental Limb' as long as they do not fit another style (such as Cuff-type).

corr collar

An adjustment device on some moderate to high power objectives to compensate for variations in the thickness of cover slips. This device, usually calibrated, is the chrome-finished part surrounding the objective on the left; it was required in the past because of great variation in the thickness of coverslips. It is a device seldom used today, though highly corrected objectives are often equipped with one because coverslip thickness still varies slightly and the optimal correction lessens spherical, and to a lesser extent, chromatic aberration. The correction collar adjusts the spacing of the optical elements in the objective for optimal correction of spherical aberration which is affected by cover slip thickness. Although cover slip thickness is now 'standardized,' standard cover slips can still vary a little bit, and for very critical work special high precision-thickness coverslips are available (at additional cost). For ordinary work, just using the standard coverslip thickness that the objective is rated for is usually sufficient. For extremely critical work, objectives with correction collars are still available.

The idea of a correction collar dates back to work by Lister and Ross. Once Lister had developed the theory of how to correct spherical and chromatic aberrations, he realized that the distance between the front element and the back elements of the objective would need to be different depending on the thickness of the coverslip. Ross came up with the idea of an adjustable distance between the fron and back elements, and constructed objectives that allowed this distance to be varied. It is uncertain when Ross first made these adjustable fronts. Ross published the first description of the correction collar in 1837, but earlier examples than shown in his publication were likely produced before that. The first type simply consisted of a outer collar holding the front element which slides over the inner barrel housing the other elements. Clamping screws were used to fix the optimal distance once determined. This design poses three problems. One is that it is easy to disturb the specimen and the alignment of the elements when simply pushing or pulling the outer collar. The second is that the focus to the specimen had to be changed with the change of the front element position which neccessarily moved the front element closer or further away from the object being studied. Third, moving the front element carelessly on a high power objective could cause it to crush the coverslip and specimen, not to mention the front element of the objective. His improvement in the arrangement as published in 1837, made the adjustment more precise utilizing a knurled knob around the objective. This did not solve the above problems though. A later and more satisfactory arrangement, first introduced by Francis Wenham about 1855, was to build a system into the interior of the objective that moves the back elements away from or closer to the front elements-a much more satisfactory design which was quickly adopted.

COVER SLIP(previously a cover glass):
A thin piece of glass used to cover a liquid or preserved specimen on a microscope slide. Most standard objectives specify 0.17 mm thickness coverslips; these are commonly called number 1.5. It is a common mistake to use number 1 coverslips with objectives rated for the 1.5.

CROSSED POLARS: using a polarizer and analyzer with their optical axes oriented 90 degrees to each other resulting in a dark background unless an anisotropic specimen is in between them in the field of view.


DARK GROUND ILLUMINATION: previously Dark Field. A method of illuminating an object only with oblique light from all sides, so that only the objects being studied will be seen against a dark background. At low magnifications this requires only a simple central stop below the stage. At high power, special dark ground condensers and other equipment are required. For more about Oblique as well as Dark Ground Illumination, and the history of the methods, see my article about the History of Dark Ground Illumination on this web site.

DARK WELL: A small cylindrical container device for holding an opaque object on or in the center of the stage. They are painted black inside to avoid reflections. The device must be used with downward or vertical illumination.


EYEPIECE: (also an ocular) A lens or set of lenses of a microscope nearest to the observer during actual use. In a compound microscope, it magnifies a real image formed by the objective lens to form an enlarged virtual image.


FLATNESS OF FIELD: when one area is in focus, the rest of the area in the same plane being observed is in focus from edge to edge. NOT the same as Aplanatic.

FOCAL LENGTH: The distance from the objective to the subject being studied when the object is in focus; in general the shorter the focal length of a lens, the higher the magnification.

FOOT: The base of a microscope; several varieties exist including the classic 'horseshoe', 'English or Crouch type', and both Raised and flat tripods.

A cone shaped device limiting the internal aperture of a dark field objective. In use, the device is dropped into the rear opening of the optical part of the objective to provide a reduction of the field of view to eliminate stray light from interfering with the dark background. This device is not needed in objectives which have a built-in iris diaphragm. In the example shown to the left, the chrome part is unscrewed from the brass part and the funnel stop is dropped into it, so that it restricts the aperture at the top of the optical elements. In this example all the optical elements are housed in the lower chrome portion of the objective, so the stop must be placed in that part in order to reach the rear or top end of those elements.


HASTINGS TRIPLET: One type of simple microscope lens which is composed of a combination of three lenses cemented into a single unit which provides achromatic and aplanatic magnification. The central element is biconvex and the two outer elements are convex-concave.There are slight differences between a Hastings and a Steinheil triplet.


IMMERSION: using a liquid medium between the objective and the subject, eliminating the air-glass interface and thereby, when properly designed, improving resolution and reducing aberrations. Various forms of immersion have been developed, using water, oil, and glycerin, as examples. Today oil immersion is commonly used, water only for special situations.


KEROSENE: also known as paraffin oil(England), paraffin(England), or coal oil(archaic: U.S.A.), the oil mixture that made oil lamps easier and safer to use, and was commonly available starting about 1860. It is a mixture of 6-16 carbon molecules obtained from the fractional distillation of petroleum. Almost all oil lamps were powered by this after about 1860.

This is a system named after its inventor, the famous German optician August Kohler, which is designed to achieve optimal illumination while minimizing glare, distortion, and extraneous reflections which would degrade the image, especially during photography. In order to achieve proper Kohler illumination, two iris diaphragms, a focusable condenser, and the ability to center the light source as well as the condenser are required. The ability to vary the distance of the light source from the condenser is also sometimes needed. Kohler illumination is also important when optimizing the use of phase contrast or interference contrast illumination. If you want to get the most out of your microscope, it should be capable of Kohler illumination.


LIMB: The main part of the microscope which supports the stage and optical tube. Generally there are three main types. These include the Lister Limb, Bar Limb, and Continental Limb.

LISTER LIMB: A limb which is based on the design of Joseph Jackson Lister. The basic construction features a single solid piece which is the support for both the optical tube and the stage. The Jackson Limb is similar except for the extension of the same piece of metal to below the stage to support the substage.

LEIBERKUHN: A reflector which surrounds an objective and allows vertical illumination of an opaque object by reflection of light coming from underneath the stage. This was the earliest method of vertical illumination, though a bullseye condenser could concentrate lighting at an angle. The latter technique cannot be used to illuminate opaque objects when using objectives of short focal length (high power).

A glass-enclosed chamber to restrain a subject, often in liquid. Some forms of live box can serve as a compressor, others are simply an enclosed chamber. Dedicated compressoria are more easily used as compressors than live boxes. Cornelius Varley made a major improvement in the live box by creating a trough around the edge of the glass surface on which the subject was placed. An example of his live box, which he called a 'Capillary Tablet' or 'Cage' is shown on this web site. An article discussing the history and types of livebox is available on this site.


LOUPE: A handheld magnifier, usually without a handle and of higher power and smaller diameter than a magnifying glass. Different types of lenses can be used as loupes including Coddington lenses, and Triplets. In use, Loupes require the user to bring their eye close to the lens to properly see the image.


MECHANICAL STAGE: A stage that is controlled for fine movements by knobs or other controls, most commonly via rack and pinion or worm screw. Mechanical stages were often built-in but many attachable varieties were also made.


NUMERICAL APERTURE (n.a.): The modern term which reflects the resolving power of an objective lens.


OBJECTIVE: The lens or group of lenses nearest the object being studie ed with a microscope.


Illuminating a subject only with light rays directed from an oblique direction so that only light reflected by the objects on the slide will be directed towards the objective. Oblique illumination differs from dark ground illumination in that the oblique rays illuminate the object from only one direction; in Dark Ground illumination, the oblique light comes from all sides of the object.

OCULAR: an eyepiece.

OIL IMMERSION: See immersion.


PARAFFIN OIL:KEROSENE;paraffin oil was also sometimes simply called 'paraffin.'

PARCENTRIC:Refers to the fact that, after changing an objective, the object being studied is still in the center of the field. All high quality microscopes eventually had this feature.

PARFOCAL: Refers to designs of objectives which allows objectives of different focal lengths to be in focus without the need to change the coarse focus of the microscope once one parfocal objective is in focus.

PLANO or PLANAR: Flatness of field; when one area is in focus, the rest of the area in the same plane being observed is in focus from edge to edge. Not to be confused with Aplanatic.

The (often rectangular) compartment which houses the pinion, driven by a knob outside the box. The coarse focus pinion acts on the rack of the limb.

POLARIZER: An optical element fitted below a specimen which project polarized light on the specimen. The polarizer is often an integral part of a polarizing condenser.

projection eyepiece

A projection eyepiece projects the image as seen through the microscope onto a paper, screen or the imaging plane of a camera. This accessory generally requires a strong light source for the microscope, which was not easily available until the advent of the Carbon Arc Lamp late in the nineteenth century. Solar microscopes were generally used for this purpose until strong artificial light became available. Early versions used a prism, while later versions used lenses to project the image optimally. An example of a version from the early twentieth century is the projection eyepiece by Watson.


RACK & PINION: A method of moving a part in a controlled fashion, making use of a gear (pinion) controlled by a knob, acting on a toothed part attached to the part being moved. The most common use of this is in the Coarse Focus control of most compound microscopes. Initially the rack and pinion were both straight, but eventually a diagonal rack and a spiral pinion were used to increase the accuracy of the movement. Racks and pinions were also often used to drive the substage, and also mechanical stage. Mechanical stages become more important because as the magnification increases, the field of view becomes so small, so slight movements are required to adjust the location of an object without moving it outside the field of view.

The ability of an objective to resolve details, usually expressed as a distance between two points or alternatively by the related term, Numerical Aperture. Without an increase in resolving power (resolution), increased magnification serves no purpose as the image would be increasingly distorted with the increase in magnification. This is termed 'empty magnification' and was common with use of higher power objectives in early (pre-achromatic) microscopes.


A simple microscope is a microscope that uses a lens or set of lenses to enlarge an object through angular magnification alone, giving the viewer an erect enlarged virtual image. The image is erect, and if more than one lens is used, the virtual image of the first is simply enlarged, unlike in a compound microscope where the second set of lenses focuses on the projected real image and the resulting image is inverted unless a special image-erecting system is used. This type of microscope is relatively limited to low to low-medium power magnification before aberrations limit its usefulness. Van Leeuwoenhoek's most powerful simple microscope magnified 250X where as compound microscopes easily reach 400X and with oil immersion 1000X.

solar microscope

An instrument which uses the bright light of the sun to project the image of a microscopic subject onto a wall.


Blurring of the image with loss of resolving power, resulting from the fact that parallel light rays passing through the curved surface of a simple lens are refracted to different degrees depending on their distance from the central optical axis. These different rays then reach focus at different distances, leading to a blurred image even at best focus. The effect is worse at higher magnifications. This occurs because the thickness of a biconvex lens is greater in the center and progressively thinner towards the outer edges. In the images shown, the one to the left illustrates the effect of spherical aberration, while the right shows the improvement with aplanatic optics.

STAGE: The flat surface of a microscope on which the specimen is placed, usually on a slide.

STAGE CLIP(S):A device usually in pairs, that holds the slide on the stage putting slight pressure on it from above.

STAGE FORCEPS: A microscope accessory for holding an opaque or three dimensional object over the stage of a microscope.

STAND: The main components of a microscope which sits on a table. It does not neccesarily include the optical parts like objectives, eyepieces, etc. The term is not usually used for hand-held microscopes without a support for use on a table.


Below the stage of the microscope is the substage. This area, sitting between the mirror or light source, and the stage, can be simple or complex. The simplest substage apparatus consists of a fitting which holds an aperture to regulate the cone of light reflected up through the stage. The basic type of substage housing is a ring attached to the bottom of the stage. In its more complex form, it is an adjustable housing that can be moved up or down and centered in the optical axis of the objective. In the most sophisticated forms it can also have a calibrated rotation mechanism as is seen on the Grand Van Heurck Microscope in this collection. This housing can contain a condenser, or a polarizer, and in its ultimate form has provision for regulating the diameter of the light passing to and from the condenser. Some housings have adjustments for purposely creating a off-center light path. Many have a provision for adding a stop or filter under the condenser.


A cylindrical lens with each end convex, but unequally, with the lesser curved lens being placed on or very near the object being studied. This lens is not as free of aberration as the Coddington and has a very small field of view.

One type of simple microscope lens which is composed of a combination of three lenses cemented into a single unit which provides achromatic and aplanatic magnification. The central element is biconvex and the two outer elements are convex-concave. There are slight differences between a Hastings and a Steinheil triplet.

solid disks which block a portion of the central-most light rays of a condenser or light source. Simple stops can allow dark ground illumination at the lower powers.


From the German, Literally 'pocket microscope,' the term has been used to describe both small pocket-sized microscopes but also folding or compact portable microscopes, which folded, even in their cases, are usually larger than a modern pocket. Some algensuschers have been named or called 'Taschenmikroskops.'

Illumination of an object from above, which may be from an external light source such as a SIDE REFLECTOR, or from a VERTICAL ILLUMINATION SYSTEM.

In simple terms, the distance from the eyepiece to the objective. Microscope objectives are designed to work best with specific mechanical tube lengths (MTL). The mechanical tube length of an optical microscope is defined as the distance from the nosepiece opening, where the objective is mounted, to the top edge of the observation tubes where the eyepieces (oculars) are inserted. The optical tube length (OTL) is a slightly different measurement that varies with the eyepiece and objective used. This measurement is taken from the back focal plane of the objective (usually located somewhere inside the barrel of the objective housing), and the primary image plane of the eyepiece, which is usually about a cm below the top of the optical tube. To calculate the magnification of a given objective, one needs the focal length of the objective and the optical tube length. Magnification of an objective (power)=OTL/FL for the objective. The total magnification of a compound microscope is then the objective power times the ocular power. Since the OTL will vary even with type of objective, two objectives with the same focal length will magnify to a different degree if they have different optical designs. Tube lengths were standardized to different lengths depending on country and time period. The most recent finite standard, set by the Royal Microscopical Society, was 160 mm until it was changed to infinity in the 1980's. In this system a correcting lens known as a 'tube lens' is needed to allow focus. The magnification yielded by the objective with infinity corrected systems is the quotient of the focal length of the 'tube lens' divided by the focal length of the objective. For example, in a microscope system with a tube lens having a focal length of 180 millimeters, a 9 millimeter focal length objective will project a 20X magnified image onto the plane of the eyepiece diaphragm.


A system of viewing an opaque object in which the illuminating light is directed towards the object from the same direction that the image is being viewed from. This may be accomplished by the use of a VERTICAL ILLUMINATOR, or, in more modern instruments a light source that surrounds the objective. Vertical illumination is one type of TOP LIGHTING, but requires a light source that is oriented vertically, not from the side or an angle.

vertical illuminator

A device for supplying vertical illumination. The earliest type simply used a thin piece of glass, like a coverslip, angled at 45 degrees within the device which was usually placed on the nosepiece, just above the objective. Light could then enter from the side and be reflected down towards the subject, while the light reflected from the subject would pass through the reflector back up the optical axis to be viewed in the usual way. Other types of vertical illuminators used tiny prisms in the center of the device, the majority of the reflected light then passing around the prism up the tube. Vertical illuminators are still used today, but make use of specialized coatings to enhance the light paths desired, and reduce light paths that degrade the image. Further improvements included fittings such as iris diaphragms and condenser lenses to permit reflected Kohler illumination. Modern vertical illuminators also have built-in electric light sources which is very convenient when the optical tube rather than the stage is being moved for focusing, as using an external unattached light source in this situation requires adjusting the position of the light every time the focus is changed so that it continues to line up with the side opening of the vertical illuminator.


The distance between the end of the objective and the object being studied when the object is in focus. In general, the working distance shortens as the magnification increases.