The base of a microscope; several varieties exist including
the classic 'horseshoe', 'English or Crouch type', and both Raised and flat tripods. The horseshoe foot was invented by Edmund Hartnack in the early 1860's and became a standard F of foot from the late 19th century. It remained a standard form through the first half of the 20th century and beyond. Student microscopes continued to use the horseshoe foot in the third quarter of the 20th century.
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.
Fusee chains were popular as part of the mechanism to keep clocks and watches running at the same rate throughout the day. An entire industry making these chains arose in places like Christchurch, England. From about the 1830s to about 1880, several microscope makers, in Italy, England and the USA used fusee chain mechanisms for coarse focusing of the main microscope tube, or for the Y-axis of some mechanical stages. This is a completely different use from what they were originally developed for. When used as they were in microscopes, they were prone to malfunction and breakage, and so their use for microscope construction was abandoned, probably about 1880.
A stage controlled by a lever to allow one to easily follow a moving object. Two main types are known; the White lever controls the stage from above, while the Varley type controls the stage from below. Both were invented at about the same time. Often both types of lever stage are refered to as 'Varley' stages but this is inappropriate and unfair to the inventer, as White invented the above stage form. Some later inexpensive stages controlled by levers are simplified and less convenient to use than the original version by White.
LIEBERKUHN: A reflector which surrounds an objective and allows reflected illumination of an opaque object by reflection of light coming from beyond the object, .e.g. underneath the stage and in back of the specimen. This was the earliest method of vertical illumination, though a bullseye condenser could concentrate lighting from above at
an angle. The latter technique cannot be used to illuminate opaque objects when using objectives of short focal length (high power). Although it was used by Van Leeuwoenhoek in the early 1700's, it was popularized in the mid-late 1700's by Johann Lieberkuhn. With this device, the light source from beyond the object passes around it and is reflected back down upon it by a polished mirrored parabolic surface that surrounds the objective. The chief limitation of this technique is that the object must be small enough to allow the light to pass around it, and this is often not the case, especially for large metal samples etc.
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.
LYING DROP SLIDE:
A special slide usually used to study tiny moving creatures without any pressure applied to them. The traditional method is a hanging drop slide. With the lying drop slide, an inverted optical path is required, such as with an inverted microscope or a McArthur type microscope. In this type the liquid simply rests on a coverslip at the bottom of a hole in the slide. The objective, pointing up from beneath allows visualization through the coverslip. The lying drop slide has advantages over the hanging drop in that the drop is not as distorted and therefore the medium is more uniform and refocusing is not required when moving the slide, only when observing the subject(s) at a different depth within the 'drop'.
MECHANICAL STAGE: A stage that allows movement of the subject, usually on its glass slide, 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. These simply clamped on to the pre-existing plane stage after the stage clips were removed.
MICROPHOTOGRAPH: A greatly reduced photographic image requiring magnification to be clearly seen. Microphotographs are transparent, like photographic negatives, but are positives. Not to be confused with photomicrograph
MONOCHROMATIC LIGHT: Light of a single color (wavelength).
NICKEL PLATING: Nickel plating was a technique discovered by Golding Bird, the famous Guy's Hospital Physician about 1837. It was not popular as a technique used for microscope parts until the last quarter of the 19th Century. It then became common to see Nickel-plated drawtubes and eventually other parts. The first nickel plating done in the USA started with a process in 1869, but this process did not become popular in the USA until the last quarter of the 19th century and reached its peak after 1915. For an example of nickel plating on a draw tube see the drawtube of the Spencer No. 1 microscope of c. 1900.
NICKEL SILVER: Also called German silver, or electrum, is a special type of brass alloy composed of copper,nickel and often zinc. The usual formulation is 60% copper, 20% nickel and 20% zinc. Nickel silver is named for its silvery appearance, but it contains no elemental silver unless plated. The name "German silver" refers to its development by 19th-century German metalworkers in imitation of the Chinese alloy known as paktong. Nickel silver quickly forms a duller appearance than nickel plating due to oxidation. For an example of an instrument made of nickel silver, see the Browning Miniature Microscope on this site.
NELSONIAN ILLUMINATION: Also known as Critical Illumination. This method of adjusting illumination, used to study difficult objects at high power, was first ascribed to Edward Miles Nelson in the late 19th century. It requires a perfectly uniform light source such as a good flame or an electric ribbon filiment. Using this kind of illumination, the light source is focused in precisely the same plane as the subject being viewed. In practice, the edge of the light source is focused, and the the light source is centered over the object. To do this requires very precise focusing of the condenser, hence the need for a substage fine focus. Such mechanisms were supplied either as standard equipment or options on such high end stands as the Grand Van Heurck, the Royal and of course the Nelson-Curties models. For more information, see the article on Critical Illumination.
NOSEPIECE: The bottom part of the main optical tube of a microscope that attaches to the objective or to an objective changer. A single thread serves this purpose in microscopes without an objective changer. The objective or an objective changer, screw into the nosepiece of the optical tube.
NUMERICAL APERTURE (n.a.): The modern term which reflects the resolving power of an objective lens. The numerical aperture of a microscope objective is a measure of its ability to gather light and resolve fine specimen detail at a fixed object distance. The equation that expresses this is:
n.a.= n (sin m),
where n is the refractive index of the medium between the objective lens and the coverslip and m is one half of the angular aperture of the lens.
For air, (dry objectives), n=1.0. The higher the n.a., the better the resolution and the higher the magnification that will allow more details to be seen. A high power objective with a low n.a. will not show any more detail than a lower power objective with the same n.a. This failure of a higher power objective with a low n.a. to improve detail is often referred to as 'empty magnification' and is common with high power non-achromatic objectives. Numerical aperture can be calculated for any given objective when the optical design of the objective is known. It can also be measured directly with an apertometer. Unlike angular aperture which is a nonlinear scale, numerical aperture is linear and also unlike angular aperture, can be used to compare all objectives, regardless of whether immersion or not. Note that, with all other things equal, the higher the refractive index of the medium, the higher the N.A. Hence an oil immersion objective with angular aperture of 134 degrees has a N.A. of 1.38, but a
dry objective meant for use without an immersion fluid, with the same angular aperture will have a N.A. of only 0.92.
OBJECTIVE: The lens or group of lenses nearest the object being studied 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.
OIL IMMERSION: See immersion.
A translucent milky white glass that is prepared by adding impurities (as fluorine compounds) which disperse as crystallites within the matrix of glass and that is used for diffusing light without serious loss of lighting efficiency; used for the X-marked 'window' of the cursor of the Abbe Apertometer
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.
Phase contrast is a type of microscopy employing an optical mechanism to translate minute variations in the phase of light waves(invisible to the naked eye), into corresponding changes in amplitude, which can be visualized as differences in image contrast. This allows details that would not otherwise be visible with ordinary transmitted light microscopy to be easily and quickly visualized. Phase contrast utilizes a special condenser and special objectives. There are phase plates in the condenser and a phase ring visible at the back focal plane of each objective. For the technique to work properly the rings must be aligned. A small accessory eyepiece to view an enlarged image of the rear focal plane of the objective called a phase telescope is used for this purpose.
PHOTOMICROGRAPH (PHOTOMICROGRAPHY)A recorded image of an object as seen through a microscope. Not to be confused with microphotograph.
A phase telescope is a special device that takes the place of the eyepiece and allows the user to focus on an enlarged image of the back focal plane of the objective to align the phase ring of the objective with the phase plate of the condenser. A Bertrand lens, when present, may also be used for the same purpose, but is usually found only on petrographic microscopes equipped for polarized light obervations. Occasionally a Bertrand lens was made as an attachable accessory between the main optical head of the microscope and the body tube below.
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. A pinion box was usually part of the coarse focus mechanism. The coarse focus pinion acts on the rack of the limb, to adjust the distance of the objective from the object being studied, thereby allowing focus. See Rack and Pinion
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.
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.
A mechanical arrangement for moving a part in a controlled
fashion, making use of a gear (pinion) controlled by a knob, acting on a toothed part (the rack), 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 cut straight, but eventually a diagonal rack and a diagonal pinion were used to increase the accuracy of the movement. This use of the diagonal rack and pinion for the microscope was first introduced by James Swift in England. Racks and pinions were also often used to drive the substage, and often, also the mechanical stage. Mechanical stages became more important because as the magnification increases, the field of view becomes so small that slight movements are required to adjust the location of an object but without moving it outside the field of view. The pinion was often contained within a pinion box often with adjustments to keep the pinion's contact with the rack optimized.
When a light beam passes from one medium to another, or near the edge of an object, its tradjectory is changed. This effect is called refraction. The refractive index of any medium is the ratio of the phase velocity of the light waves in a vacuum to their phase velocity in the medium itself. The refractive index allows comparison of this effect from one medium to the next (as in comparing two types of glass). In general, mediums with higher refractive index have higher amounts of dispersion. Some important refractive indices: Air=1.0, Water=1.33, Glycerin=1.46, Glass=1.52, Cedar wood oil=1.516, Immersion oil=1.51, Canada Balsam=1.545.
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.
Shagreen, as now defined in the world of antique microscopes, is a decorative firm material often used as a wrap-around covering of the body tubes of antique microscopes and telescopes. This material is derived from the skin of a ray. Its surface is covered with pearly circles. Ray skin in its original state is quite rough. However after the skin has been treated and dried and polished to a high gloss, it leaves the appearance seen here. The final shagreen was often dyed. The most common color used for microscopes was gray, followed by red, green, and blue. In the image to the left the outer tube is covered in gray shagreen; the inner tube is covered in dyed vellum. See the page on history of Shagreen for more details and history.
Sharkskin, as defined in the world of antique microscopes, is hard rough surface derived from the skin of a shark. Its surface is pitted and the pitting may either be smoothed or rough and abrasive. The simple microscope case shown here is covered in sharkskin which is pitted but not abrasive. See the page on history of Shagreen for more details and history.
A simple microscope is a
microscope that uses a single lens, or a set of lenses in close proximity, 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 medium power magnification before
aberrations limit its usefulness. Van Leeuwoenhoek's most powerful
simple microscope magnified 250X whereas compound microscopes easily
reach 600X and with oil immersion 1200X or even more.
A flat device on which specimens are placed for study. Starting in the mid-19th century glass slides were most often a standard 1 x 3 inches, but before this, and sometimes later, especially for cheap commercial slides, smaller slides were used. Even earlier, bone 'sliders' were most commonly used. These could be made either solid with slight depressions, or with holes for transparent objects. Opaque objects on bone sliders were usually not covered. Transparent objects shown on bone sliders were mounted on tiny pieces of mica and covered with these as well, the top talc held in place with a springy piece of brass wire known as a 'circlip.' Early on specimens mounted on glass slides were also covered with mica 'talcs,' soon thin glass cover slips were used. Although modern slides usually have a label, Victorian glass slides often are covered in paper with the subject name either written on the paper or an attached label. Some relatively early Victorian slides are simply labeled by engraving them with a diamond.
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.
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.
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.
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.
STOPS or PATCH STOPS:
Patch stops, or simply stops, are solid disks which block the central light rays of a condenser or light source. They are usually supported by a
spider frame with two or three struts, which fits into the swing-out filter holder of a condenser. In some condensers, like the Powell & Lealand High Power Acromatic Condenser, there is a wheel of stops to select from built in. There are also stops which allow different types of oblique illumination by using various patterns such as a crescent-shaped opening. Simple stops can allow dark ground illumination at the lower powers, but for high power dark ground illumination, a more sophisticated system is required. Depending on the condenser, the swing-out filter holder could have different diameters, hence the support of the stops was made in different diameter to fit the condensers in question.
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.
An optical element or group of
elements located under the stage, and 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. Good examples of the various types of substage condensers can be seen in the accessory section of the Grand Van Heurk Microscope on this site.
A small cylindrical container, housing mica cover slips ('talcs') on one side, and brass retaining clips ('circlips') on the other, with screw-on caps at each end if ivory, or one end if brass. Most were made of ivory but occasionally in brass. They could be quite plain, as the example to the left, or like the one shown to the right, have fancy turned caps. The talcbox to the right measures 20 mm in length and 12 mm in diameter with the caps having a slightly larger outer diameter. Talcboxes were commonly supplied with screwbarrel microscopes,(like this one), as well as Culpeper-type microscopes.
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.'
Since it was first realized that the optics of microscopes were subject to artifacts, especially spherical and chromatic aberration, slides to test the quality of microscope optics were devised. These could be natural objects with regular markings like scales of certain butterflies, hairs of antennae of insects, or most commonly, certain diatoms. But complicated machines were also devised to engrave rulings closer and closer together on slides. Most famous among these were the rulings by Nobert, who was able to scribe lines so close together that the finest of them could not be resolved by microcopes of the time, and they were only successfully resolved in modern times when the electron microscope became available.
TOP LIGHTING (EPISCOPIC ILLUMINATION):
Illumination of an object from above, which may be from an external light source such as a SIDE REFLECTOR, or from a vertical illumination device. The illumination can be axial (parallel to the optical path), or oblique to any degree, depending on the way the light is directed on the object and the desired effect. Some opaque objects are best illuminated obliquely, while for other purposes purely axial rays may be prefered.
An instrument used to smoothly rotate a microscope slide while applying cement or ringing compound to seal the edges of a coverslip or build up successive layers to create a
cell to enclose a specimen without crushing it.
A.K.A. TURTLESHELL, is a material made from the (now highly endangered) Hawksbill Sea Turtle. When polished it is a redish brown colored material with light and dark areas. It was used to house pocket magnifiers, most notably the Browning Platyscopic Magnifiers. It should be distinguished from Horn, which generally has a black and white coloration and has fibers on close examination.
TRANSMITTED LIGHT OR TRANSMITTED ILLUMINATION:
Illumination by light passing through a transparent or semi-transparent specimen.
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 type of inclination mechanism in which an axle passes through one or more cylinders.
A type of mechanical stage in which the right-left and forward-backward controls are concentric, eliminating the need to move the hand when controlling a different axis. It also allows both knobs to be turned simultaneously to allow movement in a diagonal direction. This type of control is found on many instruments on this site including the 1843 Powell & Lealand microscope microscope.
A Type Slide is a microscope slide containing different types of diatoms, foraminifera, or similar small organisms, and intended to allow comparison with other examples as an aid to identification or to show the variety of types of diatoms at a specific geographical location. The Germans called this
Typen Platte. Type slides may have just a few types or number into the many hundreds. The slide shown here was made by Klaus Kemp in the third quarter of the 20th Century and contains 530 different forms. Mounters like Thum of Leipzig were famous makers in Germany in the 19th century. Often type slides were supplied with a key list. Well preserved examples, especially with their lists, often command high prices on the market today.
VARLEY STAGE or VARLEY LEVER STAGE: see LEVER STAGE
Vellum(arrow) is a type of parchement, a paper-like material made from goatskin. It was seldom used to cover the outside tubes of microscopes, but was a common covering for early drawtubes or the inner sliding optical tube of older microscopes. It was commonly dyed green. Old vellum which has not been dyed appears to have a tannish tint. The outer tube of this culpeper type microscope is covered with shagreen.
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.
A device for supplying vertical illumination, a form of episcopic 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.
WHITE STAGE or WHITE LEVER STAGE: see LEVER STAGE
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. Special long working distance objectives have been developed though, to increase this distance so as to allow more flexibility in working with the subject being studied.