Achromatic – Describes lenses which are corrected for chromatic aberration at two wavelengths, and for spherical aberration for one wavelength. These objectives are used for most routine observations. They can be used with either normal or compensating eyepieces. If used with a green filter they will provide resolution almost comparable with an apochromatic objective of the same NA.
Analyzer – A polar located above the objective, it can normally be placed in, or removed from, the light path.
Anisotropic – An anisotropic substance is one through which light (also heat etc) travels at speeds which are dependent on the direction of travel. In most directions, the light ray is split into two plane polarized rays which vibrate at right angles to each other, and which travel at different speeds.
Anomalous colours – When interference colours do not correspond to the standard Newton scale of colours, they are described as anomalous. The following are examples: The restricted range of colour that is seen in vermilion or iron red where the interference colour is influenced by the colour of the pigment. The orange brown that occurs where one would expect the bluish first order red in orpiment. The second order peacock green or blue that one sees in red lead, where one might expect a first order grey.
Aperture diaphragm – This is used to set the effective N.A. of the substage condenser so that it is slightly smaller than the objective that is being used.
Apochromatic – Describes lenses which are corrected for chromatic aberration at three wavelengths, and for spherical aberration for two wavelengths. Apochromatic objectives have a larger NA, as well as a smaller working distance than achromatic lenses of the same magnification. They should always be used with a ‘compensating’ eyepiece.
Artifact – An image which has been unintentionally altered or created by the microscopist. If its irrelevance is not recognised, an artifact may be misleading. Artifacts are usually caused by faulty specimen preparation, or by the incorrect use of the microscope. Examples: dust near the plane of the field diaphragm will be projected into the plane of the image and appear to be part of it, or the image of the dust may be superimposed on and change the colour of a particle in the field. The use of too small an aperture can result in artifacts in the form of colour fringes and lines that are not present in the object being examined.
Bertrand lens – This is a lens that can be inserted between the objective and the eyepiece. It allows one to view the back focal plane of the objective and an image of the aperture diaphragm. In pigment work it is primarily used to help set the aperture diaphragm to the correct size. It can also be used when centring the lamp. Its main use occurs in the study of rock sections, when it is used to view interference figures. These can allow one to determine whether a material is uniaxial or biaxial, and also establish whether it is positive or negative. Unfortunately pigment particles are almost always too small and irregularly shaped for this to be done.
Biaxial crystals – These are crystals belonging to the orthorhombic, monoclinic, or triclinic systems. They have two optic axes or directions that will not affect plane polarised light.
Birefringence – This refers to a difference in refractive index. It can refer to the difference which is observed for a particle lying in a particular orientation. This is sometimes referred to as the birefringence of a section. It can also refer to the maximum difference that is possible for the substance. For example: the refractive indices of calcite are 1.486 for e and 1.658 for w; the birefringence is consequently 0.172, but because of the way in which calcite cleaves, e’ is usually 1.566 and the birefringence that is usually seen is 0.118.
Chelsea filter – A filter that passes two narrow bands of light one in the yellow green at about 5400 Å and the other in the deep red at about 6900 Å. It was developed by the London Chamber of Commerce in 1934 and is marketed by the Gemmological Association. It was originally intended to differentiate emeralds, which appeared red if viewed through it, from glass imitations, which did not. However, it has been found that a number of blue pigments including: ultramarine, cobalt blue and smalt appear red if they are illuminated by light that has passed through the filter.
Cleavage – A pigment mineral has good cleavage if well-defined planes which relate to its structure are formed when it is ground.
Compensators – There are other compensators; but the three compensators that are most often used by polarised light microscopists are: the ‘first order red plate’ (also called a ‘gypsum plate’, ‘lambda plate’, or ‘sensitive tint plate’), the ‘quarter wave plate’ (also known as a ‘mica plate’), and the ‘quartz wedge’ (normally only the first order red plate is used in pigment microscopy). These are sliders inset with thin pieces of crystal (or plastic) of known orientation that can be inserted into the microscope tube below the analyzer in a fixed direction, and which allow the direction of the fast and slow rays in an anisotropic particle to be determined. To establish the direction, the particle is examined between crossed polars, the compensator is inserted and the stage is rotated. When the directions of the particle and compensator correspond the colour seen will be higher on the Newtonian scale than that shown by either the particle or the compensator alone. If the stage is rotated so that the directions are contrary, the colour will become lower than when the directions correspond. For example a particle which showed a grey between crossed polars might, with the red plate inserted, show a blue or green when the directions corresponded, and yellow when they were contrary. The first order red plate is mainly used with small particles and those that have a low birefringence. The quarter wave plate is used with particles where the colours between crossed polars are in the second or third order. The quartz wedge can be used to establish the order of colour that is being observed.
Conchoidal – The term is used to describe the type of fracture that is typically found in glass and quartz. The form is made up of a series of shallow concave areas and the edges are usually sharp.
Cover slip – A thin piece of circular or rectangular glass applied over a preparation, and usually held onto the slide by a temporary or permanent mountant.
Dispersion – The variation of the refractive index according to the colour or wavelength of the light.
Dispersion staining – A technique for establishing the wavelength that has the same refractive index for both particle and the fluid or mountant in which it is immersed, without the use of monochromatic light. In suitable circumstances the edges of a particle will appear coloured. The coloured edges were first noticed by Christianson and the effect was named after him. Cherkasov described several methods of obtaining the effect, and called the technique ‘focal screening’. McCrone coined the term ‘dispersion staining’.
Diffraction – This refers to the way that a beam of light is bent when it passes an edge, and spreads when it passes through a narrow slit or small aperture.
EF – These initials are found on some objectives and stand for ‘extended field’. When used on a flat field objective they may indicate a field that is smaller than that of an objective merely marked ‘plan’ or ‘flat field’.
Elongation – Elongation is positive when the long axis of the crystal is parallel to the higher refractive index of the compensator, and negative when it is parallel to the lower refractive index. With small particles such as pigments, it is checked by crossing the polars and inserting the ‘red plate’. When the ‘slow’ (i.e. higher refractive index) direction of the red plate is parallel to the direction of greater refractive index in the crystal, the colour of the crystal will be higher on the Newton scale than the red plate (e.g. second order blue). Conversely, when the slow direction is parallel to the lower refractive index, the colour will be lower (e.g. first order yellow).
Extinction – When an anisotropic substance is rotated between crossed polars there are four positions, at 90 degrees to each other, where it passes little or no light; these are the extinction positions. Extinction can be described as sharp, undulose, or tabby. cf., parallel, oblique, symmetrical.
Graticule – (See also measurement) A transparent plate that is marked with cross lines, a scale, grid, or other geometric figures. It is placed in a suitable eyepiece so that it lies in the plane in which the principal image is focused. Scales need to be calibrated for each objective by comparing them with the calibrations on a stage micrometer. q.v. The eyepiece graticule must be calibrated for each objective with which it will be used.
To Calibrate. Arrange the stage micrometer so that two lines on it correspond with two lines on the eyepiece graticule. The lines should preferably be well separated, and at high magnifications the left or right hand edge of the stage micrometer lines should be lined up with the lines of the eyepiece micrometer.
Let x = the number of divisions on the eyepiece graticule between the two matching lines
Let y = the number of divisions on the stage micrometer between the two matching lines
Then each eyepiece division will equal y divided by x, multiplied by the size of the divisions of the stage micrometer.
e.g. If 15 eyepiece divisions are equal to 12 divisions on the stage micrometer, and if each stage micrometer division is equal to 0.01 mm; then each eyepiece division will be equal to 12/15 x 0.01 mm. It follows that if a crystal is 7 eyepiece divisions long it will be 12/15 x 0.01 x 7 mm long = 0.056 mm And since 1 µ (micron or micrometre) = 0.001 mm the crystal will be 56 µ long.
Gypsum plate – See compensators.
HI – These initials may be found inscribed on some oil immersion lenses and stand for ‘homologus immersion’ i.e. they are to be used with an oil that has a refractive index corresponding to the glass of the cover slip.
Immersion lenses – Today three types of immersion lenses are made; oil, glycerine, and water. Replacing the air between the objective and slide with a fluid of higher refractive index allows a lens to be made with a greater NA than is possible for a dry lens. A small drop of a suitable liquid is placed on the slide and the lens lowered into it. Oil is likely to fluoresce in U.V. light, so where this would create a problem, glycerine is used. Water immersion lenses are mainly used to examine living organisms that live in water environments. In the past cedar wood oil was used with oil immersion lenses, but today this has been replaced by a non-drying synthetic oil. Immersion lenses must be carefully cleaned after use. To obtain the maximum resolution of which the lens is capable, the substage condenser and lamp condenser should be well corrected and the slide should also be connected to the substage condenser preferably with oil, but water is better than air. In routine work air is often left between the slide and the substage condenser, this means that the full NA of the lens cannot be used.
Interference colours – The colours that one sees in an anisotropic crystal when it is viewed between crossed polars. It is caused by the interference between the two rays, which have been travelling at different speeds, when they recombine in the analyzer.
Isotropic – An isotropic substance is one through which light (also heat and electricity) travels at the same speed irrespective of direction. Isotropic substances are either amorphous, or belong to the cubic crystal system.
Magnification – The magnification of an objective/eyepiece combination, can be said to be the magnification of the objective, multiplied by the magnification of the eyepiece, multiplied by the ‘tube factor’ (if there is one). Generally the magnification should not be allowed to exceed 1000 x NA of the objective.
Measurement – A stage micrometer and an eyepiece graticule will enable any particle to be measured. If the stage is rotated length, breadth, etc. can be measured reasonably accurately. A calibrated rotating stage also enables angles to be recorded. Measurements are often made for statistical purposes, and then the particles are counted, and measured ‘as they lie’ without rotating the stage. It can be done in a number of ways. See under graticule for a description of how to calibrate the eyepiece and use the graticule for measuring.
Micrometre – (Usually written as the Greek letter µm mu followed by ‘m’). The usual unit of length of measurement in light microscopy, and equal to 0.001 mm. It is still sometimes referred to by its former name which was micron.
Micron – See ‘micrometre’.
NA – see numerical aperture.
Nanometer (nm) – The usual unit for measuring light wavelengths, equal to 0.000001 mm. Occasionally still referred to by its former name of millimicron.
Nicols – Old name for polars, q.v.
Numerical aperture – (Often abbreviated to N.A.). This is defined as the sine of half the angle of the maximum cone of light which the lens is capable of receiving multiplied by the refractive index of the medium between the lens and the slide. The numerical aperture provides a measure of the resolution of the lens. Thus a lens with a numerical aperture of 0.45 will only separate points which are twice as far apart as a lens with an NA of 0.9. In practice manufacturers relate the NA to the magnification of the lens, so that the total magnification of objective and eyepiece etc., is not likely to exceed 1000 x NA. Although the theoretical maximum NA of a dry objective is 1, the maximum practical NA is .95. Today few oil immersion lenses have an NA which is greater than 1.3.
Objectives – These are the main lenses of a microscope. They can range in magnification from x1 to x100, and can be achromatic, or apochromatic. They can have a flat or curved field of view. Their resolution is indicated by the N.A. Low power objectives are usualy designed to be used “dry” but the highest powers are always immersion objectives. An objective with a high N.A. has a small working distance.
Oblique extinction – This occurs when a particle extinguishes and neither an edge nor the line bisecting the angle of the crystal or cleavage flake is parallel with the polars. The main example in pigments is gypsum.
Parallel extinction – This occurs if a crystal or cleavage flake extinguishes when one of its sides is parallel to the planes of the polars. Most notably elongated particles extinguish in this way. Also called ‘straight extinction’.
Plan – This is found engraved on some objectives and indicates that the objective provides a ‘flat field’ and that the image is not merely sharp in the center.
Plane polarized or polarised light – Light vibrating in a single plane.
Pleochroism – A particle exhibits pleochroism if, when it is viewed in plane polarized light, it changes in colour, or becomes darker or lighter when the stage (or polar) is rotated. The effect only occurs in anisotropic substances, and is the result of the rays which vibrate in one direction being more strongly absorbed than those which vibrate in the other. The ray which vibrates at right angles to the polariser is usually more strongly absorbed than the ray that vibrates parallel to it, and is consequently the more definite colour.
Polarization colours – These are the result of the interference of the two light waves, one of which has been retarded, when they are brought together in the analyzer. The colour depends on the difference between the two refractive indexes and also on the thickness of the particle. The highest colours are seen when a particle is half way between extinction positions. With many materials the colours follow the order of the Newton scale of colours. With strongly coloured materials the colours can be influenced by the colour of the material itself. With some other materials, the colours do not follow the order of the Newton scale. Where the colours do not follow the Newton order, either because of the strong colour of the material, or for other reasons, the colours are described as anomalous.
Polariser – A polar located below the substage condenser. In a polarising microscope it is usually left in the light path permanently. In continental microscopes the vibration direction which is passed is normally E–W. On American microscopes and on old British ones, the direction is usually N–S.
Polars – Optical mechanisms which convert light vibrating in more than one plane into light vibrating in a single plane. Today they are usually made of ‘polaroid’, in the past they were made of calcite and called ‘Nicol prisms’ or ‘Nicols’. ‘Nicols’ is a term sometimes used instead of polars. On a polarizing or petrological microscope, there are two polars, a ‘polarizer’ and an ‘analyzer’ q.v.
Quartz wedge – See Compensators.
Red plate – (First order red plate) See Compensators.
Refraction – Refers to the way in which the path of a beam of light is bent when it travels from a medium of one refractive index to another. This only occurs when the beam is not normal to the interface.
Refractive index – The refractive index is the speed of light in the substance divided by the speed of light in a vacuum.
Refractive index fluids – Fluids, often in sets, that have a known refractive index, and which are used to determine the refractive index of materials immersed in them.
Relief – High or low relief refers to the degree of contrast which can be seen at the edges of a particle when it is immersed in a medium. Low relief occurs when the refractive index of the medium and the particle are almost the same. In plane polarized light, q.v. the degree of relief may change as the stage is rotated.
Symmetrical extinction – This describes extinction which occurs when the line bisecting the angle of the crystal or cleavage flake is parallel with the planes of the polars. The main example in pigments is calcite.
Sensitive tint plate – See Compensators.
Sharp extinction – This occurs when an anisotropic crystal is only dark for a few degrees when the stage is rotated between crossed polars.
Stage micrometer – A scale (usually 1 mm in length) which has been divided into a known number of parts (usually 100). It is used when the graticule is calibrated. See under graticule for an example.
Tabby extinction – This can occur in both isotropic and anisotropic materials when they are under strain. Instead of becoming dark, or changing in colour or tone in a regular manner, at the extinction position, they exhibit an irregular pattern of different intensities of grey.
Tube factor – The magnification factor that occurs when lenses are introduced to compensate for the increase in mechanical tube length that is needed for the slots that take the compensators on a polarizing microscope. The factor is usually x 1.2, or x 1.25.
Tube length (mechanical) – The distance from the shoulder of the objective to the upper end of the open tube where the eyepiece is inserted. On most microscopes this is 160 mm, but on Leitz microscopes it was 170 mm until recently.
Tube length (optical) – The distance from the back focal plane of the objective to the image formed by the objective (the intermediate image).
Twinkling – The name for the phenomenon which occurs when the w refractive index of a uniaxial crystal corresponds to the refractive index of the mountant: the particle will ‘disappear’ twice during a complete rotation of the microscope stage.
Undulose extinction – Extinction is undulose when the position of extinction is different for different wavelengths. The particle may therefore change in colour or tone and never become completely black.
Uniaxial crystal – A crystal in the tetragonal, trigonal or hexagonal system. It has a single optic axis.
Mactaggart, P. & Mactaggart, A. (June 2007) ‘Glossary’ In: Pigment ID using Polarised Light Microscopy from: https://academicprojects.co.uk/glossary/
