Part of The Grand Glossary: A Collector's Reference. This section covers the specialist vocabulary of antique sundials and scientific instruments, from the geometry of dialling and the classification of dial types to the principal categories of optical, navigational, and philosophical instruments, and the great makers whose names define the field. Terms are arranged alphabetically within thematic groupings.
Sundials: Theory and Terminology
Analemma
A figure-of-eight curve, derived from the equation of time, used on certain sundials to correct the reading of solar (apparent) time to mean time. When inscribed on a dial face and used in conjunction with a table or scale, the analemma allows the user to obtain mean time directly from the shadow of the gnomon without a separate calculation. The term is also used for the figure-of-eight pattern traced by the sun’s position in the sky at the same clock time each day over the course of a year, resulting from the combination of the earth’s axial tilt and the eccentricity of its orbit.
Declination
In the context of dialling, the angle by which a vertical dial face deviates from true south (or north), measured in degrees east or west. A south-facing vertical dial has zero declination; a wall dial facing south-east has an eastward declination. The hour lines on a declining dial must be calculated for the specific declination of the surface on which the dial is mounted, making declining dials more complex to construct and more precisely site-specific than horizontal dials. The declination of a wall dial is determined by compass and corrected for magnetic variation.
Equation of Time
The difference, varying throughout the year between approximately −16 and +14 minutes, between solar time (as shown by a sundial) and mean time (as shown by a clock keeping equal hours). The equation of time arises from two astronomical causes: the eccentricity of the earth’s orbit (which causes the earth to move faster when closer to the sun in winter and slower when farther away in summer) and the obliquity of the ecliptic (the tilt of the earth’s axis). On four days each year — approximately 15 April, 13 June, 1 September, and 25 December — the equation of time is zero and the sundial and clock agree. The equation of time is tabulated on many 18th and 19th-century dial faces and on the printed cards supplied with portable dials.
Gnomon
The part of a sundial that casts the shadow used to read the time. In the most precise usage, the gnomon is the entire shadow-casting structure; the style is the thin edge of the gnomon whose shadow falls on the hour lines. On a correctly constructed horizontal or vertical dial, the style is parallel to the earth’s axis and therefore makes an angle with the horizontal equal to the latitude of the place for which the dial was made. A dial with a gnomon set at the wrong angle, or used at a significantly different latitude from that for which it was made, will not keep correct time.
Hour Lines
The lines engraved, scribed, or marked on the dial face from which the time is read by reference to the shadow of the gnomon or style. On a correctly made dial, the hour lines are spaced according to precise mathematical calculation based on the dial’s type, orientation, and the latitude for which it was designed. The spacing of hour lines on a horizontal dial differs from that on a vertical or declining dial; lines that are evenly spaced indicate a decorative rather than functional dial. Half-hour and quarter-hour lines, and sometimes lines for every five minutes, are found on finely made dials.
Latitude
The geographic latitude for which a sundial is designed, which determines the angle of the style (on polar-pointing dials), the spacing of the hour lines, and the configuration of declining dials. A sundial used at a latitude significantly different from that for which it was made will show incorrect time. Many portable dials incorporate a latitude adjustment mechanism — a sliding or pivoting gnomon — to allow the dial to be set for the user’s latitude. Fixed garden dials are made for a specific latitude and their provenance and design latitude are important considerations.
Meridian
The north–south line passing through the observer’s position, with which a horizontal sundial must be aligned for correct operation. The local meridian is the line along which the sun crosses its highest point each day (solar noon, when the gnomon’s shadow falls due north in the northern hemisphere). Many horizontal dials incorporate a compass rose or alignment marks to assist in setting the dial on the meridian. The difference between magnetic north and true north (magnetic variation or declination) must be applied when aligning a dial by compass.
Sundial Types
Altitude Dial
A dial that tells the time by measuring the altitude (angular height) of the sun above the horizon rather than by the direction of a shadow. Altitude dials include the shepherd’s dial (a cylinder dial), the ring dial, and the quadrant dial. They require the user to orient the instrument towards the sun and read the time from a scale corresponding to the season. Altitude dials are less precise than gnomonic (shadow-casting) dials but have the advantage of not requiring alignment with the meridian.
Analemmatic Dial
A horizontal sundial in which the hour points are arranged on an ellipse and the gnomon is a vertical rod whose position on a central scale must be adjusted according to the date. The analemmatic dial gives mean solar time directly when the rod is set to the correct date position. Large analemmatic dials are sometimes laid out as garden features, with the observer standing at the correct date position on the central scale and acting as the gnomon. The form was developed in the 17th century and described by the French mathematician De Vaulezard.
Diptych Dial
A portable folding dial consisting of two rectangular leaves joined by a hinge, with dial faces on the inner surfaces and a string gnomon stretched between them when the leaves are opened to a fixed angle. Diptych dials were produced in large numbers in Nuremberg and Augsburg from the late 15th to the mid-17th century, typically in boxwood, ivory, or fruitwood, with printed paper or engraved scales. They are among the most widely collected of all antique instruments. The finest examples incorporate wind vanes, compasses, lunar calendars, and multiple dial types within a compact, finely crafted object. The principal Nuremberg makers include Hans Tucher, Paul Reinmann, and Leonhart Miller.
Equatorial Dial
A dial in which the dial plate is set parallel to the plane of the earth’s equator, with the gnomon perpendicular to the plate and parallel to the earth’s axis. The hour lines on an equatorial dial are evenly spaced at 15-degree intervals — a direct consequence of the earth’s rotation — making equatorial dials the simplest to construct mathematically. The shadow falls on the upper surface of the dial plate for six months of the year (when the sun is north of the equator) and on the lower surface for the other six months, so a complete equatorial dial has scales on both faces. Equatorial dials in brass or silvered brass were produced in considerable numbers in England and on the continent from the 17th century onwards.
Horizontal Dial
The most familiar type of garden and table sundial, in which the dial plate is set horizontally and the gnomon is a triangular plate whose hypotenuse (the style) points towards the celestial pole. The hour lines on a horizontal dial are unequally spaced, calculated for the specific latitude of the dial’s intended location. English horizontal dials were produced in stone, lead, brass, and slate for garden use and in brass, silver, and ivory for table use. The finest English horizontal dials were engraved by specialist dial makers and instrument makers and are important examples of the engraver’s and mathematical instrument maker’s art.
Pillar Dial (Shepherd’s Dial)
A portable altitude dial in the form of a cylinder, with a horizontal gnomon projecting from its side at a fixed point. The time is read by rotating the cylinder until the gnomon’s shadow falls vertically, then reading the time from the seasonal scale on the cylinder’s surface. Pillar dials were made in wood, ivory, and boxwood and are among the simplest and most portable of all dial types. They were produced across Europe from the medieval period and are often associated with rural use, hence the alternative name shepherd’s dial.
Polyhedral Dial
A multi-faced dial in which several dial types — horizontal, vertical, declining, and equatorial faces — are combined on the faces of a geometric solid (cube, octahedron, or other form), each face carrying its own gnomon and hour scale. Polyhedral dials were produced as demonstrations of the dialler’s art and as luxury scientific objects, and are among the most technically complex and visually impressive of all sundial types. They were produced in stone for garden use and in gilt brass and silver for cabinet use. Notable English examples are found in the collections of Oxford and Cambridge colleges.
Scientific Instruments: Categories and Terminology
Orrery
A mechanical model of the solar system in which the planets (and sometimes the moon) are driven around a central sun by a clockwork mechanism, their relative orbital periods maintained by gearing. The orrery demonstrates the relative motions of the planets, the phases of the moon, and the phenomena of eclipses. It takes its name from Charles Boyle, fourth Earl of Orrery, for whom one of the earliest examples was made by John Rowley around 1713. The finest orreries — by makers including Benjamin Martin, George Adams, and W. and S. Jones — are elaborate objects of great mechanical ingenuity and decorative quality. Planetaria, telluria, and lunaria are related instruments demonstrating specific aspects of the earth–moon–sun system.
Quadrant
An instrument consisting of a quarter-circle arc graduated in degrees, used for measuring angles — particularly the altitude of the sun or stars above the horizon — and for performing trigonometric calculations. The simple geometric quadrant, with a plumb line, was the standard instrument of the medieval astronomer and surveyor. The mural quadrant — a large quadrant fixed to a wall aligned precisely on the meridian — was the principal instrument of positional astronomy from the 15th to the late 18th century, used by Tycho Brahe, John Flamsteed, and Edmond Halley. The horary quadrant, with scales for telling the time by the sun, is a distinct and collected instrument type.
Sector
A mathematical instrument consisting of two flat rules joined at one end by a pivot, with a variety of scales engraved on their faces for performing calculations in arithmetic, geometry, trigonometry, and gunnery. The sector was invented independently by Thomas Hood in England and by Galileo in Italy in the late 16th century and remained in use until superseded by the slide rule and logarithmic tables. English sectors were typically made in ivory, boxwood, or brass; the finest examples, by makers including Henry Sutton and Edmund Culpeper, are finely engraved instruments of great precision. The identification of the scales and their use requires specialist knowledge of the history of mathematical instruments.
Slide Rule
A calculating instrument in which two logarithmic scales are arranged to slide relative to each other, enabling multiplication, division, and the extraction of roots to be performed by simple addition and subtraction of lengths on the scales. The slide rule was developed from the logarithmic scales of Edmund Gunter by William Oughtred in the 1620s and remained the standard calculating instrument of engineers, scientists, and navigators until the introduction of the electronic calculator in the early 1970s. Antique slide rules — in boxwood, ivory, and later aluminium and plastic — by makers including Coggeshall, Stanley, and Faber-Castell are collected as instruments of scientific history.
Optical Instruments
Microscope
An optical instrument for the magnification and examination of small objects, comprising one or more lenses in a mechanical stage and focusing arrangement. The compound microscope — using an objective and eyepiece lens in combination — was developed in the early 17th century and improved steadily throughout the 18th and 19th centuries. English microscopes of the 18th and early 19th centuries — in lacquered brass, with mahogany cases and finely engraved scales — by makers including John Cuff, George Adams, and Andrew Ross are important and widely collected scientific instruments. The condition of the optics (freedom from fungal growth, delamination, and scratching) and the completeness of the original accessories are the primary assessment criteria.
Telescope
An optical instrument for magnifying distant objects, using a combination of lenses (refracting telescope) or mirrors and lenses (reflecting telescope). The refracting telescope was developed in the Netherlands in the early 17th century and rapidly adopted for astronomical and terrestrial use. The reflecting telescope, overcoming the chromatic aberration of simple refracting designs, was invented by James Gregory (1663) and perfected by Isaac Newton (1668). Fine antique telescopes — in brass, with shagreen or vellum-covered tubes — by makers including Dollond, Ramsden, and Troughton are significant collecting objects. The clarity and condition of the optics, and the completeness and originality of the instrument, are the primary criteria.
Navigational Instruments
Compass
An instrument for determining direction by reference to the earth’s magnetic field. The magnetic compass consists of a magnetised needle pivoted to rotate freely above a graduated card (the compass rose) indicating the cardinal and intermediate points. Marine compasses are mounted in a binnacle and suspended in gimbals to remain horizontal at sea. Pocket compasses for surveying and exploration are typically mounted in a brass case with a hinged lid. The finest antique compasses — by makers including Thomas Heath, George Adams, and the Dollond firm — incorporate vernier scales and are instruments of considerable precision as well as decorative quality.
Octant
A reflecting navigational instrument for measuring angles — particularly the altitude of the sun or stars above the horizon for the purpose of determining latitude — using mirrors to bring two objects simultaneously into the field of view. The octant (so called because its arc is one-eighth of a circle, subtending 45 degrees) was invented independently by John Hadley in England and Thomas Godfrey in America in 1731 and rapidly superseded earlier instruments including the cross-staff and back-staff. It was typically made in ebony with a brass arc and ivory scales. The octant was itself largely replaced by the sextant (with a larger arc of one-sixth of a circle, subtending 60 degrees) in the late 18th and early 19th centuries.
Sextant
A reflecting navigational instrument of greater range and precision than the octant, with an arc of one-sixth of a circle (60 degrees) allowing the measurement of angles up to 120 degrees. The sextant became the standard navigational instrument for determining latitude and longitude at sea from the late 18th century. Fine sextants — in brass with silver or gold scales and mounted in fitted mahogany cases with full sets of coloured and plain shade glasses — by the great London makers are among the most technically sophisticated and aesthetically refined of all scientific instruments. The condition of the mirrors (silvering), the scales, and the vernier and micrometer reading systems, and the completeness of the shade glasses and accessories, are the primary assessment criteria.
Principal Makers
Troughton and Simms
One of the most distinguished firms in the history of English scientific instrument making, formed by the merger of the business of Edward Troughton (1753–1835) with that of William Simms (1793–1860) in 1826. Edward Troughton was the leading maker of precision astronomical and surveying instruments of his generation, responsible for major instruments at Greenwich, Dublin, and observatories worldwide. The firm of Troughton and Simms continued his tradition of excellence, making theodolites, levels, sextants, transits, and astronomical circles of the highest accuracy for scientific institutions and the surveying profession throughout the 19th century. Instruments signed Troughton and Simms, or the successor firm Cooke, Troughton and Simms, are among the most sought by collectors of precision instruments.
Jesse Ramsden (1735–1800)
One of the greatest English instrument makers of the 18th century, Ramsden was a pioneer in the development of precision dividing engines for the accurate graduation of circular and linear scales. His dividing engine of 1775, supported by the Board of Longitude, allowed instrument scales to be divided with an accuracy previously unattainable and transformed the standard of navigational, astronomical, and surveying instruments. Ramsden made theodolites, sextants, and astronomical circles for the leading scientists and institutions of his day, including the Great Theodolite used in the Trigonometrical Survey of Britain. His instruments are marked by their exceptional accuracy, their clean mechanical design, and the quality of their workmanship.
George Adams (1709–1772) and George Adams the Younger (1750–1795)
Father and son instrument makers trading as mathematical instrument makers to King George III, the Adams firm produced a remarkable range of philosophical, optical, and mathematical instruments for the scientific and educational market. Their published works — including the elder Adams’ Micrographia Illustrata and the younger Adams’ Essays on the Microscope and Geometrical and Graphical Essays — established them as leading figures in the popularisation of science. Adams microscopes, telescopes, orreries, and electrical machines are widely collected and are typically characterised by their fine lacquered brass work and the quality of their mahogany cases.
John Dollond (1706–1761) and Peter Dollond (1731–1820)
The Dollond firm was responsible for one of the most important advances in optical instrument making of the 18th century: the practical development of the achromatic lens, in which crown and flint glass elements are combined to cancel chromatic aberration and produce a sharp, colour-free image. John Dollond patented the achromatic doublet in 1758, transforming the quality of refracting telescopes and later of microscopes and other optical instruments. The firm, continued by Peter Dollond and later by the firm of Dollond and Aitchison, became the leading supplier of optical instruments to the Royal Navy and the scientific establishment. Dollond telescopes and optical instruments are among the most widely encountered and collected of all English scientific instruments.
Richard Melville (fl. late 18th–early 19th century)
A Scottish instrument maker working in the tradition of the great Edinburgh and Glasgow scientific instrument trade, Melville produced sundials, mathematical instruments, and surveying equipment of fine quality. Scottish instrument makers of this period — working in a tradition that produced some of the finest precision instruments in Britain — are increasingly recognised by collectors as significant figures in the history of British science and technology. Melville’s dials and instruments, when encountered, are notable for the quality of their engraving and the precision of their construction, reflecting the high standards of the Scottish mathematical instrument trade.
Benjamin Martin (1704–1782)
A London instrument maker, optician, and prolific scientific writer, Martin was one of the most important figures in the popularisation of natural philosophy in 18th-century Britain. His shop on Fleet Street sold a wide range of optical, mathematical, and philosophical instruments and his publications — including the General Magazine of Arts and Sciences (1755–65) and the Philosophia Britannica — reached a broad educated audience. Martin’s microscopes, orreries, electrical machines, and telescopes are widely collected; his ‘drum’ microscope is one of the most recognisable English microscope designs of the 18th century.
W. and S. Jones
A leading London firm of mathematical and optical instrument makers trading from Holborn in the late 18th and early 19th centuries, W. and S. Jones (William Jones, 1763–1831, and his brother Samuel) produced a broad range of instruments for the scientific, educational, and navigational markets. Their published catalogues and instruction books provide important documentation of the instruments they supplied. Jones microscopes, telescopes, and portable scientific instruments are commonly encountered in the market and represent a good standard of English instrument making at a period of high production.
Materials and Construction
Lacquered Brass
The standard material for English scientific instruments from the late 17th century onwards, in which cast, turned, or sheet brass components are finished to a high polish and coated with a clear or amber lacquer to preserve the surface. The quality of the lacquering — its evenness, its adhesion, and the colour it imparts to the brass — varies between makers and periods. Original lacquer in good condition, showing an even, warm amber tone, is preferable to relacquered or unfinished surfaces. The condition of the lacquer is an important consideration in the assessment of antique instruments.
Shagreen
A type of untanned leather with a distinctive granular surface, made from the skin of the shark, ray, or stingray, used as a covering material for the tubes of telescopes and microscopes, the cases of portable instruments, and the handles of surgical instruments. Shagreen has a natural green-grey colour (though it may be dyed) and a characteristic texture of small, regular nodules. It provides a secure, non-slip grip and protects the instrument from moisture and impact. The condition of shagreen coverings — the integrity of the surface, the security of the adhesion, and the colour — is an important factor in the assessment of covered instruments.
Vellum and Paper Scales
Many antique portable instruments — diptych dials, drawing instruments, and early compasses — incorporate printed or hand-drawn paper or vellum scales pasted onto wooden or ivory bases. The condition of these scales — their completeness, their legibility, and the security of their adhesion — is a critical factor in the assessment of these instruments, as missing or damaged paper scales significantly impair both the function and the appearance of the object. Replacement paper scales, even when well executed, should be noted and disclosed.
Condition and Assessment
Completeness
The assessment of whether all original components of an instrument are present is fundamental to its valuation. Most antique instruments were originally supplied with accessories — shade glasses, eyepieces, draw tubes, fittings, cleaning equipment, and instruction papers — in fitted mahogany or brass-bound cases. The loss of accessories, even minor ones, reduces value; the loss of major functional components (the objective lens of a microscope, the index mirror of a sextant) may render the instrument non-functional. Instruments in original cases with complete original accessories are significantly more valuable than the same instrument without case or accessories.
Engraving Quality
The quality of the engraved scales, lettering, and decorative work on a scientific instrument is both a functional and an aesthetic consideration. Scales that are finely and evenly divided, with clear, precise numerals and graduations, reflect the skill of the divider and the quality of the instrument. Decorative engraving — the maker’s signature cartouche, the border work, and any pictorial ornament — varies widely in quality between makers and between different grades of instrument from the same maker. The sharpness and evenness of the engraving, and its freedom from later re-engraving or alteration, are important assessment criteria.
Later Alterations
Alterations to antique scientific instruments — the replacement of original optics with later lenses, the substitution of later scales for original ones, the addition of later fittings — reduce value and should be disclosed. The detection of later alterations requires careful examination of the consistency of the materials, the style of the engraving, the tool marks on mechanical parts, and the patina. Instruments that have been cosmetically restored — relacquered, replated, or cleaned to remove original patina — are less desirable to serious collectors than unrestored examples showing natural age.
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