Seligue - Chevalier early achromatic experimental microscope, ~1820

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This is an enigmatic and thus far unparalleled microscope. It features many 18th century characteristics, yet the optics point at a very early 19th century date.

This microscope is undoubtedly inspired by the English Nairne type Chest Microscope, a term referring to a Cuff-style compound microscope mounted to a mahogany box, or "chest". This form was introduced ca. 1760 by Thomas Nairne (worked: 1748-93) and became a staple instrument throughout Nairne's solo career and well into his partnership with Thomas Blunt (1774-93), his former apprentice.. It was quickly copied by most of the leading microscope makers both within Britain and in continental Europe. However, the form seen here, still showing many 18th century characteristics, depicts many improvements and it is by far more sophisticated than Nairne's original design. Also in terms of optical quality it suppresses Nairne's microscope and some other pre-achromatic models (i.e., the Culpeper type). 

The microscope seen here is housed in a solidly made oak chest with few inner compartments for the accessories. Five objectives and two peculiar lieberkuhn reflectors undoubtedly belong originally to the set. The brass forceps, two tiny unfitting stage stops and few other brass parts that came with it seem to be unoriginal additions. When opened, a dovetailed part of the sidewall can be removed in order to allow free movement of the single-sided plane mirror as the stand is erected. The  stand is fixed in the base of the box by a compass joint. The pillar rises or collapses into the chest, enabling convenient use of the microscope at any angle. Atop the pillar is another compass joint secured by two knurled knobs, allowing for the tilting of the optical tube forward and downward, suggesting that perhaps it was designed for a special inspection or aquarium purpose. While this feature is known from Benjamin Martin's Universal Microscope, it is unique amongst chest microscopes. The small stage can be rotated into position or out of the way when the stand is resting into the chest. It is double sided, one side is designed to take the archaic bone sliders, while the other was perhaps holding a plate for opaque specimens, or a wet cell for aquatic or botanical inspections. 

The most astonishing detail is revealed when the optics are examined. Today there are five objectives, having a unique and very small thread of merely 9 mm. in diameter. Three of the objectives are tiny button type, two are longer and contain doublets, presumably of an early achromatic type. Two Lieberkühns are included, one (for the lower magnifications) can be screwed to a special thread around the objective base (similarly to Delebarre's microscope), the other, for the higher magnifications, can be slid along the objective's tube. When the eyepiece sliding cover unit, a typical 18th century feature, is screwed out, the field of view becomes very wide and the optical quality is impressive.

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The big question about this mysterious microscope relates to the identity of the maker and the date of production. So far, we neither found a similar instrument in any of the collection catalogues, nor in auction or private collection records. If indeed it is an achromatic microscope, it must be placed at a very early phase of the 19th century. This issue is still under investigation and updates will follow.

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Analyses of the optical system

The early achromatic chest microscope came with an assemblage of objectives and other components of optical parts. Two objectives (A, B, seen above with their inner parts dismantled) were achromatic, another three (C, D, E) were non-achromatic. The role of the other brass components is unclear. The two achromatic objectives contain each a small cylindrical tube, each one of which contains one doublet (see below). In objective A the cylinder slides freely into the tube while in objective B it screws into the end of it. Four discoidal lenses fitting the internal diameters of the tubes were found, two were still inserted into objective A. It was unclear whether each two of the lenses were originally inserted into each objective, or whether all four of them originally came from one of them. All four lenses bear a film of some sticky matter on one side, now blackened with dirt. Therefore, they may have been glued to each other in the past in pairs by some optical resin and inserted into the objectives.

The two composite objectives were dismantled and their internal brass cylinders containing the assumed doublets, as well as the four free lenses, were taken out. The internal lenses within the brass cylinders were held in place by brass circlips (photo below), and it was decided to open only the one from objective B. It contains two lenses, dry-set together, one is double convex and the other is a double-concave cylinder. No glue or balsam was holding these lenses together.

The results of the chemical analysis of the lenses are provided in the table below. The "Balance" is the concentration of the light elements of atomic number 1-11, which the portable XRF is incapable of identifying. The ± sign refers to the limits of detection for each element.

Physical and chemical analyses of the lenses

The optical components were subjected to scientific analyses in the Laboratory for Microarchaeology at the Ben Gurion University of the Negev, Israel. Non-intrusive elemental analysis of the lenses was carried-out using a Niton™ XL3t GOLDD+ XRF Analyzer set to the "Mining" calibration, providing a list of elements from Mg to U. Screening was made for 30 seconds per "filter", summing up in 120 seconds for each analysis. The data was set to be displayed in weight percent. These tests were aimed at providing some preliminary information about the optical system, first and foremost to clarify if it is indeed made of achromatic doublets and if so, try to reconstruct its general setting.

The lenses were photographed under a Zeiss Stemi-2000 stereomicroscope equipped with upper LED ring illumination. 

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© Microscope History all rights reserved

© Microscope History all rights reserved

Molecular study of the coating on the lenses

The resin remains were attempted to be analyzed using a Rikagu™ Progeny ResQ 1064nm handheld Raman spectrometer. The aim of this study was to reveal the composition of the matter remains coating lenses 1-4.

The Raman indicated that the four lenses were coated by turpentine made from tree resin. It indicates that the lenses were glued (presumably in couples) by an optical-quality mounting resin. 

The elemental concentrations of the lenses indicate that while the double-convex lens of the doublet in objective B is made of high silica-lime "crown glass", the double concave cylindrical "negative" lens attached to it was made high lead-alkaline + silica "flint glass". This clearly testifies to an achromatic doublet of an early and very simple type. The four free lenses bearing the resin/balsam remains were all found to be of a more or less similar composition, all being made of crown glass but with somewhat lower contents of calcium (Ca) as compared with the "positive" lens from the doublet.

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