DESCRIPTION:
This is an example of the massive version of Andrew Pritchard s Achromatic engiscope and is signed: Andrew Pritchard, 162 Fleet street, London on the foot as well as on the body tube, and numbered No 560 on the front limb of the foot. This dates this instrument as having been made towards the end of the period 1838-1854 during which Pritchard traded from this address.

The microscope, alone weighs 9 lb, 9 1/2 ounces (4.35 Kg). It is supported by a massive tripod foot which has a span of 210 mm wide and 190 mm long, the weight bearing surfaces of which underneath are covered in thick leather. Unlike earlier Pritchard models, there is no swinging arm arising from the pillar to support a candle holder/condenser. Instead, there are 8.2 mm diameter holes at the end of each foot, presumably to take a vertical rod carrying these accessories. There is a collar clamp at the top of the heavy pillar allowing for an increase in height. When fully extended, this brings the height of the eye piece to 500 mm above the surface in working position. The inclination joint at the top of the pillar forms a clamp, which is tightened by a large knurled knob. This clamp extends to encircle the limb, which contains a triangular bar into which the rack is cut for the coarse focus, which is operated by a single large knurled head on the right similar in size as the one which tightens the clamp.

There is a ring at the top of the limb, the tension on which is adjusted with a knurled knob, which when loosened, allows for rotation of the entire limb for 90 in both directions allowing for the use of a phial-holder attached by means of a bayonet fitting to the stage, so that a tube of liquid containing organisms can be held upright during examination. The image shown to the left illustrates this configuration using a re-created vial holder, the original one lacking from this outfit. Note that the limb in this configuration is both inclined and rotated.

The solid 11.4 mm thick brass stage measuring 105 x 87 mm has multiple apertures for insertion of stage accessories and is principally used to carry one of the accessory stages which are a push-fit into its 41 mm diameter aperture. There is also provision for a dove-tailed substage slider to carry a wheel of apertures, now no longer present. The tailpiece carries a sleeve with fastening screw for the plano-concave mirror. The horizontal arm is affixed to the top of the triangular bar and is fastened with a large knurled head, with another aperture at its proximal end for insertion of stage accessories. The end of this arm carries the 174 mm long body tube which has an internal diameter of 31.4 mm to take the eye pieces.

On this example the fine adjustment is of a unique design, and different from other Pritchard microscopes known to be extant. It consists of a sliding inner sprung nosepiece, at its bottom end in contact with a forked leaf-shaped counter spring. The latter is affixed to a tensioned trunnion placed along the under surface of the arm, whereby fine adjustment is achieved by turning a knurled head operating a fine screw, placed diagonally at an angle of 45 , which causes downward displacement of the fork mechanism. This appears to be a departure from fine adjustment arrangements as present in other examples of this model, including those of a large ring situated at the lower end of the body tube, or a lateral screw half-way up the body tube activating a cone mechanism which lowers and raises the nosepiece. This is illustrative of this period of experimentation, during which a definitive form of fine adjustment was yet to be finalized.

 

COMPARISON TO THE SMALLER ENGISCOPE: This microscope is supported on a much larger stand than the smaller Pritchard Achromatic Engiscope as can be seen on the Pritchard Compound Microscope Comparison Page.

CONDITION:
The microscope and its accessories are in excellent condition, with little or no loss of their semi-gloss golden lacquer finish.

HISTORY OF ANDREW PRITCHARD AND HIS MICROSCOPES:

As an apprentice, Pritchard became familiar with optical glass working, which he and Varley were to extend into making lenses of diamond, which has a higher refractive index, and lower dispersion properties than glass. It was when Cornelius was away on patent business in 1824 that Andrew attempted to make the first diamond lens at the request of C.R. Goring, a medical practitioner with a great interest in improving the optical performance of the microscope. Their association was to last 16 years and resulted in a number of simple microscopes being made with diamond and sapphire lenses, which outperformed those with glass lenses by a degree, and numerical apertures of some 0.26 were achieved. These were time-consuming and expensive to produce however and were never made in great numbers.

Around that same time John Cuthbert (1783-1854)was making reflecting microscopes, using mirror optics, while the optician William Tulley (1789-1835) was working on the development of achromatic microscope objectives at the request first of C.R. Goring and subsequently of the wine merchant Joseph Jackson Lister (1786-1869). The history of development during this period is complex, but the breakthroughs were the Lister/Tulley microscope fitted with a combination of objective lenses made by Tulley himself, and those sourced from France made by Chevalier, who had mastered the technique of making small achromatic doublets. Chevalier made is achromats of a bonded combination of a biconvex crown glass- and a plano-concave lens made of flint glass, contained in a small brass cell, which could be combined with other such units, thereafter known as French buttons. The key to further development appeared in 1830 with Lister's paper on the theory of microscope objectives, highlighting the need to combine objective elements at a specific distance, so that their aplanatic foci coincided, in order to produce an objective which was as free of chromatic and spherical aberration, as possible.

It is worth noting that it was normal for an optical instrument maker/retailer such as Pritchard to outsource some of his components and to obtain these from others such as from Hugh Powell and Andrew Ross, who were at that stage still working for the trade . It is possible, that Hugh Powell was involved in the design of what later became known as the distinctive Pritchard model microscope, an image of which was first printed in The Natural History of The Animalcules in 1834. This also explains why there are examples of microscopes of this model(which, in itself bears a resemblance to early Chevalier models) which display many features of Hugh Powell's work; Powell later started signing his own microscopes bearing this design, and there are also those signed by Pritchard which are fitted with a short side-lever fine focus characteristic of the work of Ross. To complicate matters further, Pritchard himself must also have provided to the trade as there are Pritchard model microscopes which are signed with the names of other opticians, such as Dollond and Bate, whereas others are unsigned.

The skill required to make and assemble the minute components of the new achromatic objectives represented a considerable challenge, and it appears that Pritchard outsourced this work wherever possible, importing achromatic doublets made in Paris by Vincent & Charles Chevalier, while some of his earlier objectives may have been made by Ross. Later during the 1830 s he also sourced high quality objectives from Hugh Powell, whereas some prominent customers, such as Lady Caledon turned to Powell directly to obtain a 1/8 inch objective for her Pritchard microscope.

Pritchard moved his business from 18 Pickett street to 312 Strand in 1829, and was trading from both addresses between 1831-1835, before occupying the 263 Strand address between 1835-1838. Microscopes signed by him and signed with one of these locations are in private and museum collections, which assists with dating.

Many of these instruments conform to the general Pritchard design , but with the development of more complex and sophisticated instruments which emerged from Ross , Smith s, and Powell s workshops from the late 1830 s onwards (and beginning to be signed by themselves), Pritchard attempted his own improvements by selling much more massive versions of his design, such as the microscope illustrated on this page. Nuttall has remarked that 'Visually these lacked the appeal of those introduced by his former workmen and survive only in small numbers', the simplified versions without inclination joint being more widely sold. Pritchard also sold simple microscopes of various kinds, an example of which from about 1837 is a 'pocket microscope' as shown on this site.

By 1838 Andrew Prichard had transferred his business to 162 Fleet street, from where he continued to trade until 1854, and it is likely, that Hugh Powell continued to supply him until about 1840, where after it is likely that the brass work was undertaken in house by his own workers, including his apprentice Alfred Eden.

Towards the second part of the Fleet street period, Pritchard also started engraving his microscopes with a serial number. The numbering system may have started much earlier, as according to Nuttall: A letter accompanying a compound microscope he sold in December 1835, from 263 Strand, perhaps provides some indication of overall production numbers by this date: for though the instrument itself is un-numbered it is referred to as No.66 in the instructions for its use.

From about 1840 onward, Pritchard's business started to suffer from the widespread economic depression, and he branched out into the Patent Business, which was at the time a more lucrative source of income. Inevitably this shift in emphasis caused Pritchard to fall from the first rank of microscope suppliers. He did continue to sell his microscopes, as well as his characteristic paper-covered slides in small leather-covered boxes, and mahogany Microscopic cabinets , in addition to the glass slides containing wood sections sandwiched in between, sealed on the edges by red wax, a method thought to have been originated by William Darker who likely made these for Pritchard. Imported optics were subsequently obtained from Camille Nachet, who had previously worked for Chevalier. Pritchard also sold prepared slides made by Bourgogne.

By now, Pritchard s standing as a microscope manufacturer was on the wane. Nuttall recounts that: His reputation was scarred by the Jury's comments at the 1851 Exhibition, at which, though he gained an honourable mention, his microscopes were dismissed as old fashioned. G.L E Turner adds to this: and his objectives indifferent, which is not surprising, as they were still made out of imported French buttons , albeit with elements of different focal lengths, and sort-of complying with Lister's principles of combining aplanatic foci. As such, a Prichard 1/10 objective with an N.A. of 0.54 could no longer compete with its Ross or Powell equivalent, with a shorter working distance, larger top lens diameter, and an N.A. in the region of 0.90. When his first compound microscopes were sold to an eager public, his work was much more regarded as state-of-the-art, and Pritchard was the first supplier to fulfil a need. His contribution to microscopy as viewed from the present remains considerable however. As an entrepreneur and popularizer of the microscope, summed up rather well by the palaeontologist Gideon Mantell, who had praised Pritchard as a supplier of effective microscopes to his geological friends as late as 1846: By his various works he has greatly promoted the taste for microscopical observations.

Pritchard retired from the microscopical business in 1854, then placing the Fleet street business in the hands of his brother in law, Samuel Straker, his manager from no later than 1849. Straker continued to advertise Pritchard's microscopes, but by January 1855 they had parted their ways. Pritchard kept the Fleet street business open until the end of 1858, only opening to customers on Tuesdays and Saturday mornings. By then, the substantial building was sub-tenanted. Andrew Pritchard died in 1882, after a long retirement. This may not have been the end of microscopes with features resembling those of Pritchard's. This will be the topic of a future feature on this site.

BIBLIOGRAPHY

Family Portrait of Andrew Pritchard with an Example of The Same Model as the Microscope Featured on This Page Brewster, D. A Treatise on the Microscope (Edinburgh, 1837), Reprinted in the 7th edition of the Encyclopaedia Britannica.

Frank, A. Microscopes from the Frank Collection 1800-1860 (Jersey, 1979)

Frank, A. Early Achromatic Microscopes by Hugh Powell. Microscopy 32,July-December 1972

Nuttall, R.H. C.R. Goring, J.J. Lister and the Achromatic Microscope. Microscopy 32, January-June 1973

Nuttall, R.H. Marketing the Achromatic Microscope: Andrew Pritchard s Engiscope. Quekett J Microscopy, 2006, 40, 309-330.

Turner, G.L E. Collecting Microscopes, Littlehampton book services Ltd, 1981.