DESCRIPTION:

This is an example of the standard version of Andrew Pritchard's Achromatic Engiscope and is signed on one leg of the foot: A. Pritchard, 162 Fleet Street, London, A 312. It was designed to be used as either a simple or compound microscope.

The microscope is supported by a solid brass flattened tripod foot with a span of 230 mm. From this arises a 130 mm tall brass pillar, with an internal telescoping piece which allows extension to a height of 210 mm, and is tightened by a large knurled brass ring(A). At its upper end the pillar extends into a compass joint(B) which can be tightened with a key (see below) and extends into a clamp(C) which encircles the tubular limb, allowing this to be positioned at different angles, and is itself tightened by a knurled knob(D).

Inside the top of the tubular limb is a triangular bar with rack and pinion coarse focusing, which is operated by a single knurled knob placed on the right-hand side. At its upper end, the triangular bar carries a double support arm which carries the 188 mm long body tube with an internal fitting diameter of 36.3 mm for the eyepieces. At the other end of the arm, there is a circular sprung clamp for the insertion of single lenses. The arm can be rotated and then fixed in a new position by loosening and tightening a circular knurled knob affixing it to the top of triangular bar. There is no draw tube or fine focusing arrangement on this particular example. The tubular limb extends into the tail-piece carrying a sleeve for the gimballed plano-concave mirror.

The central portion of the limb carries the support for the stage; the stage measures about 65 x 65 mm. The stage attaches to the limb with two sliding pins(shown on the bottom of the stage to the right) which are about 12 mm apart. There is a bracket on the circular limb into which these slot, and the stage can be easily removed by pulling the back of the stage upwards. The stage is fitted with a two-pronged slide clamp at its upper surface and a sprung sliding stage at its lower surface. The latter is kept under tension with two small springs, and allows for larger slides, selenites, or glass tubes to be inserted on the bottom of the stage. The bottom plate of the sprung stage also contains a cylindrical fitting for substage accessories such as a condenser lens, or a polarizer.

ACCESSORIES:

Eyepieces:
There are two eye pieces with approximate magnifications of 5X (a) and 8X (b). In typical Pritchard fashion, these have removable dust caps. The 8X eye piece is also equipped with its own container, which when fitted, keeps the field lens on the bottom dust proof.

Polarized Light Accessories:

In typical Pritchard fashion, the polarized light accessories store as a single unit (c above). For storage, the objective with built-in prism screws into the substage polarizer and dust caps cover this assembly from both above and below. The analyzer prism housing fits inside the analyzer objective.

Pritchard Slide Set:
'd' above is a cased set of slides sold by Pritchard, often supplied with his microscopes, added to this set by the present owner.

Simple Lens:
'e' is a period replacement simple lens that fits the end of the arm of the microscope designed for the use of simple lenses. This is also shown in situ in the uppermost right image above.

Adapters for Objectives:
The microscope came supplied with two adapters (f above), for which there is a matching recess in the case. One has male Pritchard thread and female pre-RMS Smith & Beck objective thread, while the other has male pre-RMS Smith & Beck thread but female Pritchard thread, so the ensemble would allow for Smith & Beck objectives to be used on this microscope, as well as the use of Pritchard objectives on an early Smith & Beck instrument.

Objectives:
There are three Pritchard Objectives stored in original cans (g, h, and i above). These all have the pre-RMS Pritchard thread of 14.5 mm diameter, 36 tpi, and include a 1/3 inch, marked A; 1/5 inch marked B; 1/12 inch marked C, and the polarizing objective containing a small Nicol prism and fitted with two French buttons. As noted above, the polarizing objective is stored with the polarizer. The objectives marked A,B, and C are stored in brass containers. The lowest magnification objective is stored in a can labeled A, 1/3^rdInch. The cap of the can for the mid-power objective is engraved B, A. Pritchard, 162 Fleet street, London. The high power objective's can is engraved C, 1/12^th inch. The polarized light objective with built-in analyzer is a divisible objective.
Optical Data Obtained with Stage Micrometer and Cheshire Apertometer:

OBJECTIVE	APPROX. NA	APPROX MAG.WITH EYEPIECES
		5X	8X
A 1/3 IN.	0.20	100	168
B 1/5 IN.	0.44	260	345
C 1/12 IN.	0.51	500	800
POL 1	0.19	104	152
POL 1+2	0.24	192	305

Stage accessories and Key:

Also included are a stage forceps(j above), and a live box(left), the latter engraved A. Pritchard, 162 Fleet St^t, London. A *key (k above), to tighten the compass joint is also present.

Substage Condenser:

The present owner has added a period replacement substage condenser(l). This is a stopped condenser lens in cylindrical sleeve, as per the design by Dr Goring, and described in The Microscopic Cabinet, fig. 15, page 170, and is shown in-situ to the right.

Nachet Objectives:

Also added are two objectives in cans inscribed 1 Nachet, A.Pritchard and 2, Nachet, A. Pritchard. These were obviously obtained from Camille Nachet in Paris and retailed by Pritchard for use with his own instruments. These carry the internal Nachet objective thread and require the use of an adapter.

Case:
The original mahogany case is in good condition and measures 314 x 235 x 110 mm. Its internal supports are covered with green velvet.

CONDITION:
When purchased the microscope and its accessories were in good original condition, with loss of lacquer and patination being largely confined to the body tube and tubular part of the limb, with scattered lacquer losses elsewhere. The mirror is a later replacement, but has a black rim, as also used in Pritchard microscopes.

HISTORY OF THE STANDARD MODEL PRITCHARD MICROSCOPE:
A first depiction(left) of what was to become the Pritchard Standard Achromatic microscope design can be found in his The Natural History of the Animalcules of 1834. The engraving shows the microscope in its original form featuring a pear-shaped stage with two pronged stage clips and a moveable condenser lens positioned half-way between the stage and the mirror. A number of examples of this model are extant signed with the 18 Pickett street (1827-1838)and 263 Strand (1835-1838) addresses.

Slightly later, the pear-shaped stage was replaced by a square shaped one, still fitted with stage clips on top, but also having a spring stage underneath, activated by two springs surrounding central rods placed in the 12 o'clock and 6 o'clock positions. Other examples are known equipped with Turrell's design of mechanical stage. Versions of the Standard Achromatic model with the more solid square stage similar to that on the Massive model probably date from later in the Fleet street (1838-1854) period, and these could be fitted with a Bonanni type spring (super) stage, or Tyrell mechanical stage. It is commonly accepted that Pritchard started numbering his microscopes at some time during the Fleet street period, and it is likely, that some instruments were numbered retrospectively, i.e. some time after their manufacture. For instance, Nos 122 and 124 both have the Massive style thick stage , whereas Nos A 310, and A 312 (this example) have earlier types of stages. It is therefore tempting to speculate, that numbers with the prefix A were numbered retrospectively, and predate some of the later more massive models.

For more information about the history of Pritchard and his Microscopes, please see the History section of the page on the Massive Engiscope on this site, and the discussion about Pritchard on Brian Stevenson's site, microscopist.net.

*In modern jargon, this would be referred to as a spanner wrench.