An English compound microscope, unsigned by Hugh Powell, London, circa 1836

Details
An English compound microscope, unsigned by Hugh Powell, London, circa 1836

A flat tripod foot, the legs with square edges, and inset with cork pads, supports a telescoping pillar with a clamping screw. The limb is connected to the pillar by a compass joint and clamping screw to grip the shaft below the stage. Rackwork moves the body-tube, which is screwed into a cranked arm at the top of the pillar. This arm can be swivelled round so bringing in place a slit collar for simple microscopic objectives. The plano-concave mirror is connected to a sliding tube by a quadrant arm. For certain applications, the limb can be set into the horizontal position limited by a peg stop.
The stage attaches to the limb by a pair of pegs inserted into a bracket adjacent to the focusing wheel. When dismounted, an aperture disc with four apertures screws into place, or this may be substituted by a substage condenser, consisting of a long tube with two lenses, one of which is in an adjustable sleeve. This example of the stage is also fitted with a lateral, X movement by micrometer screw, the micrometer wheel divided into 80 parts and numbered in tens. On the right side of the stage is another micrometer which drives a wedge under the stage to give a measure to the fine focus. The micrometer wheel is divided in 25 parts; this was a Hugh Powell invention. An upper stage plate, with a slide rest, may be fitted by means of a lug. With the aperture disc removed, a swivel arm can be centred to the optic axis bearing a dark well.

There are three eyepieces, labelled A, B, C, each with a cap incorporating a bayonet catch, a characteristic of Powell's earlier microscopes. Each eyepiece has a diaphragm between the lenses to support a crosswire (none present). The body tube has a notch to position the eyepiece when using the crosswire. Two objective cans are present, engraved with the power in a small circle: "1 in", and ".1/4". The two objectives present are late nineteenth century; both have RMS screw threads (after 1858); one is unmarked, while the other is inscribed: ".1/6in. 0.87 N.A." (i.e., after c. 1880). Also present is an adapter from the narrow Powell screw thread to the RMS standard, and a lieberkuhn.

Other accessories are: stage forceps on a jointed arm, which can peg into the lower or upper stage; tweezers; a key to tighten the compass joint on the stand; a candle holder with a bull's-eye lens on an articulated arm. The microscope is contained in a mahogany case with blue velvet on the supports, 11.3/8in. (29cm.) wide

See Colour Plate 15
Literature
G.L'E. Turner, The Great Age of the Microscope: The Collection of the Royal Microscopical Society through 150 Years (Bristol and New York: Adam Hilger, 1989), 114-119.

Lot Essay

The Maker
The design and execution of this microscope all point to Hugh Powell (1799-1883) as the maker. In fact, it is virtually identical to a microscope displayed in the Museum of the History of Science, Oxford (C93) signed: H. POWELL 24, Clarendon Str, Somers Town, London. Comparison with two other closely similar instruments, signed Hugh Powell, London, which are in the collection of the Royal Microscopical Society, Oxford (nos 113, 114) shows that all come from the same workshop. The present instrument and that at the Museum both have the same square edged feet that suggest they are earlier than the RMS microscopes. The first publication of Hugh Powell was in 1836, when he described a fine adjustment in the form of a micrometer screw acting on the stage - as here.
The mechanical stage is to the pattern of Edmund Turrell, where the X and Y motions are on concentric shafts. Turrell was an engraver at 46 Clarendon Street, Somers Town, while Powell was at 24 Clarendon Street from 1832 to 1846. The stage was published in the Transactions of the Society of Arts in 1833, Turrell having been awarded the Silver Isis Medal of the Society in 1832.

The microscope represents the transition period from the Regency models, with stands and tubes poorly mounted, and therefore subject to vibration. After they key publication in 1830 of J.J. Lister, the spherical abberation could be eliminated by the construction of a lens system for the objectives. By about 1837 the three leading London makers, Powell, Ross and Smith, were producing corrected objectives of considerable magnification, and for serious use the stand had to be redesigned. This occured gradually, as microscopy moved from natural philosophy to scientific functions. By 1840 all three makers had achieved much more stable stands.

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