Accession No
1004
Brief Description
telescope eyepiece with micrometer, by John Rowley, English, 1710 (c)
Origin
England; London
Maker
Rowley, John
Class
astronomy; optical
Earliest Date
1710
Latest Date
1710
Inscription Date
Material
metal (brass, steel); ivory; glass; wood
Dimensions
length 210mm; diameter of telescope end 89mm; diameter of brass circle 100mm
Special Collection
Provenance
On loan from Pembroke College, University of Cambridge from 1951.
Inscription
John Rowley Fecit (circular scale) [Jim Bennett notes in The Divided Circle, p.64, that this signature actually reads "John Ronley Fecit". This is presumably a quirk of Rowley's engraved script?]
Description Notes
Turned wooden telescope eyepiece with brass section moving wire micrometer. Eyepiece in three screw-fit sections,with one lens remaining in wooden cell. Micrometer consists of two wires mounted on two brass plates moved together by two steel screws linked by cogs to single handle set on circular brass plate. The handle is brass with an ivory handle and a pointer moving over a circular brass scale divided anticlockwise [0] - 100, numbered by 10, graduated to 1. Linear scale on eye end of micrometer, divided0 - 60, numbered by 10, graduated by 1.
Brass-bound wooden ring with thread for attachment to telescope. Micrometer secured to eyepice tube by four brass nuts.
Condition good; complete
References
Events
Description
One of the first major advances in telescope design came in about 1640, with the addition of fine threads or wires into the focal plane of the instrument. This enabled astronomers to measure the apparent size of celestial objects and small angular distances between them.
Remarkably, it is said that the English astronomer William Gascoigne invented the eyepiece micrometer when a spider spun its web in the appropriate position inside his telescope tube. Later examples, like this instrument by John Rowley, included two wires that were moved by a geared handle, with a pointer registering their displacement.
04/07/2019
Created by: Josh Nall on 04/07/2019
Description
Micrometer
Micrometers were used mainly by astronomers and microscopists to measure objects. They were first devised in about 1609 and used the exact number of turns of a screw to measure small distances. Micrometers were inserted into the path of a telescopes and microscopes, and were also used to accurately measure quadrant scales.
Early examples of the micrometers used in telescopes were calibrated geometrically using a piece of card at a distance of about 200m with lines of known separation on it. However, this introduced inaccuracies of a few seconds of arc due to the closeness of the card compared with the distance of real observations. It was not until 1672 that this problem was overcome and accurate calibration was possible.
18/10/2002
Created by: Saffron Clackson on 18/10/2002
FM:39694
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