Accession No
2126
Brief Description
divided telescope object-glass micrometer, 1st half 19th C
Origin
Maker
Class
astronomy; optical
Earliest Date
1800
Latest Date
1850
Inscription Date
Material
metal (brass; bronze); glass; wood
Dimensions
box length 340mm; breadth 156mm; height 62mm
Special Collection
Provenance
Transferred from Institute of Astronomy, University of Cambridge, Cambridge, England, 10/1975. Presented to University Observatories by Benedict Chapman, Master of Gonville & Caius College, see printed ‘Report of the Observatory Syndicate’, 06/1850 in University Library [C.U.R. 29]
Inscription
‘A DIVIDED OBJECT-GLASS MICROMETER
GIVEN TO THE
UNIVERSITY OBSERVATORY
BY
BENEDICT CHAPMAN, D.D.
Master of Caius College
1850.’ (plate on box)
Description Notes
Rack and pinion moves semi-lenses laterally in opposite directions. Square for handle (missing). Brass scale of 0 - 5.2 ins divided to 0.05 ins with vernier on brass with adjustable zero. Second rack and pinion motion rotates divided object glass; square for handle (missing). Push-fit connection to telescope tube.
Fitted wooden box.
Condition good (a little tarnishing); incomplete (handles missing)
References
Events
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:39720
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