Accession No

1066

Brief Description

divided telescope object-glass micrometer, 2nd half 18th Century

Origin

Maker

Class

astronomy

Earliest Date

1750

Latest Date

1800

Inscription Date

Material

metal (brass); glass; wood

Dimensions

box length 407mm; breadth 192mm; height 65mm

Special Collection

Provenance

Donated by the Solar Physics Laboratory, University of Cambridge in 07/1951.

Inscription

Description Notes

Rack and pinion with square for handle (missing) moves semi-lenses laterally in opposite directions. Second rack and pinion rotates objective with respect to body tube. Lens separation measured by brass scale of 0 - 5.5 ins. Vernier on brass, adjustable zero.
Fitted wooden box.

Condition good (some wear and corrosion); incomplete (handle 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:39724