Accession No

0499

Brief Description

micrometer eyepiece

Origin

Maker

Class

astronomy; optical

Earliest Date

1800

Latest Date

1900

Inscription Date

Material

metal (brass); glass

Dimensions

minimum length 114mm; diameter of tube 25mm

Special Collection

Provenance

Inscription

Description Notes

Telescope eyepiece with divided lens micrometer, brass. Three screw-fit parts. 1st the divided lens micrometer connected to telescope mount by rack and pinion focussing adjustment. Semi-lenses (cylindrical) moved in opposite directions by rack and pinion moved by knurled screw, displacement measured by linear scale on brass 0 - 20, numbered by 5, graduated to 1, with Vernier. 2nd, brass tube with one lens. 3rd eyepiece with two lenses.

Condition good; complete

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:39687