Accession No

0891

Brief Description

reflecting telescope, 2-1/4-inch, Gregorian, English, mid-18th Century

Origin

England (?)

Maker

Class

astronomy; optical

Earliest Date

1725

Latest Date

1775

Inscription Date

Material

metal (brass; speculum metal); glass

Dimensions

length of barrel 346mm; diameter of primary mirror 60mm

Special Collection

Robert Whipple collection

Provenance

Purchased by Robert Stewart Whipple in 06/1939.

Inscription

Description Notes

Gregorian reflecting telescope with 2-1/4-inch primary mirror. Barrel, friction-tight cover with threaded piece. Eye end holds speculum metal objective mirror by 3 brass springs. Screw-in eyepiece with eye stop on single draw tube. Focus by long screw moving speculum metal secondary mirror from eye end. Eyepiece lenses in threaded brass mounts. Peep hole and window sights (cross hairs missing on latter). 2 knurled screws secure barrel to altazimuth mount with wing nut clamp for vertical motion. Pillar stand with screw. Folding tripod base with cabriole feet.
In accessions register stained pine case is mentioned, now missing.

a telescope in the teaching gallery (0168) was incorrectly given this number

Condition

References

Events

Description
Isaac Newton built the first reflecting telescope in 1669. Unlike a refracting telescope a reflecting telescope uses concave mirrors to focus light. The great advantage of using mirrors is that all colours of light are reflected equally. This was an improvement on the refracting telescope which used lenses to focus light. Here the light refracted at different angles to create a distorting fringe of colour around the image being observed.

At first high quality reflecting telescopes were difficult to make as the mirrors available to use degraded quickly and were unable to transmit enough light. But by the eighteenth century methods of making mirrors had improved and instrument makers such as James Short had created hundreds of reflecting telescopes for sale.

There are three types of reflecting telescope: the Gregorian, the Cassegrain and the Newtonian. Both the Gregorian and Cassegrain telescopes are pointed directly at the object being viewed. But the Newtonian has a secondary mirror set at 45° to the objective so the observer stands at right angles to the telescope. Today modern telescopes (with apertures greater than one metre) are reflecting telescopes. Various innovations make them more powerful and accurate. Computer controlled deformable mirrors can correct against changes due to atmospheric conditions or faults in the material.

Created by: Jenny Downes

FM:40129