Accession No
1002
Brief Description
2-¼-inch refracting telescope, by Jesse Ramsden, English, 1785
Origin
England; London
Maker
Ramsden, Jesse
Class
astronomy; optical
Earliest Date
1785
Latest Date
1785
Inscription Date
1785
Material
metal (brass); wood (mahogany); glass
Dimensions
[Main broad barrel 1:] 367mm; [Mid-section linking barrel to eye tube:] 38mm; [eye tube:] 245mm [Aperture:] 2-¼-inch (57 mm) [box:] 470mm (l) x 174mm (w) x 105mm (t) [Note: with the telescope sitting horizontal on its tripod stand, the distance from the ground to the underside of the tube is 443mm.]
Special Collection
Provenance
On loan from Gonville & Caius College, University of Cambridge from 1949.
Inscription
[Main tube:] Ramsden London
[Eyepieces numbered:] 1,2,3,4
[Manuscript label pasted to inside of box’s lid:] “Ramsden 1785”
& “This excellent little telescope was made by Mr. Ramsden for the Honble Mr. Stewart Mc. Kenzie - only three of this size were ever made.
It is the most complete portable instrument I have seen - beautifully brilliant as a day telescope - & shews double stars in the finest stile. There are two eyepieces for erect vision added to the original ones -- which magnify 45 & 130 times.”
Description Notes
2-¼-inch refracting telescope, by Jesse Ramsden, English, 1785.
Brass refracting telescope. Achromatic object glass in threaded brass mount. 2 draw tubes, one with rack and pinion focussing adjustment. 4 lens erecting eyepiece in screw in brass tube, lenses in threaded brass mounts. Altazimuth mount on pillar and folding tripod stand with straight legs. Fitted Mahogany box bound with brass. Racked stay (attachment to pillar is broken). 2 telescopic stays. 6 brass mounted eyepiece fittings and shades. Brass tool for tightening mount, with turned mahogony handle.
Ms. label pasted inside box lid, giving table of magnifications etc., plus the following:
“Ramsden 1785” & “This excellent little telescope was made by Mr. Ramsden for the Honble Mr. Stewart Mc. Kenzie - only three of this size were ever made. It is the most complete portable instrument I have seen - beautifully brilliant as a day telescope - & shews double stars in the finest stile. There are two eyepieces for erect vision added to the original ones -- which magnify 45 & 130 times.”
Good/fair condition. Some tarnishing to brass.
References
James Hyslop; 'Two late 18th-century telescopes'; Explore Whipple Collections online article; Whipple Museum of the History of Science; University of Cambridge; 2008: https://www.whipplemuseum.cam.ac.uk/explore-whipple-collections/astronomy/two-telescopes
Events
Description
Refracting telescopes use glass lenses to bring the light of distant objects into focus, magnifying them. The telescope’s exact origin is disputed, but it first appeared among Dutch spectacle makers at the beginning of the seventeenth century. Lenses are not ideal for focusing light because, in the same way that a prism splits white light into its spectrum, different wavelengths (colours) of light are refracted (bent) through different angles as they pass through a lens. For this reason, images seen through a refracting telescope may suffer from a type of colourful distortion, known as chromatic aberration. An improvement to the quality of telescopic images came in 1758, when John Dollond started manufacturing a special lens made of two different types of glass. This reduced chromatic aberration by bringing two particular wavelengths of light into a common focus. Achromatic lenses and improvements in glass making made both small and large refracting telescopes popular in the eighteenth and nineteenth centuries.
The prestigious London instrument maker Jesse Ramsden (1735-1800) made this particular telescope in 1785. Through his marriage to John Dollond’s daughter, Sarah, Ramsden was able to acquire a share in Dollond’s patent for the achromatic lens, enabling him to make high-quality telescopes like this example. A later handwritten label is stuck to the inside of this telescope's box, explaining that the instrument was collected for its beauty and rarity as well as its optical ability:
"This excellent little telescope was made by Mr Ramsden for the Honourable Mr Stewart McKenzie -- only three of this size were ever made. It is the most complete portable instrument I have ever seen -- beautifully brilliant as a day telescope -- & shews double stars in the finest stile."
"Stewart McKenzie" may have been James Stuart MacKenzie, (1719-1800) the politician and amateur astronomer. He was the brother of the Prime Minister John Stuart. James McKenzie left politics in 1780 and dedicated himself to science.
19/12/2013
Created by: Joshua Nall on 19/12/2013
FM:39683
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