Accession No
4466
Brief Description
3 3/4-inch refracting telescope on wooden stand with equatorial mount, and with stool, by Dollond, English, 1767 (c)
Origin
England; London
Maker
Dollond
Class
astronomy; optical
Earliest Date
1760
Latest Date
1811
Inscription Date
Material
metal (brass, oxidised brass, steel); wood (mahogany, 2 others); glass; cloth (baize); ivory
Dimensions
height (including stand) 1770mm; length 1350mm; breadth 840mm stool height 625mm; breadth 660mm; depth 580mm box length 1472mm; box breadth 205mm; box height 192mm
Special Collection
Provenance
Loaned by St John's College, University of Cambridge. Given to St. John's College by Thomas Catton, F.R.S. and a fellow of the college.
Inscription
‘DOLLOND*LONDON’ (eye-end)
Description Notes
3 3/4-inch refracting telescope on wooden stand with equatorial mount, and with stool, by Dollond, English, c. 1767.
Brass refracting telescope with screwfit objective and pushfit objective cover; pushfit eye-end with pushfit eyepiece; eye-end adjusted by knurled brass screw - this adjusts a scale divided 0 - [4.2], numbered by 1, subdivided to 0.05, with vernier reading to 0.005; sighting telescope mounted on top of telescope near eye-end, with screwfit eyepiece and objective, pushfit objective cover. Mounted below the centre of the telescope is a brass semicircular scale divided [85˚] - 0 - [83˚], numbered by 10˚, subdivided to 30´; the knurled edge of the scale allows adjustment of the telescope in a vertical plane, by means of a wooden handled brass rod; at the same time as the altitude of the telescope is altered three telescopic brass rods are expanded or contracted (one rod set near the objective end and the other two at the eyepiece end, and all attached to the supporting frame). The telescope is mounted in a brass-shod mahogany lattice-work stand. Large brass quadrant set vertically in the stand and divided [6˚] - 100˚, numbered by 10˚, subdivided to 20´, again for adjustment of the altitude of the instrument. A large brass column has one end set movably on this quadrant; the other end supports the centre of the telescope and carries a circular scale divided I - XII, I - XII, numbered by I and by 20 minutes, and subdivided to 4 minutes, with vernier reading to 12 seconds; this scale is used for measuring the right ascension of celestial objects.
Wooden stool with circular seat and lattice-work stand with three feet.
Accessories (39 items):
Fitted wooden box with baize protectors contains
one triangular brass foot with one levelling screw; two round brass rests for other legs of tripod.
one inverting prism for eyepiece.
one adjustable eyepiece with six lenses.
six standard eyepieces.
one eyepiece fitting with adjustable filters.
eyepiece cover.
one eyepiece attachment with sliding filter.
single filter.
brass adaptor.
spare viewfinder tube.
bubble level with hooks for attachment to telescope.
micrometer for viewfinder with pushfit lens and crystal
wedge in cork mount.
divided object glass micrometer with scale divided 0 - 40, numbered by 10, graduated to 1; second scale divided 0 - [8.75], numbered by 1, graduated to 0.05 with Vernier divided 0 - 50, numbered by 10, graduated to 2.
Two 4 ft wooden rods with brass key ends, on universal joints, for tightening and loosening fastenings on telescope; also one 2ft rod.
One 3ft wooden rod with key end for adjustment of larger size fastenings on telescope.
Chisel with wooden handle and steel blade.
Narrow brass tube (purpose unknown).
Small fitted wooden box containing
quadrant attachment to main telescope with spirit level and scale divided 0 - [100˚], numbered by 10˚, graduated to 15´. Vernier moves over scale divided 0 - 15´, numbered by 5´, subdivided to 30´´.
Brass-headed steel pin.
Brass key with knurled head.
Five attachments for small micrometer.
Condition good; complete.
Key for this instrument may be on bunch of keys for St. John’s instruments. See location note and notes fields for more information.
References
Events
Description
This 3 3/4 inch refracting telescope is very similar to other Dollond telescopes in the museum and St. John's College. However, it differs from those examples in the solidity of its stand which would keep movement of the instrument to a minum. It was given to St. John's College by Thomas Catton, F.R.S. and a fellow of the college.
[Display label from ‘An University Within Ourselves’ exhibition]
07/02/2022
Created by: Morgan Bell on 07/02/2022
Description
The refracting telescope uses a lens to focus the observed image. Its exact origin is disputed, but it first appeared among Dutch spectacle makers at the beginning of the seventeenth century.
Great discoveries were made using the refracting telescope. Galileo’s work Siderius Nuncius (The Starry Messenger, 1610) describes his discoveries of the mountains on the moon, new stars and the moons of Jupiter.
Galileo’s telescopes consisted of a concave and a convex lens which gave an upright image of low magnification. The Keplerian telescope, which was the main type used in astronomy before the invention of the reflecting telescope, has two convex lenses, which gave an upside-down image with a wider field of view. It can be modified for use on land by adding an extra eyepiece lens, which gives an upright image.
The main problem with refracting telescopes is that they suffer from chromatic aberration. When light travels through an ordinary lens each colour is bent through a different angle. A spectrum of colours would appear around the image being viewed.
An improvement to the quality of telescopic images came in 1758, when John Dollond started manufacturing a special lens made of two different sorts 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 nineteenth century. Refracting telescopes are still in use today but are usually small telescopes designed for amateur users.
Created by: Jenny Downes
FM:39913
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