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A large reflecting telescope, made by William Herschel

These two different types of telescope date from a similar time period and were made by important instrument makers. They differ in their methods of focusing distant light from the stars: one uses lenses to refract (bend) incoming light, the other has mirrors to reflect light.

A 'beautifully brilliant telescope'

Refracting telescope
Image 1: Refracting telescope made by Jesse Ramsden; circa 1785. This telescope was probably owned by James Stuart MacKenzie, brother of the then-Prime Minister (Wh.1002).

This telescope (Image 1) is a refracting telescope. Glass lenses are used to bring the light of distant objects into focus, magnifying them. Different colours of light are refracted (bent) through different angles. For this reason, images seen through a refracting telescope may suffer from a type of colourful distortion, known as chromatic aberration.

Read more: aberration (in microscopes)

The body of the telescope with The maker's mark of Jesse Ramsden (1735-1800), a famous 18th-century astronomical instrument maker, is inscribed on the body of the telescope. A later handwritten label is stuck to the inside of the 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 Honble 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 style."

"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; the brothers' intimacy with the King was disliked by Members of Parliament. James McKenzie left politics in 1780 and dedicated himself to science.

A Royal reflecting telescope

One of the most prominent objects on display in the Main Gallery of the Whipple Museum is the 'Herschel' telescope (above). It is a reflecting telescope, using curved and flat mirrors to reflect light and form a magnified image. As lenses are not used, reflecting telescopes do not suffer from chromatic aberration.

The telescope takes its name from William Herschel (1738-1822), who achieved public acclaim and royal favour through his discovery of the planet Uranus. He originally called the planet the Georgium Sidus (Latin for 'George's Star'), to honour King George III in 1781.

A few years later George III requested that Herschel make a number of telescopes. The Whipple Museum's example is one of five 10ft reflecting telescopes made in response to that request. Following Herschel's standard design, the King's cabinet maker constructed the mahogany stand and tube. Herschel made the optical parts himself.

Previous owners of the reflecting telescope

Sketches of the optics of the Herschel telescope
Image 2: Sketches by Sir Howard Grubb, who examined on the telescopes's mirrors. The two discs show how an artificial star appeared, either side of the point of focus, when magnified by the 'Herschel' telescope's mirror (Wh.0012).

The history of the Whipple's Herschel telescope has been well documented. George III presented it to George Spencer, the fourth Duke of Marlborough in 1786, saying "I can answer for the excellency of this instrument, having twice compared it to the one in my possession".(1) It was held in the Observatory at Blenheim Palace until it was given to Herschel's great-grandson, Joseph Hardcastle (1868-1917). The Hardcastle family then sold it to Howard Marryat in 1927 who then gave it to Robert Whipple in 1944, to mark the foundation of his gift of 2000 scientific instruments and books to the University of Cambridge.

When the telescope was in the possession of Joseph Hardcastle he sent the mirrors to be examined by Sir Howard Grubb, who worked on the optics of periscopes for the Royal Navy during the first World War. In a letter written to Hardcastle he described the large relecting mirror's optics as 'good' (see Image 2 for a sketch drawn by Grubb). Rather than the aesthetic qualities of his great-grand father's telescope, Hardcastle was interested in how well the optics still worked.

Read more: another telescope with accompanying handwritten instruction sheet

References

M. Fowler, Blenheim: Biography of a Palace (London: Viking, 1989), p. 116.

James Hyslop

James Hyslop, 'Two late 18th-century telescopes', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2008

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