Two Telescopes

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.

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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