New light on the invention of the achromatic telescope objective

Abstract

In the spring of the year 1758 John Dollond (F.R.S., 1761) first came on the market with his new telescope object-glass, in which both the chromatic and spherical aberration were corrected: the small firm of Peter and John Dollond became famous very quickly. On 19 April 1758, Dollond was issued a patent for the duration of 14 years, and later in the year he was awarded the Copley Medal of the Royal Society. However, soon after this, a rumour spread in optical circles that Dollond had stolen the new construction from Chester Moor Hall, the real inventor. Who was Chester Moor Hall? Unfortunately we have very little knowledge of him. He was born in late 1704 on the estates of Moor Hall in Sutton, near Rochfort, in the county of Essex. His baptism took place on 9 December 1704. His parents were well- off and promoted his studies to become a lawyer. He opened a law office and became a bencher of the Inner Temple, an old London legal institution. In 1733, he was one of the subscribers to Dr Jebb’s new edition of Roger Bacon’s Opus Majus , which indicates his extensive scientific education. Later on, he occupied the position of Justice of the Peace on his estates in Essex. Hall never published any information about his research on the achromatic object glass. The only written documents to survive him are some signatures on judicial files. He died on 17 March 1771. Because John Dollond was also very reticent about his methods and train of thought, the invention of the achromatic telescope objective has remained in some mystery and nebulous darkness. The author of this paper is now in possession of three telescopes from the middle of the eighteenth century, of which the object glasses have all the same construction, but differ very much from the normal construction of English object glasses from this time. I shall describe the main optical research on these three objectives. The results of these measurements give us insight into the thoughts and the practical methods of their inventor. In this short paper, some of the details may not be intelligible to readers who are not familiar with theoretical and practical optics. I would, however, be pleased to explain any problems which may occur, for interested readers.