John Dollond was a British maker of optical and astronomical instruments who developed an achromatic refracting telescope and a practical heliometer, a telescope that used a divided lens to measure the Sun’s diameter and the angles between celestial bodies.
Background
John Dollond was born on June 21, 1706 (O.S. June 10) in London, England, of French Protestant parents who had originally lived in Normandy. The conjectured original spelling of their name as d'Hollande implies that they were of Dutch extraction. Dollond was brought up to the hereditary trade of silk-weaving, and his father's death, while he was still a child, compelled the sacrifice of his education to the necessities of his family.
Education
Dollond followed his father's trade, but found time to acquire a knowledge of Latin, Greek, mathematics, physics, anatomy and other subjects.
Career
Dollond's eldest son, Peter, joined him as a weaver but, stimulated by the knowledge of mathematics and optics learned from his father, subsequently took up the trade of optical instrument maker. This venture was successful, and John Dollond was consequently persuaded to leave the weaving trade. He joined Peter in business in 1752, and the partnership soon became fruitful.
Some ideas for the improvement of the optical arrangement of lenses in the refracting telescope were incorporated in a letter addressed to James Short, a fellow of the Royal Society, who communicated the letter to the society, where it was read on March 1, 1753. Soon after this, on May 10, 1753, another paper was read to the Royal Society, this time on an improved micrometer (heliometer) for the telescope. Dollond had modified the Savery micrometer by using one object glass cut into two equal segments instead of two whole lenses. The micrometer could now be applied to the reflecting telescope, which was immediately done by James Short.
Eventually Dollond conducted (1757-1758) a series of experiments with different kinds of glass to check Newton’s findings. The paper incorporating the results, with the conclusion that the objectives of refracting telescopes could be made “without the images formed by them being affected by the different refrangibility of the rays of light,” was read to the Royal Society in June 1758. Dollond’s composite objective was patented, but the patent was challenged by a group of London optical instrument makers after his death. In 1766 the court upheld Peter Dollond’s right to the patent on the grounds that Chester More Hall, the inventor of an achromatic lens combination in the period 1729-1733, did not exploit the invention commercially or publicize his findings.
It seems unlikely that Hall’s invention could have been known to anyone capable of realizing its significance, because the Royal Society not only published Dollond’s papers but conferred both the Copley Medal (1758) and membership (1761) upon him. The certificate proposing Dollond for membership was signed in February 1761 by ten men, including scientists of the standing of Gowin Knight, John Smeaton, James Short, William Watson, and John Ellicott. The proposal specifically refers to Dollond’s invention of “an Object-Glass, consisting of two Spherical Lenses of different densities, so contrived as to correct the Errors arising from the different refrangibility of the Rays of Light.”
Early in 1761 Dollond was appointed optician to King George III. Regrettably, he did not enjoy this honor for long; he died of apoplexy later that year.
Dollond’s early successes in optics brought him to the attention of astronomers and mathematicians. He corresponded with many, including Euler, against whom he defended Newton’s opinion that no combination of lenses could produce an image free of color, and that in this respect no improvement could be expected in the refracting telescope.
Membership
In 1761 Dollond was elected a fellow of the Royal Society.
Royal Society
,
United Kingdom
1761
Connections
Dollond had three daughters and two sons, Peter and John.