He commanded the light cavalry of the Maison du Roi (English: King's Household). In 1750 he became the king's commissioner to the Estates of Brittany and persuaded the assembly to accept the Vingtième tax.
As an astronomer and physicist he was particularly interested in scientific instruments and used most of his income to build and collect them. His estate contains a remarkable amount of rare and curious items collected from Egypt, Greece, China, including Etruscan vases of all types, antique bronzes and natural history specimens.
At a time when physicists were abandoning glass globe electrostatic generators, which used sulphur or resin, to use glass plates instead, the Duke of Chaulnes built the largest machine of this type that had yet been seen. He used it to produce the effects of lightning for the first time in France.
In 1743, he was made an honorary member of the Académie des sciences. Two years after, he published a memoir in article form which described his experiments, and was included at the beginning of Newton'sOpticks Bk. IV. In this work he described his discovery of the peculiarities of the diffraction of light rays reflected by a concave mirror and how they might be stopped by a board pierced in the middle. In 1765, he introduced an astronomical instrument fitted with two achromatic lenses. He also invented a new microscope and had it built in England; his description of the microscope included several plates. He first thought of the manufacture of artificial mineral water.[5]
His final work was a thesis on a new triquetrum which was stronger and easier to use than previous models.[6]
Personal life
[edit]His wife, the Duchess of Chaulnes (née Anne Josèphe Bonnier) depicted as the goddess Hebe, by Jean Marc Nattier, 1744
He married Anne Josèphe Bonnier (1718–1787), who replaced the Viscountess of Beaune in 1766 as one of Queen Marie Leszczynska's ladies-in-waiting. Together, they were the parents of:[7]
Nouvelle Méthode pour diviser les instruments de mathématiques, in Description des arts et métiers, published by the Académie des sciences, 1768, in-fol. 44 p. with 15 illustrations.
