Oil on canvas
Early annotation on the back in ink: "Mr William Cavendish painted at the age of around two years by Sir Joshua Reynolds."
A portrait of Cavendish as a child ("Portrait Of Master William Cavendish (1783-1812), Full-Length, Standing Astride a Black Dog, In a Landscape") was sold at Sotheby's in 2008. The written description and the portrait in question can be related to the one presented here.
The son of Lord George Augustus Henry Cavendish and Lady Elizabeth Compton, William Cavendish was painted many times by Joshua Reynolds. Here he was only two years old. According to Reynolds' records (M. Cormack, "The Ledgers of Sir Joshua Reynolds," in Walpole Society, 1968-1970, vol. 42, p. 149.), daily appointments were scheduled for the making of portraits of the young Cavendish.
Reynolds, as a portrait painter, painted those of the Dukes of Devonshire over several generations, who are part of the Cavendish family.