Cf. another blue-glazed dou illustrated by M. Medley, 'The Illustrated Regulations for Ceremonial Paraphernalia of the Ch'ing Dynasty', in the Victoria and Albert Museum, Transactions of the Oriental Ceramic Society, vol. 31, 1957-1959, pp. 95 - 105, pl. 39a. Medley noted that in the Winter Solstice sacrifice to Heaven in 1748, blue porcelain vessels were used for the first time as stipulated by Emperor Qianlong, ibid, p. 100.