Lionel Henry Cust, Dictionary of National Biography… (1892)
“ For his scientific researches he was elected a fellow of the Royal Society. Knowles is best known, however, from his long, intimate friendship with Henry Fuseli the painter, and the circle to which that artist belonged. He was the executor of Fuseli's will, and a devoted admirer of his art. In 1830 he published an edition of Fuseli's ‘Lectures on Painting,’ and in 1831, in 3 vols. 8vo, the life of Fuseli, written as a labour of love, to which was added an edition of the painter's writings on art. As a biography the work has some merit. ”
