English: Reverend Thomas Scales (left) in a detail from the The Anti-Slavery Society Convention, 1840, by Benjamin Robert Haydon (died 1846), given to the National Portrait Gallery, London in 1880 by the British and Foreign Anti-Slavery Society. Oil on canvas, 1841. 117 in. x 151 in. (2972 mm x 3836 mm). See source website for additional information. Quote from the description at the National Portrait Gallery website:

This monumental painting records the 1840 convention of the British and Foreign Anti-Slavery Society which was established to promote worldwide abolition. A frail and elderly Thomas Clarkson addresses a meeting of over 500 delegates. [...] Haydon later wrote: 'a liberated slave, now a delegate, is looking up to Clarkson with deep interest ... this is the point of interest in the picture, and illustrative of the object in painting it, the African sitting by the intellectual European, in equality and intelligence'.