The picture shows a meeting of the British and Foreign Anti-Slavery Society in 1840. The society was founded in 1839 with the aim of world-wide abolition of slavery and the slave trade. The figure on the front left-hand side is Thomas Clarkson, speaking to an audience of over 500 in which a number of historical figures can be identified. (For further information, see http://www.npg.org.uk/collections/search/portrait/mw00028/The-Anti-Slavery-Society-Convention-1840).
Benjamin Robert Haydon (1786–1846), The Anti-Slavery Society Convention, oil on canvas, 297,2 cm x 383,6 cm, 1840; source: National Portrait Gallery, https://www.npg.org.uk/collections/search/portrait/mw00028/The-Anti-Slavery-Society-Convention-1840, Attribution-NonCommercial-NoDerivs 3.0 Unported (CC BY-NC-ND 3.0), https://creativecommons.org/licenses/by-nc-nd/3.0/.