In June 1715 Thornhill was officially awarded the much-coveted commission to decorate the dome of St Paul's Cathedral in London, for which he had been competing since 1709 (see Tate L01481). This design, one of a set of eight (see also Tate L01483-9), follows the 1709 ruling that the subject matter be taken from the Acts of the Apostles. It also heeds the new stipulation of 1715 that the work be executed in 'Basso-Relievo', in other words in monochrome, simulating sculptural relief. Thornhill divided the dome into eight architectural compartments, each segment treating a different episode in the life of St Paul. All eight of the oil sketches follow almost exactly the scenes as finally finished in the cupola: it is perhaps the case, therefore, that they form a presentation set painted after the originals on the ceiling, and are not preliminary models.
Working with a team of two or three painters under him, one of whom was Robert Brown (died 1753), work started in 1715. In 1716 Thornhill is said to have hosted an entertainment on the scaffold platform erected in the dome, and a visitor in August of that year noted that the architectural decoration was almost complete but the 'history part' had yet to be begun (cited in Croft Murray 1962, p.73). The entire St Paul cycle was probably complete towards the end of 1717, for which Thornhill received £4,000, but he continued to work on the painted architectural decoration of the lantern, and further scenes from the life of St Paul in the Whispering Gallery, until 1721. The latter no longer exist.
The first episode, The Conversion of St Paul, was positioned directly to the east, in line with the altar. Thornhill chose the moment when Paul, journeying to Damascus, was struck to the ground and heard God calling him: 'as he journeyed, he came near Damascus: and suddenly there shined round about him a light from heaven: And he fell to the earth, and heard a voice saying unto him, Saul, Saul, why persecutest thou me?' (Acts 9: 3-4). Thornhill believed complete familiarity with textual sources to be an essential intellectual ingredient for the history painter, as well as the ability to depict visually a precise and intelligible moment in the narrative without departing from textual facts. In certain circumstances, however, he accepted the necessity to improve on historical moments, to render them more visually effective. No horses are mentioned on the road to Damascus in the Bible, yet their rearing and flailing forms add drama to the moment, and it had long been conventional for artists to include them.
Arline Meyer, Sir James Thornhill and the Legacy of Raphael's Tapestry Cartoons, exhibition catalogue, Miriam and Ira D. Wallach Art Gallery, Columbia University in the City of New York, 1996
Carol Gibson-Wood, 'The Political Background to Thornhill's Paintings in St Paul's Cathedral', Journal of the Warburg and Courtauld Institutes, vol. 56, 1993, pp.229-37
Edward Croft-Murray, Decorative Painting in England 1537-1837, I, London 1962, pp.73-4 and 271b