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 small oil is one of a set of eight (see Tate L01482-4 and L01486-9) which probably forms a presentation set painted after the eight scenes from the life of St Paul as finally finished in the cupola. Thornhill strictly adhered to the 1709 and 1715 rulings that the dome be painted with figurative histories taken from the Acts of the Apostles, and that they be executed in monochrome, simulating sculptural relief. Thornhill worked on the cupola until 1717, for which he was paid £4,000, and on other areas of the dome until 1721 (see Tate L01482).
The Conversion of the Gaoler at Philippi is the fourth episode in the cycle, which moves anti-clockwise round the dome starting with Paul's conversion positioned directly to the east. It is taken from Acts 16: 16-26. Paul and Silas, while preaching at Philippi, one of the chief cities of Macedonia, were beaten and cast into prison. At midnight they prayed to God, 'and suddenly there was a great earthquake, so that the foundations of the prison were shaken: and immediately all the doors were opened, and every one's bands were loosed'. The gaoler, terrified that his charges had escaped, would have killed himself save for Paul reassuring him that they were still there. Thornhill chooses the moment when the gaoler 'called for a light, and sprang in, and came trembling, and fell down before Paul and Silas'.
Thornhill heightens the drama of the moment through the inclusion of other guards, one of whom runs into the cell holding a blazing torch aloft, which illuminates the cavernous space and the figurative grouping in the foreground.
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