In 1847–48 St. Sophia underwent a thorough restoration by the Swiss architect Gaspare Fossati. In the course of this work the Byzantine mosaics, which had previously been obscured with plaster and whitewash, were temporarily revealed. Having made a record of them, Fossati was compelled to cover again all the mosaics representing Christian subjects.

In this volume the Fossati papers relating to the mosaics of St. Sophia are for the first time made the object of a comprehensive publication and discussion. This material is supplemented from other sources, namely the hitherto unpublished drawings of Cornelius Loos and excerpts from the accounts of travelers who visited Istanbul after the Turkish conquest.