In the preparation of this work I have availed myself of the aid afforded by previous students in the same field of research, and I have gratefully acknowledged my debt to them whenever there has been occasion to do so. At the same time this is a fresh study of the subject, and has been made with the hope of confirming what is true, correcting mistakes, and gathering additional information. Attention has been given to both the history and the architecture of these buildings. The materials for the former are, unfortunately, all too scanty. No continuous records of any of these churches exist. A few incidents scattered over wide tracts of time constitute all that can be known. Still, disconnected incidents though they be, they give us glimpses of the characteristic thoughts and feelings of a large mass of our humanity during a long period of history. The student of the architecture of these churches likewise labours under serious disadvantages. Turkish colour-wash frequently conceals what is necessary for a complete survey; while access to the higher parts of a building by means of scaffolding or ladders is often impossible under present circumstances. Hence the architect cannot always speak positively, and must leave many an interesting point in suspense. Care has been taken to distinguish the original parts of a building from alterations made in Byzantine days or since the Turkish conquest; while, by the prominence given to the variety of type which the churches present, the life and movement observable in Byzantine ecclesiastical art has been made clear, and the common idea that it was a stereotyped art has been proved to be without foundation. Numerous references to the church of S. Sophia occur in the course of this volume, but the reader will not find that great monument of Byzantine architectural genius dealt with in the studies here offered. The obstacles in the way of a proper treatment of that subject proved insuperable, while the writings of Salzenberg, Lethaby, and Swainson, and especially the splendid and exhaustive monograph of my friend Mr. E. M. Antoniadi, seemed to make any attempt of mine in the same direction superfluous if not presumptuous. The omission will, however, secure one advantage: the churches actually studied will not be overshadowed by the grandeur of the 'Great Church,' but will stand clear before the view in all the light that beats upon them. I recall gratefully my obligations to the Sultan's Government and to the late Sir Nicholas O'Conor, British Ambassador at Constantinople, for permission to make a scientific examination of the churches of the city. To the present British Ambassador, Sir Gerard Lowther, best thanks are due for the facilities enjoyed in the study of the church of S. Irene.