To those who are more interested in the literary and other
expressions of mediaeval culture than in its architecture, I
would suggest that they may prefer to postpone the reading
of Part I (The Visible) and open to Part II (The Invisible).
The order I have followed is, however, the preferable order:
first a viewing of the magnificent creation of rock and glass,
and later a search for its deeper meanings. These involve all
the more serious concerns of the people of the Middle Ages
and I have dealt with them at some length -- Hell, Purgatory,
Limbo, Paradise, and the Last Judgment, Eve and Mary,
saints, devils, sinners, and angels. But the cathedral was not
only a triumph of plastic art and a graphic representation of
the natural and supernatural worlds. It was a sacred theatre
for the celebration of mystic rites, and these too I have dwelt
upon -- ceremonial of fire and water, the Mystery of the
Mass, rites of dedication, songs and processions and the
great music of the Gregorian chant.

It was M. Carey Thomas, of Bryn Mawr, who suggested
to me the idea of undertaking the researches of which this
book is the result, and I wish to pay in this place grateful
tribute to her memory.

To the John Simon Guggenheim Memorial Foundation I
owe a debt of gratitude for making possible the carrying out
of my project. By its generous grant and the equally generous
policy of Henry Allen Moe, secretary of the foundation, who
allowed me unrestricted liberty in the use of that grant,
I was able to travel extensively not only in France but in
various other countries rich in architecture.

To many friends and colleagues I am deeply grateful for
criticisms, suggestions, and valuable help, and I wish I
could adequately express my appreciation of all that they
have contributed. Very specially I wish to thank three
architectural experts, F. H. Bosworth, Professor of Architec-
ture at Cornell, Joseph Hudnut, Dean of the School of Archi-
tecture at Harvard, and Heathcote Woolsey. Also very

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