In this address I use the word Euclid not as the name of the man but as a short title for the XIII books of his Elements of Geometry. The “Euclid” of our schooldays was but a small part of the whole, just as the “Newton” of our Cambridge days was a mere fragment of the Principia. We learnt the whole of Books I-IV, about half of VI and about half of XI, of the rest we knew nothing.