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PREFATORY NOTE.
About five years ago I was engaged in preparing a catalogue of the ancient books which belong to Christ's Hospital. One portion of these books consisted of a collection of ancient mathematical works presented at various times for the use of that part of the school which is known as the Royal Mathematical Foundation of King Charles II. Amongst them were some well known by name to every mathematical student, but which few have ever seen. Perhaps the most interesting of them all was a little volume, printed in London in 1653, containing Gassendi's Explanation of the Ptolemaic and Copernican Systems of Astronomy, as well as that of Tycho Brahe, Galileo's Sidereus Nuncius, and Kepler's Dioptrics. I found Galileo's account
