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414 on the Two Chief Systems of the World, either in the original or in the picturesque old translation by Salusbury: there is also a modern German version of this book, as well as of the Two New Sciences, in Ostwald's series of Klassiker der exakten Wissenschaften.

Chapter VII.—For Kepler's life I have used chiefly Wolf and the life—or rather biographical material—given by Frisch in the last volume of his edition of Kepler's works, also to a small extent Breitschwerdt's Johann Keppler. For Kepler's scientific discoveries I have used chiefly his own writings, but I am indebted to some extent to Wolf and Delambre, especially for information with regard to his minor works. The portrait is a reproduction of one by Nordling given in Frisch's edition.

The Lives of Eminent Persons, already referred to, also contains an excellent popular account of Kepler's life and work.

Chapter VIII.—I have used chiefly Wolf and Delambre; for the English writers Gascoigne and Horrocks I have used Whewell and articles in the Dict. Nat. Biog. What I have said about the work of Huygens is taken directly from the books of his which are quoted in the text; and for special points I have consulted the Principia of Descartes, and a very few of Cassini's extensive writings.

There is no obvious book to recommend to students.

Chapter IX.— For the external events of Newton's life I have relied chiefly on Brewster's Memoirs of Sir Isaac Newton; and for the history of the growth of his ideas on the subject of gravitation I have made extensive use of W. W. R. Ball's Essay on Newton's Principia, and of the original documents contained in it. I have also made some use of the articles on Newton in the Encyclopaedia Britannica and the Dictionary of National Biography; as well as of Rigaud's Correspondence of Scientific Men of the Seventeenth Century, of Edleston's Correspondence of Sir Isaac Newton and Prof. Cotes, and of Baily's ''Account of the Revd. John Flamsteed''. The portrait is a reproduction of one by Kneller.

Students are recommended to read Brewster's book, quoted above, or the abridged Life of Sir Isaac Newton by the same author. The Laws of Motion are discussed in most modern text-books of dynamics; the best treatment that I am acquainted with of the various difficulties connected with them is in an article by W. H. Macaulay in the Bulletin of the American Mathematical Society, Ser. II., Vol. III., No. 10, July 1897.

Chapter X.—For Flamsteed I have used chiefly Baily's ''Account of the Revd. John Flamsteed; for Bradley little but the Miscellaneous Works and Correspondence of the Rev. James''