414 Authorities 'and Books for Students 



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 Wissen- 

 schaften. 



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. 



TheZ/z>,y 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 Diet. Nat. Biog. What I have 

 said abjout 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 Rev*- 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 Rev*- John Flamsteed; for Bradley little but the 

 Miscellaneous Works and Correspondence of the Rev. James 



