viii Preface 



no longer be explained or concealed. For example, no 

 modern astronomer as such need be concerned with 

 the Greek scheme of epicycles, but the history of its 

 invention, of its gradual perfection as fresh observations 

 were obtained, of its subsequent failure to stand more 

 stringent tests, and of its final abandonment in favour of 

 a more satisfactory theory, is, I think, a valuable and 

 interesting object-lesson in scientific method. I have at 

 any rate written this book with that conviction, and have 

 decided very largely from that point of view what to omit 

 and what to include. 



The book makes no claim to be an original contribution 

 to the subject ; it is written largely from second-hand 

 sources, of which, however, many are not very accessible to 

 the general reader. Particulars of the authorities which 

 have been used are given in an appendix. 



It remains gratefully to acknowledge the help that I have 

 received in my work. Mr. W. W. Rouse Ball, Tutor of 

 Trinity College, whose great knowledge of the history of 

 mathematics a subject very closely connected with astro- 

 nomy has made his criticisms of special value, has been 

 kind enough to read the proofs, and has thereby saved me 

 from several errors ; he has also given me valuable infor- 

 mation with regard to portraits of astronomers. Miss H. 

 M. Johnson has undertaken the laborious and tedious task 

 of reading the whole book in manuscript as well as in 

 proof, and of verifying the cross-references. Miss F. 

 Hardcastle, of Girton College, has also read the proofs, 

 and verified most of the numerical calculations, as well as 

 the cross-references. To both I am indebted for the 

 detection of a large number of obscurities in expression, 

 as well as of clerical and other errors and of misprints. 

 Miss Johnson has also saved me much time by making the 

 Index of Names, and Miss Hardcastle has rendered me 



