vi Preface 



the dogmatic form of a text-book. It has also seemed 

 advisable to lighten the book by omitting except in a very 

 few simple and important cases all accounts of astro- 

 nomical instruments ; I do not remember ever to have 

 derived any pleasure or profit from a written description 

 of a scientific instrument before seeing the instrument 

 itself, or one very similar to it, and I have abstained 

 from attempting to give to my readers what I have never 

 succeeded in obtaining myself. The aim of the book 

 has also necessitated the omission of a number of im- 

 portant astronomical discoveries, which find their natural 

 expression in the technical language of mathematics. I 

 have on this account only been able to describe in the 

 briefest and most general way the wonderful and beautiful 

 superstructure which several generations of mathematicians 

 have erected on the foundations laid by Newton. For 

 the same reason I have been compelled occasionally 

 to occupy a good deal of space in stating in ordinary 

 English what might have been expressed much more 

 briefly, as well as more clearly, by an algebraical formula : 

 for the benefit of such mathematicians as may happen to 

 read the book I have added a few mathematical footnotes ; 

 otherwise I have tried to abstain scrupulously from the 

 use of any mathematics beyond simple arithmetic and a 

 few technical terms which are explained in the text. A 

 good deal of space has also been saved by the total 

 omission of, or the briefest possible reference to, a very 

 large number of astronomical facts which do not bear on 

 any well-established general theory ; and for similar reasons 

 I have generally abstained from noticing speculative 

 theories which have not yet been established or refuted. 

 la particular, for these and for other reasons (stated more 

 fully at the beginning of chapter XHI.), I have -dealt in the 

 briefest possible way with the immense mass of observafcons 



