xii AUTHOR'S PREFACE. 



progress of science ; I have made this work as perfect as was 

 in my power. I have taken great care with the list of autho- 

 rities. I have also indicated by their titles all the articles from 

 periodicals, reviews, or academic collections, to which I refer 

 the reader. I am sure that those who know what an ungrate- 

 ful task it is to search such badly catalogued libraries as most 

 of ours are, will give me credit for this part of my work. We 

 can only see the expression of science at a given moment in 

 Memoires. Books are, after all, merely a summary : they are 

 behind-hand even on the day they are published. 



G. P. 



