PREFACE. 



I AM aware that this book has neither the 

 fullness of personal narrative, nor the closeness 

 of scientific analysis, which its too comprehen- 

 sive title might lead the reader to expect. A 

 word of explanation is therefore needed. I 

 thought little at first of the general public, 

 when I began to weave together in narrative 

 form the facts, letters, and journals contained 

 in these volumes. My chief object was to pre- 

 vent the dispersion and final loss of scattered 

 papers which had an unquestionablg'e %om 

 value. But, as my work grew upon my 

 hands, I began to feel that the story of an in- 

 tellectual life, which was marked by such rare 

 coherence and unity of aim, might have a 

 wider interest and usefulness ; might, perhaps. 



