VI PREFACE 



The selection and preparation of the illustrations for Part I were 

 under the efficient direction of Dr. W.J. G. Land, and most of the 

 original drawings of the book were made by Miss Anna Hamilton, 

 an artist to whom great credit is due. We owe certain original illus- 

 trations to the cooperation of our colleagues, who are named in con- 

 nection with the figures ; and also some of the drawings in Part III 

 to Miss Anna M. Starr. In addition to the mutual criticism of the 

 authors, Dr. C. J. Chamberlain, Dr. William Crocker, and Mr. 

 George D. Fuller made helpful suggestions in reading the proof. 

 For such errors as remain, after all our efforts to eliminate them, the 

 authors themselves assume full responsibility. In correcting them, we 

 shall welcome the help of the wider circle of users to whom the book 



now goes. 



JOHN M. COULTER. 



CHARLES R. BARNES. 



HENRY C. COWLES. 

 The University of Chicago. 



