viii PREFACE 



For assistance in the investigations upon which these 

 conclusions are based we wish to make grateful acknowl- 

 edgments to Miss Bertha I. Barker, Miss A. P. Hale, Miss 

 M. D. Hale, and Miss Elizabeth Strongman. For the 

 color chart reproduced as the Frontispiece of this volume 

 we are indebted to Miss Strongman. 



Brookline, Massachusetts, 

 March, 1908. 



