VU1 PREFACE 



Grateful acknowledgment is due to several of my friends and colleagues, 

 especially to Professors Donaldson, Scammon, Downey, Sigerfoos, McKnight, 

 Bell and Litzenberg, who have read and criticised certain chapters; to the 

 artist, Miss Jean Hirsch, and to various authors for the illustrations used; and to 

 the Graduate School of the University of Minnesota for special support in the 

 original investigations which are utilized in the present work. 



C. M. Jackson. 



Minneapolis. 



