IV PKEFACE 



For aid in the preparation of these pages I am also much 

 indebted to those unnamed assistants to whose lot has fallen 

 the greater part of the computations for the tables and whose 

 devotion to their work has added a human interest to a task 

 otherwise monotonous. 



To the many authors whose results are here briefly cited or 

 quoted in extenso I take the opportunity to express my obliga- 

 tions — very sincere obligations — for experience shows that such 

 results come only by hard labor. 



Many of the illustrations have been taken from the journals 

 in which they were originally published and my thanks are due 

 to the editors and publishers of these journals for the privilege 

 of reprinting the illustrations here. 



During the preparation of this book my immediate colleagues 

 have given me encouragement and aid, and I cherish the hope 

 that, should the occasion arise, both of these will be again forth- 

 coming to help mend the gaps and rectify the errors which a 

 close scrutiny of these pages is certain to reveal. 



Philadelphia, 1915. 



