My acknowledgment is due to Miss Helen Douglas Irvine, who with skill and intelligent understanding Of the subject translated into English the French text Of these lectures. I wish also to express my gratitude to my friends, Professor George Lincoln Hendrickson, who took upon himself the tedious task Of reading the manuscript and the proofs Of this book and to whom I am indebted for many valuable suggestions both in matter and in form, and Professor Grant Showerman, who Obligingly consented to revise the last chapters before they were printed.