In the present edition, the author has continued to follow the course previously adopted by him, to utilize the best illustrations available rather than to use his own to the exclusion of others. Too many books purporting to be treatises on certain subjects are merely expositions of the author's views to the belittling of the opinions and work of others. Authors of treatises should be just and impartial and should not give undue prominence to their own views and Opinions. This, obviously, they owe to their readers and to the public in general. The author of this book hopes that he has succeeded in adhering faithfully to that belief and, if he has failed in ever so small a degree, it has been unintentional.