Footnotes have been moved to the end of the text and relabeled consecutively through the document.

Illustrations have been moved to paragraph breaks near where they are mentioned.

Punctuation and mathematical notation has been made consistent.

Variations in spelling and hyphenation were retained as they appear in the original publication, except that obvious typographical errors have been corrected.

The following change was made:

p. 116: , changed to = (AMB = triangle ABC)