The text of the original work (including inconsistent spelling, hyphenation, formatting etc.) has been retained, except as mentioned below.
The slight differences between the Table of Contents and the text have not been changed

Changes made to the text:
Some punctuation errors and obvious typographical errors have been corrected silently.
Several illustrations have been moved to where they are described in the text.
Page 123, formula (2): the original shows an unclear superscript after the first L. As described in the following line of the text, this has been changed to L_l.