Of the two papers here reproduced, the first was given in 1905 to
a meeting of the women science students in Oxford; the second, in
1908, to the Philosophical Society of this College. They are printed
by request, with the author’s apologies for their incompleteness.
The lecture form has been retained. I am indebted to my brother, Mr.
P. Jourdain, for help in preparing the first lecture, and for his
revision of the text.