The surveys of physics which I have endeavoured to give on several occasions during recent years, are now collected in
chronological order in the following pages.
Since it was my intention that the essays should reach a wider
circle of readers in due time, I am now publishing them in an
almost unaltered form without much fear of seriously contra-
dicting myself, at least on physical questions. With regard to my
occasional references to psychology, my own views have been
accepted in certain circles, but they have been severely criticized
in others. I do not intend to reply to the criticisms here, but I
would ask my critics not to consider that due to inability or
contempt. Perhaps an opportunity will present itself elsewhere.