on Gutenberg for online read or to download (html - convenient for web browser !)
and in EPUB format
http://www.gutenberg.org/ebooks/37589
A pdf version (horrid format !) is on Archive
http://www.archive.org/details/throughmagicglas00buck
Looks like Ms Duncan is too young to find on the free sites, so I'll have to watch ebay !
and I cant find the old Jahn either, only a revised version which is also too recent to be downloadable.