There are a number of websites where one may obtain access to copies of documents that present quaternions as they were originally taught.

One of the best sites is the collection of mathematical monographs at Cornell University, http://historical.library.cornell.edu/math/ .  There are books by W.R. Hamilton, C.J. Joly, A.S. Hardy, H.W.L.Hime, A. Macfarlane, A. McAulay.

Online Books, http://onlinebooks.library.upenn.edu/ , also has copies of a number of these books, but they are linked to the Cornell site. Search with the term 'quaternions'.