Correction
The Harvard copy is not the second edition
Front matter; Google Books Nu9lQ3uVC2YC / Internet Archive defensecharlest00johngoog
What is saidBoth copies carry a title page dated 1890 and are catalogued as the same book. The volume is usually cited as the 1890 second edition.
What the record showsThey are two different issues, and the difference is in the front matter. The Harvard copy, accession-stamped October 1890, has no preface to the second edition and no portrait plate of the author; it runs to 499 scanned images. The other copy has both, and 519. Since the added preface is dated 4 December 1890, the Harvard copy must be the earlier issue of that year.
What to doCite pagination freely — the text is identically paginated in both, and every citation on this site has been checked in both. But do not cite Johnson's own corrections from the Harvard copy: they are not in it.