I learn from Wieland Wilker at the Textualcriticism list that all four volumes of this (vol. 1 pts 1 and 2, and vol. 2 pts 1 and 2) are online. I was able to find three of them easily enough. But the one I could not find was vol. 1 part 2; that is here:
The work is very valuable still, because Harnack crammed it full of erudition which later compendia such as Quasten, good as they are, tend to omit.