How does Dimmesdale torture himself in The Scarlet Letter?

There is
another instrument of torture that should be mentioned as well:  's "bloody scourge." 
There is quite a large section of the text that deals with Dimmesdale's scourge.  Let's begin
there:

His inward trouble drove him to practices, more in
accordance with theold, corrupted faith of Rome, than with the better light of the churchin
which he had been born and bred. In Mr. Dimmesdale's secret closet,under lock and key, there was
a bloody scourge. Oftentimes, thisProtestant and Puritan divine had plied it on his own
shoulders;laughing bitterly at himself the while, and smiting so much the morepitilessly,
because of that bitter laugh. It was his custom, too, as ithas been that of many other pious
Puritans, to fast--not however, likethem, in order to purify the body and render it the fitter
medium ofcelestial illumination,--but rigorously, and until his knees trembledbeneath him, as an
act of penance. He kept vigils, likewise, night afternight, sometimes in utter darkness;
sometimes with a glimmering lamp;and sometimes, viewing his own face in a looking-glass, by the
mostpowerful light which he could throw upon it. He thus typified theconstant introspection
wherewith he tortured, but could not purify,himself.

A
scourge, of course, is a " whip or lash, especially for the infliction of punishment or
torture."  So Dimmesdale's vigils are full of a lot more than prayer and fasting. 
Dimmesdale is so bothered by his guilt that he actually whips himself in order to do penance for
his sin with .  Dimmesdale is often called a Christ-figure.  Dimmesdale's use of the scourge is
one of the first implements that connect him to Christ (although the scourge is used to purify
Dimmesdale for his own sins instead of the sins of humanity).  There are other connections as
well, such as Dimmesdale's "three temptations" in the "desert" and his
ultimate sacrifice.