I bought this Recta DP6 compass quite a few years back, when I had a
boat. I drug it out of the closet the other day to get a bearing for a
sat. dish and it occured to me that I had forgotten what the slide out
mirror is for and how does one use it.

Any ideas are appreciated.

This is a really nice compass, marketed by the Swiss Army Knife people.
The case and the compass are both first rate. As of right now, it's
smarter than me, too.