DLO LIve! Ep. 69 PU Scopes. How do riflescopes work?

Last week someone dug up an old article I did on WWII era PU scopes that were on Soviet 91/30 Sniper rifles sttarting in 1942 and onward. Altogether, there were well in excess of 300,000 of these that were made.
While we are at it, this is a good time to re-visit how riflescopes work in general. There have been some questions about that.