Found this recently on cloudynights and I can't help feeling it warrants a sticky for the un-initiated:
http://www.brayebrookobservatory.org...fEYEPIECES.pdf
http://www.brayebrookobservatory.org...S/treediag.jpg
http://www.brayebrookobservatory.org...arlow%20Lenses
It's interesting to note the Plossl dates from the 1860's and the orthoscopic design dates from 1880... considering what the prices of these were 30, 40 years ago the current prices for some of these 4-element designs seem a bit steep these days.