Of possible interest to those interested in the history of the telescope and optical principles, here is an online transcription of the 1922 classic by Louis Bell, "The Telescope": http://www.archive.org/stream/telescope029809mbp/telescope029809mbp_djvu.txt Unfortunately there are no illustrations, but it's still a good read. I love the language and idioms of the day. Historians and modern engineers have since corrected some of the details, but much of it is still relevant.