Tom Mchugh wrote: > > Tim Yu wrote: > > >One factor I haven't seen written about...For the incident and transmitted > >rays to be parallel, we also need to have the pane(s) of glass of uniform > >thickness throughout. Glass is a **very** slow fluid; over time, any > >non-horizontal sheet of glass will get thicker and thicker at the bottom. > >[Haven't mic'd any old glass, but that's what I've been told...]. For rays > >that strike such a non-uniformly thick pane (with, say, air on either side) > >from above the horizontal line, the transmitted ray will bend ever so > >slightly up toward the horizontal line.
Most of this misconception has arisen from how sheet glass was originally manufactured. It was plate glass. A bulb of glass was spun to make a plate. The middle bulls eye was the cheap part, now much beloved of theme pubs. The outside was the higher quality. It has a natural wedge shape. Usually the thicker, and heavier part was placed at the bottom. However, there are plenty of instances of the thicker part being found at the top of a stained glass panel. -- Nick
