I know - the link doesn't always work.It's odd - sometimes it does load. It's very good and worth reading.
If not try typing in manually - or doing a google even. It's one of a series of factsheets published in the USA - I think on behalf of the govenment by the National Parks authority.
I don't (as yet) really know! I think it's glass which has a raised surface, pressed into prisms... it makes sense, given what the fact sheet says about how it was used to try to direct more natural light into shop interiors. I haven't come across this in this country as yet.
Biff any glass that is wedge shaped is prismatic. It refracts light towards the thin part of the wedge shae and could be used to direct sunlight through a roof light etc.
ah yes. Try the link below. There are some fine pictures and a good explanation of what became a common feature of shop fronts in the USA after Frank Lloyd Wright patented the idea in 1897.
Some of the links to American heritage sites seem very dodgy. The URL I gave is right but sometimes doesn't seem to work. Here's the chain of links that I followed:
Start at
http://www.louisianahp.org/
Then choose Main Street Program at bottom left of page to get to:
http://www.louisianahp.org/lmsintro.htm
Near bottom of page click on Technical Information to get to:
http://www.louisianahp.org/MainStreet/techinical.html
Then click near top of page for Your Transoms Are Showing, We Hope! to get to:
http://www.louisianahp.org/MainStreet/techinical_1.html
They do seem dodgy - wonder why? There seems to be some very good stuff however, and worth following though. I've not had this trouble before (I've done a great deal of research in the past year about an aspect of construction whech originated in the USA) but in that case it's been through direct google searches.
Right - will try what you did when I get a mo - many thanks!