-----Original Message----- From: Bru <[email protected]> To: [email protected] Sent: Thu, 10 Jun 2010 20:03 Subject: RE: [canals-list] Re: Roof lights and fires :-(
Hmm, I think the potential mistake here has to be the assumption that a ullseye decklight is a perfect plano-convex lens. I don't think it is ... r at least I don't think it *should* be. As best as I can recall, the old riginal ones I've seen seemed to be a bit more complex with what looked ike a Fresnel lens on the flatter side and a very pronounced central bump n the reverse (as opposed to a surface curved all the way across). However, looking at the only (fairly small) pics I can find of modern ersions, the bumped side does seem to be more like a plano-convex lens in hape however the "flat" side seems to be ridged or indented at the outer dge and the inner part, it seems to me, dished. =========================== This is probably because the way of making sheet glass has changed over the years. In the past the molten glass was poured onto a rotating(?) table. The glass would spread out producing sheet glass around the edges but the centre would remain lumpy and uneven at the point the molten glass hit the table. Panes would be cut from the flat area, leaving the lumpy bit (the bullseye) to be disposed of. It tended to be sold off cheaply, but why it couldn't be re-melted I don't know. There would also be a 'stalk' of glass where it had been poured and this was usually just broken off, leaving a flattish centre. Nowadays, molten glass is floated on a bed of molten tin producing much large sheets of higher quality 'plate' glass and the bullseye is not produced. To get over this problem (for those people who feel that the bullseye makes a nice 'traditional' window) the glass industry produce a moulded approximation of the traditional waste product. DaveD [Non-text portions of this message have been removed]
