large glass positive plates

I occasionally buy old glass half plate negatives to print and look into the past, and they arent expensive.

As I now have a whole plate contact printing frame, I bought some whole plate ones (8.5x6.75 inches), but it turned out that all except one are positives. They are clearly contact printed back onto plates for some reason (one from a half plate size with a black border; the rest same size). How would these have been displayed? A bit large for a projection one would think. Not sure how I can display them for that matter.

You might have got yourself a nice set of lantern slides! These were the original multimedia show, and way before photography existed, people had figured out how to project a transparent painted image on glass through the use of a projection device. It was a very popular entertainment from the 17th century until the early 20th century, where it was gradually displaced by cinema.

It was very common to project large glass slides. At the onset of the magic lantern era, it made painting them easier, and during the photographic era it sidestepped the need to make enlargements when only slow emulsions like collodion were available. I gotta say that the size you decribe seems rather big for projection, though, but who knows.

I've seen some projection devices from the 19th and they are just monsters compared to our modern slide projectors. You have to realize first that the light source for a long time was not a bulb, but lime light, which is produced by the heating of quicklime. So you need some kind of chimney and a combustion chamber. The slides were projected at all sorts of events, sometimes church lectures or public assemblies, so you can imagine the light power that was involved, and the heat it could produce! Plus, you could have projectors with fadeout between slides by using multiple projecting heads (the "stereopticon"). Dazzling!

There's a great illustrated book about the history of the magic lantern, it's called "Realms of light : uses and perceptions of the magic lantern from the 17th to the 21st century : an illustrated collection of essays by 27 authors from six countrie" ISBN 0951044168.

Ah fascinating. I had only seen the small square lantern slides, hadnt realised that they came so big. Must have been very impressive on display.

Another possibility is that these were an intermediate stage in making enlarged negatives for contact printing. This was the technique that Edward Weston and Paul Strand (among others) used. The original negative from a "small" camera such as Weston's 3.25x4.25 Graflex would be contact printed onto a plate, which would then be copied with a studio camera, producing 8x10 or larger negatives, which could then be contact printed onto paper.