Day 24, A mystery lens assembly
Back when I was totally obsessed with all things related to telescope and optics, I would often acquire random optics for things, even when I didn't much know about what they were or what they were used for. This lens was something that came to me via Paul Zurakowski, the head of the Chabot Telescope Makers (friend and mentor). The label on the can says '3 3/4" Refractor Lense' which hinted that maybe he thought it could be used as a main lens in a telescope.
Close inspection reveals that the tube is made from solid brass, and has a lovely patina. I have not completely
disassembled the lens but I believe it consists of two elements, each a plano convex lens, with the flat sides
facing outward. The barrel is smooth except for a lip which has a narrow edge and is lightly knurled. The front
part of the lens does unscrew, revealing a fine brass thread, but the lens appears to be held in place by some
form of cement, possibly Canada balsam or the like? The lenses
appear to be simple single element lenses, and I suspect are of the same type of glass.
The weight of the overall assembly is a little over 850 grams, or about 1lb 13oz. It feels quite heavy and dense.
I see some light chipping
around the edges, which don't appear to be damage, but may be original to the manufacture.
This implies that
these may have been made at a time before mass production of lenses. I doubt that this could be much younger than
one hundred years old.
The focal length is quite short. Focusing the light from the overhead flourescent light fixtures shows it comes to a focus just about three or four inches away from the rear of the lens assembly. This means that it was unlikely to be a telescope lens (it would be pretty much rubbish for that purpose.)
So, what was it used for?
I have only speculation. Because I believe it is constructed of a single type of glass, it likely lacks any kind of correction for chromatic aberration, and also lacks internal stops or the like which would make me think that it was used as a camera lens. It has too short a focal length to be intended as a telescope objective. It makes a reasonable magnifier, and there were vintage loupes from the Victorian age, but they would be much smaller and lighter, and/or have legs on it to rest on the desk.
My guess was that these were used as part of a collimation system, or perhaps part of a projection system. In digging around the Internet, looking for similar examples, I found this set of four lenses which are listed as "Victorian projection lenses" but without any hint of the manufacturer.
The overall size, shape and profile appear to be pretty much identical to the example I have, which makes me think that the projector lens idea is probably correct. They probably date to the late 19th century, back in the days of "magic lantern" projectors. But this is where my research stops...
Does anyone else have a better idea about what they could have come from, their manufacturer, or the date?
Hope you are all having a good weekend.