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Thread: Bulls eye glass

  1. #16

    Default Re: Bulls eye glass (By: Tom McIntyre)

    Quote Originally Posted by Tom McIntyre View Post
    I am not sure that anyone other than a chemist would recognize the term "watch glass" as a small dish used to monitor the evaporation process of a solution.

    I once bought a watch from a friend who had ground a "watch glass" to make a crystal for a late 18th century watch case. It was awfully thick and I replaced it with a correct crystal eventually.
    We use them at school, so pretty much anybody who goes to secondary school in the UK will be familiar with them.
    Nick, lots to learn, late starter.

  2. #17

    Default Re: Bulls eye glass

    While sorting some ancient glass crystals that had jumped between bins during a move, I have also encountered the term "Bullseye" used in reference to some pocket watch crystals with a roughly 1cm diameter thicker portion in the middle inner surface. (for "hunter case" pocket watches that had a hole in the middle of the cover to view the time without opening)

    While the re-ground "watch glass" lens mentioned yesterday might not have been appropriate for the watch it got mounted in, it seems that some pocket watch models from my grandparents era were designed for window-glass thicknesses: some of those ancient 20 ligne crystals were 4mm thick with a 60 degree beveled edge.

    Although that is where the term still survives, "watch glass" has not always been confined to the chemical sciences. Some of the older watch parts catalogs from the early 1960s also referred to the smaller domed clock crystals in the 2 inch area as "watch glasses"
    Last edited by Jim Shewchuk; 08-08-2017 at 12:47 PM.

  3. #18

    Default Re: Bulls eye glass (By: Jim Shewchuk)

    watch glasses for use in Chemistry are generally around 2mm thick, just under. We use them in Physics at school as bearings for electrostatic rods.
    Nick, lots to learn, late starter.

  4. #19

    Default Re: Bulls eye glass (By: novicetimekeeper)

    Quote Originally Posted by novicetimekeeper View Post
    watch glasses for use in Chemistry are generally around 2mm thick, just under....
    As are most of the domed mantel clock crystals I've encountered. The 4mm example referred to a super-thick style of pocket watch crystal I had encountered during sorting those mixed-up crystals. (presumably rarely used because any impact sufficient to break one would likely also severely trash the movement)

  5. #20

    Default Re: Bulls eye glass (By: Jim Shewchuk)

    Quote Originally Posted by Jim Shewchuk View Post
    As are most of the domed mantel clock crystals I've encountered. The 4mm example referred to a super-thick style of pocket watch crystal I had encountered during sorting those mixed-up crystals. (presumably rarely used because any impact sufficient to break one would likely also severely trash the movement)
    I've always wondered why they were called watch glasses, they are generally clock sized.
    Nick, lots to learn, late starter.

  6. #21

    Default Re: Bulls eye glass (By: novicetimekeeper)

    Quote Originally Posted by novicetimekeeper View Post
    I've always wondered why they were called watch glasses, they are generally clock sized.
    This is just a guess, but since some of the largest pocket watches did come close (I've encountered a single surviving example of a pocket watch case using a 22 ligne glass [49.6 mm / 1.95 inches]) to crossing over into using the smaller end of the clock glasses, I suspect that some even larger examples existed in the distant past similar to today's "wrist clocks". (a local nickname for wrist watches with 40+ mm crystals)

  7. #22

    Default Re: Bulls eye glass (By: Jim Shewchuk)

    Quote Originally Posted by Jim Shewchuk View Post
    This is just a guess, but since some of the largest pocket watches did come close (I've encountered a single surviving example of a pocket watch case using a 22 ligne glass [49.6 mm / 1.95 inches]) to crossing over into using the smaller end of the clock glasses, I suspect that some even larger examples existed in the distant past similar to today's "wrist clocks". (a local nickname for wrist watches with 40+ mm crystals)
    Yes, I think you are right, it's probably a very old term.
    Nick, lots to learn, late starter.

  8. #23

    Default Re: Bulls eye glass (By: Les harland)

    I always presumed that the true bulls eye crystals were used to let the watch be placed dial down and stay level, and to make the watch a bit thinner. I have a number of old bulls eye crystals and they are from quite thick glass originally, then ground flat from the outside on the top.

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