Ivy,
        The 3 1/4X4 glass slides you have are called "Lantern
Slides" and they were used a lot ca. 1870-1920; they are still
used in some medical schools and fine arts programs (because of
their large size/great resolution of detail.
        I dont know a modern source for projectors (I have one
old one); a call around to the larger houses that advertise in
Shutterbug Magazine should yield a return for you.  Or someone
else on the net may know a house that specializes in selling
antique equipment.
        Originally, they were a photographic update of the
painted glass plates and scenes sold for Magic Lantern
presentations; Albert S. Bickmore (Amer Mus Natl Hist) held a
lot of lectures for teachers with them in the 1870's.
        Regards,
        John Bunch
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