Good guesses both! And close!
It's a Geiger counter. (I'm not sure if what you posted is exactly the same thing, but I'm guessing the two are a little different?)
As for why I bought one:
There are a significant number of camera lenses produced from the 1940s through the 1970s that are measurably radioactive. Main source of radioactivity is the use of thorium oxide (up to 30% by weight) as a component of the glass used in the lens elements. Thorium oxide has a crystalline structural similar to calcium fluoride (fluorite). Like fluorite, its optical properties of high refractivity and low dispersion allows lens designers to minimize chromatic aberration and use lenses of lower curvature, which are less expensive to produce.
Typical radiation levels can approach 10 mR/hr (100 μSv/h) as measured at the lens element's surface, decreasing substantially with distance; at a distance of 3 ft. (.9 m.) the radiation level is difficult to detect over typical background levels. For reference, a typical chest x-ray consists of about about 10 mR, a round-trip cross country airline flight exposes a passenger to 5 mR, and a full set of dental x-rays exposes the patient to 10 mR to 40mR.
However, as thorium 232 ages, it begins to decay to “daughter particles” which emit beta radiation, and apparently also gamma radiation, making them more dangerous as they age.
Basically, tl;dr:
Many vintage camera lens manufacturers doped the glass with thorium.
As these things age, they emit more and more beta and gamma radiation, making them more dangerous, especially to your eyes.
Overall, actual level of risk from these things, depending on whom you ask, ranges from "it's a ridiculous thing to worry about" to "might wanna keep an eye on them, they get more dangerous over time."
My thing is, because I can choose to either own or not own camera lenses that are fucking radioactive, I'm going to choose the latter option.
I tested all of mine, and all of my FIL's. All were fine except for one, which was emitting twice as much radiation as background levels. Not enough to be a cause for alarm, but nonetheless we're going to sell it. Why even bother.
