About an hour later, dawn broke over Marblehead.
This pops up with some regularity in the Pentax world. Turns out Pentax made a lot of radioactive lenses back in the day, and they sold them in considerable quantity.
I happen to own one (a Super-Multi-Coated Takumar 50mm f/1.4), and can confirm that sunlight works to clear the yellowing.
If you're trying to avoid radioactive lenses, I believe pretty much everything Pentax made after the SMC PENTAX-M is non-radioactive. I have a SMC PENTAX-M 50mm f/1.4 as well that shows no signs of yellowing.
 https://news.ycombinator.com/item?id=20612636 <- scroll down ginko's link, there are scads of them.
alpha particles are Helium nuclei. They are big, slow, and are easily absorbed by a couple of centimeters of air.
beta particles are high speed electrons (or positrons). They can be stopped by a couple of millimeters of aluminum, but they emit gamma radiation while being slowed.
gamma radiation is high energy photons, everything stronger than X-rays. You need a lot of dense material to stop them. Some elements, when absorbing gamma photons, will emit a free neutron.
Slow neutrons get absorbed by atoms; fast neutrons knock atoms apart. Either of these can produce unstable isotopes that may themselves emit radiation.
The fact that the lens is radioactive is not particularly interesting. However, it is an incredibly fast lens for large format: on 4x5 film, f/4.5 produces a similar depth of field to f/1.2 on a 135* or equivalent digital camera. The 7" Aero Ektar produces a depth of field similar to f/.85 on 135.
Now, mounting one of these lenses to a rangefinder camera (typically a Graflex Pacemaker Speed Graphic press camera from the 1940s) for use in the field is a chore, and results in a very heavy and rather clumsy-to-use piece of kit.
David Burnett goes through the trouble, and the results   are rather spectacular.
*135 is the normal "35mm" film format, the same size as "full frame" digital.
When I first heard the term "radioactive glass" I thought this was just a term used to refer to very sharp lenses. Boy was I wrong. :)
This isn't true. Thallium 208, for example, is on the decay chain. Yeah, it beta decays to lead, but it also emits a 2.6 MeV gamma ray in the process . That's one of the highest energy gamma rays you typically see from radioactive decay, and it's highly penetrating. That's one example, and there's also lead 212 and actinium 228, which also peaks in the 100s of keVs.
 See the "gammas" part of http://nucleardata.nuclear.lu.se/toi/nuclide.asp?iZA=810208
Tl208 to Pb is beta decay, there are no gamma particles involved. I would think the 2.6 MeV energy listed is for the electron emitted.
Look at the last page of this table: http://www.nucleide.org/DDEP_WG/Nuclides/Tl-208_tables.pdf
The green arrows are possible beta decays to lead 208. But 208Tl never decays to the nuclear ground state of lead 208. And so after the beta decay, there are gamma rays (the black arrows) as the nucleus relaxes from whatever excited state it ended up in.
There is some good data, but no conclusion on why this would be the case. Anybody know of any followups that explain this effect?
The multi-coated versions all have a little bit more saturated colors. Other than that I have never noticed a difference.
> First of all the most commonly used films in the early 1950s were slower (less sensitive) than modern emulsions, and consequently less prone to fogging due to radiation. Second, all Leica rangefinder cameras use a focal-plane shutter, which means the film behind the lens is shielded from the radiation it emits except for the instant the shutter is open when making an exposure, which would produce negligible fogging. Since the decay chain of thorium consists exclusively of alpha and beta particle emission, neither of which is very penetrating, the closed shutter protects the film from the radiation from the rear of the lens.