Just as there was times when radioactive substances were put into everything from medicines, foods and drinks (Marie-Curie gives it a thumbs down), so too did the manufacturers of optics (but here it had a positive effect, Jiří Zouhar gives it a thumbs up).

In order to achieve a higher refractive index of light and lower dispersion (blah blah, it just made the photos of the proverbial Röentgen prettier), so-called lanthanum and later thorium glasses were produced - optics with thorium dioxide. As a result of the decay of the thorium isotope, the Canada balsam used to glue the lenses was dyed yellow, and among other things it gave the photograph a nice golden tint.

• Canada balsam (marginally also "Canadian turpentine") is a mixture of oil and resin obtained from the Balsam Fir (Abies balsamea). It is traditionally used, for example, as a medium for fixing laboratory specimens, as it is invisible when dry. When it is mixed in a 1:1 ratio with xylene, it produces a mixture with almost identical refractive index (which is very similar even without xylene) as a crown glass (n = 1.55). This is why it has found applications in the binding of lenses and optical systems. When you don't have anything to do during the long winter evenings and you feel like burning the mucous membranes or teasing the nervous system is the thing, I recommend opening a bottle of xylene.
• Today's lenses and optics are no longer cemented this way - UV fixation is used. It's an irreversible method (which can be a disadvantage, older lenses could be re-cemented by heat) and does not change over time, so you no longer encounter element separation caused by balm degradation.

Some lenses have emitted over 3 μSv/h, which is "not great, not terrible" - however, thorium emits "only" α and β radiation, so as long as you don't eat those lenses or roll around naked in them for long periods of time, you're fine. I guess. Either way, the lens is very nice, it just needs a really big camera or a really long bellows because of the long focal length. In any case - really big muscles. However, it's fun to work with and it's radiating joy and emmiting fun. Heh.

• For every millimetre of lens focal length, you need the same number of millimetres of bellows to achieve focus at infinity. That is, for a Claron that has a focal length of 610mm, you will need a bellows of at least 610mm. And if you want to focus closer, it eats more and more millimeters. In addition, if you want the subject to be the same size in the output as it is in reality, i.e. 1:1, you'll need a lift length of double that - 1220mm.
• But wait, there's more! The exposure is only valid for focusing at infinity - with a longer bellows, light is lost and you need more of it - or longer times. So comes the exposure correction for the bellows lift. And exposure correction for filters. And film reciprocity correction - the Schwarzschild effect. Well, what a fuckery. But that's another story.