Anyone who's looked at used Leica stereos knows that the plastic they made the shells out of likes to yellow. Their initial color is much like the previous generation of metal-shelled, enamel-painted Wild stereos, essentially white, as seen in this Leica stock photo of an MZ6:
But after a while if they're exposed to UV (+chemicals, fumes, skin oils???) they begin to yellow. First a little (thanks to Microscope Central for the next two images)
But then a lot
And after a while it can get out of control (image from eBay seller hagemmi):
This does not affect function per se, but boy does it affect aesthetics! It's especially annoying because a) some parts like ocular tubes were still made with enamel-painted metal which does not yellow, and b) if you mix and match from different scopes you get obvious mismatches:
This is not a problem unique to Leica stereos, or microscopes in general (though I suspect their formulation is particularly bad for it). Plastics over time yellow fairly inevitably from partial breakdown of the polymers that make them up. This pops up in retro computer and gaming hardware as well, and people in those communities found an effective solution, known as "retr0brighting". Basically you either immerse the yellowed plastic in a solution of hydrogen peroxide or apply a hydrogen peroxide-based gel to the surface (as are used for bleaching hair) and expose them to UV, either from LEDs or sunlight. This can reverse the yellowing and partially or entirely restore the original color.
I've done this a couple times now on Leica stereos and it has worked great. Here's an example with my latest Leica acquisition, my MS5. This one was not badly yellowed but it was particularly noticeable with my dead white 3d printed replacement knobs.
There are more involved approaches using either higher concentration H2O2, the aforementioned gels and also lining the tub with foil and using LED UVs. I've typically found using simple off-the-shelf 3% H2O2 and a few hours of direct summer sunlight to be sufficient.
Plastic shell shucked, cleaned and immersed in H2O2 with weighs to hold it down.
The weights are particularly important since the plastic will generate bubbles, so even if it sinks initially they will get trapped in the hollow underside of the shell and float it over the surface.
Final result:
There's still a hint of color but it's very subtle. It was not super yellowed in the first place but now it's very close the original shade. This is typically where the color has ended up on the Leica plastic I've tested, regardless of how yellow they started.
Any drawbacks? Not that I've seen. There have been concerns that this might weaken the plastic but I haven't observed any tendency to crack etc in the couple pieces I've treated. Maybe with higher concentration H2O2, maybe with more time or UV? Leica shells can get brittle over time on their own anyway. Obvious
