25mm vs 12.5mm vs 8mm for 10x magnification

[Craig Gerard]

ChrisLilley wrote:

Are any macro lenses or microscope objectives known to be made with thoriated glass?

Quote from Wikipedia:
http://en.wikipedia.org/wiki/Thorium_dioxide

Thorium dioxide was formerly added to glasses during manufacture to increase their refractive index, producing thoriated glass with up to 40% ThO2 content. These glasses were used in the construction of high-quality photographic lenses. However, the radioactivity of the thorium caused both a safety and pollution hazard and self-degradation of the glass (turning it yellow or brown over time). Lanthanum oxide has replaced thorium dioxide in almost all modern high-index glasses.

Craig

[ChrisLilley]

Yes, I know what it is

> Are any macro lenses or microscope objectives known to be made with thoriated glass?

and could that be the cause of the glass darkening here?

[rjlittlefield]

I don't know about thoriated glass.

But I can easily believe that the darkening we see in the images (whatever its origin) explains the discrepancy in exposure. So I'm a lot happier now. Thanks!

--Rik

[enricosavazzi]

That makes me think of an old PZO stereomicroscope (actually almost identical to old Zeiss models, but branded PZO) that made its way to me several years ago. The image seen through the eyepieces was clear but distinctly yellowish, and there were no obvious filters anywhere. I sold it years ago. So the answer is that thoriated glass may have been used in microscopes.