Piece of blue glass on a string?

[athanasius80]

I was just going through an old LF setup I bought off of e_ay and found a piece of deep blue glass not quite 2" in diamater hanging from a string. Its just a piece of glass. I've stumbled across these before and I'm wondering if people used these for metering before modern light meters.
Any ideas?
Thanks!

 

[David A. Goldfarb]

That's probably a monochrome viewing filter. Zone VI sold one. The idea is that you can look through the filter to get a better idea of the tonal relationships that B&W film sees.

P.S.--it's okay to say "eBay" here.

 

[glbeas]

Actually they are used for helping to visualise the scene in B&W. This works to show it like blue sensitive film will see it