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What is the matte gray filter on the back of the refractometer?

What is the “matte” gray filter on the back of my old refractometer?

My guess is that this is a sodium light filter, which will give you just a light/dark boundary instead of a rainbow at the point of the refractive index. Often this is put over the eyepiece, but there is no reason it can’t be put at the entrance for the light. It’s a yellow (actually more brown) filter or a sodium interference filter, either passing mostly 589 nm light. Very convenient, you won’t lose it there, as I lost my filter a few days ago. Luckily I found it under some papers on the table…-royjohn

I would try sliding that filter into the other position (to our right in your top photo, then shine a white light on it: If you then see yellow through the eyepiece, it’s got to be a 589 nm yellow sodium filter.

If that doesn’t happen, with the filter out of position (as shown in your top photo) put a birefringent stone on the hemicylinder in the usual fashion (R.I. fluid, etc), shine a white light through the rear port, also in the usual fashion, and observe two shadows through the eyepiece, then slide that filter into position behind the port on the rear, so the illumination passes through both, then look through the eyepiece again: if one of the shadows disappears, the filter is a polarizer.