[kensher]

Hello!

I am working with enantiomers in organic chemistry, and there is this one picture from the book which I don't fully understand. The picture is attached.

There are supposedly two lenses that are perpendicular to each other. I assume this is the two almost-transparent discs which overlap in one area. However, I don't understand what the black device is or what its purpose is. Can somebody please explain this to me?

Thanks in advance.

[Borek]

Very poor example (and poorly drawn). It tries to say more or less the same thing this video shows: https://www.youtube.com/watch?v=MhhHPOxTUy8

[kensher]

Thank you for the answer