Can’t tell if I’m missing a joke here, but see those two small knife looking protruding from the opposite side (above) where they’re measuring, those are used for measuring internal diameter.
The side they’re using is for outer diameter.
And though you can’t see it in the pic, the thin bit of metal that extends out from the bottom can be used for measuring depth.
What is it supposed to be?
I always thought they were ultra fine dust/fibres/particulate that stick to your eye and float around in the fluid?