Those round things in ships' bridge windows

On most large ships and many large boats I’ve noticed a round transparent disc about a foot across in the front bridge window. I seem to remember seeing a vertical radius in them. What is this? I assume it is some sort of navigation device whereby an object’s position can be identified as so-many degrees off of the bow. Is that right? What is it called?

On some ships that disk is a type of windscreen(shield) wiper. What happens is that the disk spins at high speed and throws all the water off. Thus the crew on the bridge always have an clear area to look through even in very heavy rain or spray

Here’s a picture of one for those who are curious.

That makes sense. They have the same sort of thing for movie cameras.

The same things are used on the enclosures of some high-speed production milling machines, so the operator can see what’s going on through the “rainstorm” of coolant.