(1) "Rogues Gallery"
These are not what you are looking for:
Air Bubbles:

	Notice the very round outline. Also, it does not move.
	This movie shows what the dirt and air bubble look like as you focus up and down. Click the arrow button on the bottom left to play the movie.

Dirt on the slides also has a distinctive appearance.

This shows what it looks like as you focus up and down through a whole slide. You may want to go frame-by-frame using the arrow buttons on the bottom right.