Double-click on the movie to PLAY.

Click once to PAUSE.

RELOAD the page or double-click to return to the START of the movie.

In this movie Q moves through a more extensive range of points on the line through BD.

It is fairly clear that the image points lie on a circle.
What happens to the image of Q as Q moves further and further up the page?
What can we say about the circle?

On the next page we see what happens to the image when the path of Q (ie the red vertical line) is moved slightly to the right.