I've been looking for 10 minutes and still can't see the 4th

I've been looking for 10 minutes and still can't see the 4th

Take a close look at one specific point and then gradually move your eyes to other points in the image. By carefully observing, the answer will become apparent. It’s almost too easy to respond to this question once you apply this simple technique.

For those who haven't seen it yet, take a look: copse