Look at the dome itself. Now look at the little ring immediately on top of the dome. That's where we were in the photos in the posts below. It's called the Golden Gallery.
From this vantage point, there appears to be a ring immediately beneath the dome, the same size as the dome, and then beneath that, there is a wider ring. The wider ring is call the Stone Gallery. We also stopped there.
See how this all works out at: http://www.stpauls.co.uk/Cathedral-History/Climb-the-Dome