A small convex mirror that instead of being silvered was blackened at the back. The idea was that being convex it would reduce the scene and by being blackened it would only reflect the main masses of the subject. The artist would sit with his back to the view and hold the glass in front of him so that he could look over his shoulder. It is said to have been devised by Claude Lorrain (1600-82). It was popular in the 17th and 18th centuries and may still be seen in action today.