In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.

In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.

Instead of a concave lens near the eye, Kepler used a convex ... Eventually, most astronomers simply accepted the inverted view. After all, did it really matter that they were seeing planets ...