Title: Hand with Reflection Sphere (Self-Portrait in Spherical Mirror)

( M.C. Escher/ Cordon Art-Baarn-Holland. All rights reserved )
Copyright 1995-1999 World of Escher, Inc.  This picture has been used without permission.

"The picture shows a spherical mirror, resting on a left hand.   But as a print is the reverse of the original drawing on stone, it was my right hand that you see depicted. (Being left-handed, I needed my left hand to make the drawing.)  Such a globe reflection collects almost one's whole surroundings in one disk-shaped image.  the whole room, four walls, the floor, and the ceiling, everything, albeit distorted, is compressed into that one small circle.  Your own head, or more exactly the point between your eyes, is the absolute center.  No matter how you turn or twist yourself, you can't get out of that central point.  You are immovably the focus, the unshakable core, of your world."  - M. C. Escher

Is Escher's statement about the point between the eyes always being at the center of the sphere accurate?  Considering the eye as a lens, how does this statement relate to the object and image in mirrors and lenses?

Q:    In what ways are satellite dishes similar to concave mirrors?  In what ways are they different?

A:    Both satellite dishes and concave mirrors have close to spherical shapes.  Their common purpose is to concentrate a parallel beam of rays into a focus.  The dissimilarity between satellite dishes and mirrors is related to the fact that satellite dishes receive and focus radio signals, while mirrors receive and focus visible light.  In fact, both radio signals and visible light are electromagnetic waves that differ only by frequency.

Q:    Why are mirrors used in large telescopes instead of lenses?

A:    It is easier to correct aberrations in mirrors by modifying the mirrors shapes, than to correct aberrations in lenses.