Figure 1 – Stereoscope Mirrors

The mirrors of Wheatstone's stereoscope flip the images left-to-right.  When this happens the 3D illusion turns inside-out (e.g. from convex to concave) – a fascinating phenomenon that he also explored with another device called a pseudoscope.  To ensure that the volumes "read" correctly in a mirror-stereoscope you have to switch the left and right images.