I have it - everyone so far is wrong - the mirror does actually do a vertical inversion however the image is invisible. Not only is it invisible but it also has the additional following property :- it cannot be seen. Despite the fact that it is both invisible and cannot be seen it projects an image of itself onto the retina. Unfortunately this image always focusses on the blind-spot so no-one knows it exists. I only know of its existence from an equation I saw in a physics book that was written before writing was invented.In this non-existant book there was an equation that linked something to something else and slap bang in the middle of it, right at the end, there was added the constant of variability. This was followed by a footnote that was explained in the next volume. The footnote referenced an obscure Ghanian piece of music. The music when transposed to the western octave and the notes written down yielded the following sentence - 5 years hard labour.

Then he lost the plot and went back to work ....