Bruce, Dennis is right, yes the image is Upside down & Reversed left to Right on the screen, but turn it 180 degrees and it's correct. Just try holding a negative with the emulsion side away from you, image upside down, thats the same as a focus screen, now revolve it the 180 degrees and everything is correct.

Add a mirror and you only correct the one dimension hence left & right are switched (backards image).

Ian