Complex sentence: The Mapparium was designed to allow the countries of the world to be viewed in accurate geographical relationship to each other, hence the design of the Mapparium--a mirror-image, concave reversal of the Earth, viewed from within.
Split into simpler sentences: The Mapparium was designed to allow the countries of the world to be viewed in accurate geographical relationship to each other. Hence the specially accurate effect gained by viewing the Mapparium--a mirror-image, concave reversal of the Earth--from within.