It is particularly useful to be able to display in three dimensions the interior structure of human, animal and other organisms.  Practically all of the data available on such organisms has at some time been collected from cross-sections through the organism.  From such sections, over the years, data has been collected and stored as pictures in books, on photographic slides and the like.  It is an object of this aspect of the invention to display this information in a multi-dimensional pictorial form.  It is known (U.K. Specifications No.  633712 and 634316) to provide a set of parallel transparent panels mounted to a support frame to represent different heights above the ground, for the purpose of plotting the movements of aircraft.  However, the pictorial representation is continually changing and does not represent a three-dimensional structure.