in all such cases, the boundaries between the classes are fuzzy and overlap inevitably occurs. Visualisation The graphic potential of computers has been recognised almost since computing was first used, but it is only comparatively recently that their possibilities as devices for the visualisation of complex. and largely ab­ stract phenomena has begun to be more fully appreciated. Some workers stress the need to be able to model photographic reality in order to assist in this task. They look to better algorithms and more resolution to achieve this end. Others-Alan Mackay for instance-suggest that it is "not just a matter of providing more and more pixels. It is a matter of providing congenial clues which employ to the greatest extent what we already know.