Abstract: A reference display system that receives information from an imaging system (e.g., a CT scanner or the like), that extracts or derives three-dimensional anatomical and/or pathological information about a part of a body (e.g., the brain or other organ) of a patient. The information is digitized in the imaging system and is introduced to a computer that is programmed to reformat the digitized information to provide as output electric signal representative of the digitized information. An optical display system (e.g., a cathode ray tube, CRT, and related circuitry) is connected to receive the output of the computer and is operable to present the reformatted information at a determined plane during an operative procedure. An operating microscope is freely located in the operative location relative to the patient during the operative procedure, the focal plane of the microscope establishing the determined plane.