1. A display apparatus for presenting a three-dimensional image of an object to a viewer, comprising: display means for presenting a two-dimensional image representative of the cross section of the object at different positions along the third dimension thereof; a fluid filled, variable focal length lens disposed in front of the display means through which the viewer directly views the display means and the two-dimensional image presented thereby, one optical surface of the lens comprising an elastic membrane facing said display means, the shape of which controls the focal length of the lens, the membrane shape being responsive to the pressure of the fluid within the lens such that modulating the fluid pressure changes the shape of the membrane thereby changing the focal length of the lens, the optical surface of the lens opposite the elastic membrane comprising an optically transparent, rigid surface facing the viewer, wherein the rigid optical surface is shaped to correct for aberrations in the lens; and control means for controlling the lens so as to change its focal length to a plurality of focal lengths in synchronization with the sequence of images presented by the display means to result in an apparent three-dimensional display of the object.