1. A viewing system for a transmission electron microscope, comprising: a converting means for converting an image of a specimen focused by a focusing lens system of said electron microscope into an electric signal; frame memory means for processing the electric signal obtained from said converting means for each image frame to thereby derive image frame data to be stored therein; display means for displaying an electron-microscope image on the basis of the contents of said frame memory means; means for deriving a stimulated image corresponding to said electron microscope image. simulation frame memory means for processing a simulated image data derived through said simulated image deriving means for each image frame to thereby store data resulting from the processing; display means for displaying a simulated image on the basis of the output of said simulation frame memory means; means for controlling said electron microscope; and means for transmitting conditions required for deriving said simulated image from said means for controlling said electron microscope to said simulated image deriving means so that at least portions of the electron-microscope image and the corresponding simulated image, respectively, are displayed concurrently.