A device using a Macintosh microcomputer and a light microscope equipped with a drawing tube is described. The mirror of the drawing tube is turned to the screen of the microcomputer. An example of its use in morphometry of peripheral nerves is given. At a 1000X magnification, the resolution of the system is 10 pixels per 2.6-.mu.m. The coefficient of variation between 20 measurements of diameter of thin (3 .mu.m in diameter) and large (12 .mu.m in diameter) myelinated fibers was 8% and 2% respectively. This precise and unexpensive system could be easily run in laboratories of morphology.