The emerging of a single dislocation at a surface creates a step with a height equal to the Burgers vector component normal to the surface. By coupling an atomic force microscope with a tensile tester, the fine slip line structure can be analyzed. This equipment is particularly suitable to follow the course of plastic flow from the emergence of the first few dislocations from bulk crystal to the stages of work hardening. The motivations, instrumentation and results on LiF single crystal are described.