“…This idea was put forward in the XX-th century, in the 30's, by Taylor [64], Orowan [50] and Polanyi [54] and was confirmed in 1956 by the first direct observations of dislocations, based on electron microscopy, by Hirsch, Horne, Whelan [34] and Bollmann [11]. We find a description of static equilibrium of dislocations in classical Physics books already written in the late 60's (see for instance Cottrell [17], Read [56], Friedel [29], Nabarro [49], and Hirth and Lothe [35] for a recent presentation).…”