SummaryA direct colour superposition technique has been used to study two dislocations in W, using He field‐ion imaging at 77°K. Limitations due to instrumentation and field‐ion microscopy are discussed. A single spiral and a double spiral on (112) W were each dissected, by field evaporation, through three successive atom layers, and apparent core shapes and dimensions were determined. The shapes were irregular with diameters of at most 0.5–2 nm for the single spiral and 0.5–1.7 nm for the double spiral. Observed displacements in the positions of the core regions may be associated with dislocation movement induced by the high electric field needed for the obse