Abstract
Experimental evidence demonstrates the incompleteness of the usual rate constant explanation of field-ion image appearance, and suggests that gas distribution effects may be the dominant influence on image contrast, certainly in some circumstances. Further evidence shows that gas concentration variations across the surface would not be given by any simple formal rule. However, significant features of image behaviour in the helium-on-tungsten system can be rationalized by closer examination of gas behaviour during and after accommodation. For the normal imaging range a provisional working rule is suggested, that regional contrast may be primarily determined by supply-and-capture effects, but local contrast by a tendency for local gas equilibria to be established.