VI PERSISTENCE 91
readjusted, so that they shall be oriented exactly alike. From each point of reference, in succession, the spines of the ridges are then to be followed with a fine pencil, in the two prints alternately, neatly marking each new point of comparison with a numeral in coloured ink (Plate 13). When both of the prints are good and clear, this is rapidly done ; wherever the impressions are faulty, there may be many ambiguities requiring patience to unravel. At first I was timid, and proceeded too hesitatingly when one of the impressions was indistinct, making short alternate traces. Afterwards on gaining confidence, I traced boldly, starting from any well-defined point of reference and not stopping until there were reasonable grounds for hesitation, and found it easy in this way to trace the unions between opposite and incompletely printed ends of ridges, and to disentangle many bad impressions.
An exact correspondence between the details of two minutiae is of secondary importance. Thus, the commonest point of reference is a bifurcation ; now the neck or point of divergence of a new ridge is apt to be a little low, and sometimes fails to take the ink ; hence a new ridge may appear in one of the prints to have an independent origin, and in the other to be a branch. The apparent origin is therefore of little importance, the main fact to be attended to is that a new ridge comes into existence at a particular point ; how it came into existence is a secondary matter. Similarly, an apparently broken ridge may in reality be due to an imperfectly printed
