v PATTERNS : THEIR OUTLINES AND CORES 83
and repeated prints from so soft a substance as flesh are often somewhat dissimilar, the one being more or less broadened out than the other, owing to unequal pressure. It is therefore well to use such other more convenient points of reference as the particular pattern may present. In loops, the intersection of the axis with the summit of the innermost bend, whether it be a staple or the envelope to a rod (Fig. 14, second and third rows of diagrams), is a well-defined position. In spirals, the centre of the pattern is fairly well defined ; also a perpendicular erected from the middle of the base to the outline above and below (Fig. 8, 4) is precise and convenient.
In prints of adults, measurements may be made in absolute units of length, as in fractions of an inch, or else in millimetres. An average ridge-interval makes, however, a better unit, being independent of growth ; it is strictly necessary to adopt it in prints made by children, if present measurements are hereafter to be compared with future ones. The simplest plan of determining and employing this unit is to count the number of ridges to the nearest half-ridge, within the space of one-tenth of an inch, measured along the axis of the finger at and about the point where it cuts the summit of the outline ; then, -having already prepared scales suitable for the various likely numbers, to make the measurements with the appropriate scale. Thus, if five ridges were crossed by the axis at that part, in the space of one-tenth of an inch, each unit of the scale to be used would be onefiftieth of an inch ; if there were four ridges, each
