Very briefly stated, the system is as follows. All persons convicted of crime against whom a previous criminal conviction has been proved, or who are subject to police supervision, are carefully measured before they are liberated, and the results tabulated on what are called card-registers. The parts of the body selected for measurement are those which in an adult are the least liable to alteration; the length and breadth of the head, and the length of the foot, for instance, being reliable indicia by means of which thousands of individuals may readily be classified. Whilst M. Bertillon's system of anthropometry is especially well adapted for purposes of classification, Mr Galton's finger-print method is preferable for purposes of identification. The minute lines which may be noticed on the skin covering the under side of the top joint of the human finger or thumb invariably display a well defined series of curved ridges, which, though never quite alike in different subjects, always approximate to one of four types, that is to say, they assume the form of an arch, a whorl, a right loop, or a left loop. The sum of the combinations which can be formed of these types and their modifications on the ten digits being a practically inexhaustible quantity, every human being carries on his finger-tips an infallible record of his personal identity. Accordingly the criminal is required to make signature by pressing with his thumb, fore, and middle fingers of both hands (previously smeared with printer's ink) on the reverse side of the card-register; whilst to make assurance trebly sure, the exact location and measurement of any distinctive marks that may be found are noted, and his photograph, both full-face and in profile, is added. When completed, the card-registers are filed in cabinets on an ingenious plan which enables the searcher to lay his hand on any particular "dossier" in the space of a very few minutes.[245]