The principle of the proposed method of index-headings is, that they should depend upon a few conspicuous differences of pattern in many fingers, and not upon many minute differences in a few fingers. It is carried into effect by distinguishing the A. L. W. class of pattern on each digit in succession, by a letter,--_a_ for Arch, _l_ for Loop, _w_ for Whorl; or else, as an alternative method, to subdivide _l_ by using _i_ for a loop with an Inner slope, and _o_ for one with an Outer slope, as the case may be. In this way, the class of pattern in each set of ten digits is described by a sequence of ten letters, the various combinations of which are alphabetically arranged and form the different index-headings. Let us now discuss the best method of carrying out this principle, by collating the results of alternative methods of applying it. We have to consider the utility of the _i_ and _o_ as compared to the simple _l_, and the gain through taking all ten digits into account, instead of only some of them. It will be instructive to print here an actual index to the finger prints of 100 different persons, who were not in any way selected, but taken as they came, and to use it as the basis of a considerable portion of the following remarks, to be checked where necessary, by results derived from an index to 500 cases, in which these hundred are included. This index is compiled on the principle shortly to be explained, entitled the "_i_ and _o_ fore-finger" method. TABLE IX.--INDEX TO 100 SETS OF FINGER PRINTS.