CHAPTER IX

METHODS OF INDEXING

In this chapter the system of classification by Arches, Loops, and Whorls described in Chapter V. will be used for indexing two, three, six or ten digits, as the case may be.

An index to each set of finger marks made by the same person, is needful in almost every kind of inquiry, whether it be for descriptive purposes, for investigations into race and heredity, or into questions of symmetry and correlation. It is essential to possess an index to the finger marks of known criminals before the method of finger prints can be utilised as an organised means of detection.

The ideal index might be conceived to consist of a considerable number of compartments, or their equivalents, each bearing a different index-heading, into which the sets of finger prints of different persons may be severally sorted, so that all similar sets shall lie in the same compartment.

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.

+------------------------------------+ | | A B C D | |Order | Right. Left. Rt. Lt. | | of |-----------------------------| |Entry.| F.M.R. F.M.R. T.L. T.L. | |------|-----------------------------| | 1 | _a a a a a a a a l a_ | | 2 | _ " " a l a l_ | | 3 | _ " " " " _ | | 4 | _ " " w l l l_ | | 5 | _a a l a a l a l a l_ | | 6 | _ " " l l l l_ | | 7 | _ " " " " _ | | 8 | _ " a a w l l l l_ | | 9 | _ " a l l l l l l_ | | 10 | _ " " l w w l_ | | 11 | _ " o l l l l l l_ | | 12 | _a a w a a l l l l l_ | | 13 | _ " a l l l l l l_ | | 14 | _a l a a a a l a l a_ | | 15 | _ " " l a l w_ | | 16 | _ " o l l w l l l_ | | 17 | _a l l a a l l l a l_ | | 18 | _ " " l l l l_ | | 19 | _ " " " " _ | | 20 | _ " " " " _ | | 21 | _ " " " " _ | | 22 | _ " " " " _ | | 23 | _ " a l w l l l l_ | | 24 | _ " i l l l l l l_ | | 25 | _ " " " " _ | | 26 | _a l l i l l w l l l_ | | 27 | _ " o a l w l l l_ | | 28 | _ " o l l w l l l_ | | 29 | _ " w w w w l l l_ | | 30 | _a l w i l w l l l l_ | | 31 | _ " o a l l l l l_ | | 32 | _ " o l l l w l l_ | | 33 | _ " " w l w l_ | | 34 | _ " o l w a l a l_ | | 35 | _i l l a l l w l l l_ | | 36 | _ " " w l w l_ | | 37 | _ " i l l l l l l_ | | 38 | _ " " " " _ | | 39 | _ " " " " _ | | 40 | _ " " " " _ | | 41 | _i l l i l l w l l l_ | | 42 | _ " i w w w l w l_ | | 43 | _i l w i l l l l w l_ | | 44 | _ " " w w w l_ | | 45 | _ " i l w w w w l_ | | 46 | _ " i w l l l l l_ | | 47 | _ " w l w w l w l_ | | 48 | _ " w w l l l l l_ | | 49 | _i w w a l l w l w l_ | | 50 | _ " w w w w l w l_ | | 51 | _ " " " " _ | | 52 | _o a w o l l l l l l_ | | 53 | _o l l o l l l l l l_ | | 54 | _ " " " " _ | | 55 | _ " " " " _ | | 56 | _ " " w l w l_ | | 57 | _ " i l l l l l l_ | | 58 | _ " " " " _ | | 59 | _ " " " " _ | | 60 | _ " o l l l l l l_ | | 61 | _ " " " " _ | | 62 | _ " " " " _ | | 63 | _ " " " " _ | | 64 | _ " " " " _ | | 65 | _ " " " " _ | | 66 | _ " w a l l l w l_ | | 67 | _ " w w w l l w l_ | | 68 | _o l w a l l l l l l_ | | 69 | _ " " w l w l_ | | 70 | _ " i l l w l w l_ | | 71 | _ " o l l l l l l_ | | 72 | _ " " " " _ | | 73 | _ " o l w l l l l_ | | 74 | _ " " " " _ | | 75 | _w l l i l l l l w l_ | | 76 | _ " " " " _ | | 77 | _w l l w l l l l l l_ | | 78 | _ " " " " _ | | 79 | _ " " w l w l_ | | 80 | _ " w l w l l l l_ | | 81 | _w l w o l w l l l l_ | | 82 | _ " " l l a l_ | | 83 | _ " " w l l l_ | | 84 | _ " w w w w l w l_ | | 85 | _ " " w w l l_ | | 86 | _ " " w w l w_ | | 87 | _ " " w w w w_ | | 88 | _ " " " " _ | | 89 | _w w l i l l l l l l_ | | 90 | _ " w l l w l l l_ | | 91 | _w w w o l w w l l l_ | | 92 | _ " w l w w l w l_ | | 93 | _ " " " " _ | | 94 | _ " w w l l l l w_ | | 95 | _ " w w w i l l l_ | | 96 | _ " " w l l l_ | | 97 | _ " " w l w l_ | | 98 | _ " " w w w l_ | | 99 | _ " " " " _ | | 100 | _ " " w w w w_ | +------------------------------------+

The sequence in which the digits have been registered is not from the thumb outwards to the little finger, but, on account of various good reasons that will be appreciated as we proceed, in the following order.

The ten digits are registered in four groups, which are distinguished in the Index by the letters A, B, C, D:--

A. _First._ The fore, middle, and ring-fingers of the _right_ hand taken in that order.

B. _Second._ The fore, middle, and ring-fingers of the _left_ hand taken in that order.

C. _Third._ The thumb and little finger of the _right_ hand.

D. _Fourth._ The thumb and little finger of the _left_ hand.

Consequently an index-heading will be of the form--

First Second Third Fourth group. group. group. group.

_a a l_ _a a w_ _l l_ _l l_

These index-headings are catalogued in alphabetical order. The method used in the Index is that which takes note of no slopes, except those of loops in the fore-finger of either hand. Consequently the index-heading for my own digits, printed on the title-page, is _wlw oll wl wl_. Those of the eight sets in Plate VI. are as follows:--

_i l w i l l w w w l_ _o l w o l w w l l l_ _o l w o l w w l l l_ _o l w o l l l l l l_ _i l w i l w w l w l_ _i l w i w l l l l l_ _i l l w w l l l l l_ _o l l a a l l l a l_ _o a a a a a l a l a_

For convenience of description and reference, the successive entries in the specimen index have been numbered from 1 to 100, but that is no part of the system: those figures would be replaced in a real index by names and addresses.

A preliminary way of obtaining an idea of the differentiating power of an index is to count the number of the different headings that are required to classify a specified number of cases. A table is appended which shows the numbers of the headings in the three alternative methods (1) of noting slopes of all kinds in all digits, (2) of noting slopes of Loops only and in the fore-fingers only, and (3) of disregarding the slopes altogether. Also in each of these three cases taking account of--

(_a_) All the ten digits;

(_b_) the fore, middle, and ring-fingers of both hands;

(_c_) those same three fingers, but of the right hand only;

(_d_) the fore and middle fingers of the right hand.

TABLE X.

_No. of different index-heads in 100 sets of Finger Prints._

+---------------------------------------------------------------+ | | | Account taken of | | No. of | |------------------------------| | digits | Digits noted. | All |_i_ and _o_| No | | regarded. | | slopes. | in fore- | slope. | | | | | fingers. | | |-----------|--------------------|---------|-----------|--------| | 10 | All the 10 digits | 82 | 76 | 71 | | | | | | | | | Fore, middle, | | | | | 6 | and ring-fingers | 65 | 50 | 43 | | | of both hands | | | | | | | | | | | 3 | Of right hand only | 25 | 16 | 14 | | | | | | | | 2 | Fore and middle of | 12 | 8 | 7 | | | right hand only | | | | +---------------------------------------------------------------+

The column headed "all slopes" refers to the method first used with success, and described in my Memoir, already alluded to (_Proc. Roy. Soc._, 1891), accompanied by a specimen index, from which the present one was derived. There the direction of the slope of every pattern that has one, is taken into account, and in order to give as much scope as possible to the method, the term Arch (I then called it a Primary) was construed somewhat over-liberally (see p. 114). It was made to include the forked-arch Fig. 12 (~2~), and even the nascent-loop (~9~), so long as not more than a single recurved ridge lay within the outline of the pattern; therefore many of the so-called arches had slopes. It is not necessary to trouble the reader with the numerical nomenclature that was then used, the method itself being now obsolete. Full particulars of it are, however, given in the Memoir.

A somewhat large experience in sorting finger prints in various ways and repeatedly, made it only too evident that the mental strain and risk of error caused by taking all slopes into account was considerable. The judgment became fatigued and the eye puzzled by having to assign opposite meanings to the same actual direction of a slope in the right and left hands respectively. There was also a frequent doubt as to the existence of a slope in large whorls of the spiral- and circlet-in-loop patterns (Fig. 13, ~21~, ~22~) when the impressions had not been rolled. A third objection is the rarity of the inner slopes in any other digit than the fore-finger. It acted like a soporific to the judgment not only of myself but of others, so that when an inner slope did occur it was apt to be overlooked. The first idea was to discard slopes altogether, notwithstanding the accompanying loss of index power, but this would be an unnecessarily trenchant measure. The slope of a loop, though it be on the fore-finger alone, decidedly merits recognition, for it differentiates such loops into two not very unequal classes. Again, there is little chance of mistake in noting it, the impression of the thumb on the one side and those of the remaining fingers on the other, affording easy guidance to the eye and judgment. These considerations determined the method I now use exclusively, by which Table IX. was compiled, and to which the second column of Table X., headed "_i_ and _o_ in fore-fingers," refers.

The heading of the third column, "no slope," explains itself, no account having been there taken of any slopes whatever, so _i_ and _o_ disappear, having become merged under _l_.

The table gives a very favourable impression of the differentiating power of all these methods of indexing. By the "_i_ and _o_ fore-finger" method, it requires as many as 76 different index-headings to include the finger prints of 100 different persons, 195 of 300 persons, and 285 of 500.

The number of entries under each index-heading varies greatly; reference to the index of 100 sets showing no less than six entries (Nos. 60-65) under one of them, and four entries (Nos. 18-21 and 37-40) under each of two others. Thus, although a large portion of the 100 sets are solitary entries under their several headings, and can be found by a single reference, the remainder are grouped together like the commoner surnames in a directory. They are troublesome to distinguish, and cannot be subdivided at all except by supplementary characteristics, such as the number of ridges in some specified part of the pattern, or the character of the cores.

In other respects the difference of merit between the three methods is somewhat greater, as is succinctly indicated by the next table.

TABLE XI.--_In 100 Sets._

+-----------------------------------------------------+ | |No. of different index-headings.| | Number of Entries |--------------------------------| |under the same head.| All | _i_ and _o_ | No | | | slopes. | fore-fingers | slope.| | | | only. | | |--------------------|---------|--------------|-------| | 1 | 71 | 63 | 58 | | 2 | 10 | 8 | 9 | | 3 | 1 | 3 | 1 | | 4 | ... | 2 | 2 | | 5 | ... | ... | ... | | 6 | 1 | ... | ... | | 13 | ... | ... | 1 | |--------------------|---------|--------------|-------| | Total | 83 | 76 | 71 | +-----------------------------------------------------+

Hence it is evident that the second method of "_i-o_ fore-finger" is capable of dealing rapidly with 100 cases, but that the method of "no slope" will give trouble in twelve out of the hundred cases.

TABLE XII.

_Index-headings under which more than 1 per cent of the sets of Finger Prints were registered._

(500 sets observed.)

+---------------------------------------------------- | _i_ and _o_ in fore-fingers. || |---------------------------------------------------- | No. | |Frequency|| | for | Index-heading. | per || |Reference.| | cent. || |----------|-----------------------------|---------|| | 1 | _a l l a l l l l l l_ | 1.2 || | 2 | _a l l i l l " " _ | 1.6 || |----------|-----------------------------|---------|| | 3 | _i l l i l l " " _ | 2.8 || | 4 | _o l l i l l " " _ | 1.4 || | 5 | _o l l o l l " " _ | 4.0 || |----------|-----------------------------|---------|| | 6 | _i l l o l l w l l l_ | 1.2 || | 7 | _o l l o l l " " _ | 1.4 || |----------|-----------------------------|---------|| | 8 | _o l l a l l l l l l_ | 2.2 || | 9 | _o l w u l l " " _ | 2.0 || | 10 | _w l l w l l " " _ | 1.2 || | 11 | _w w w w w w w w w w_ | 1.4 || +----------------------------------------------------

--------------------------------------------------+ No slope. | --------------------------------------------------| No. | |Frequency| for | Index-heading. | per | Reference.| | cent. | ----------|-----------------------------|---------| I. | _a l l a l l l l l l_ | 1.2 | II. | _a l l l l l " " _ | 2.2 | ----------|-----------------------------|---------| | | | III. | _l l l l l l " " _ | 9.2 | | | | ----------|-----------------------------|---------| IV. | _l l l l l l w l l l_ | 3.2 | | | | ----------|-----------------------------|---------| V. | _l l l a l l l l l l_ | 3.0 | VI. | _l l w l l l " " _ | 3.0 | VII. | _w l l w l l " " _ | 1.2 | VIII. | _w w w w w w w w w w_ | 1.4 | --------------------------------------------------+

The headings in the right half of the table include more cases than the left half, because a combination of two or more cases that severally contain less than 1 per cent of the finger prints, and are therefore ignored in the first half of the table, may exceed 1 per cent and find a place in the second half.

The entries in Table XII. are derived from a catalogue of 500 sets, and include all entries that appeared more than five times; in other words, whose frequency exceeded 1 per cent. These are the index-headings that give enough trouble to deserve notice in catalogues of, say, from 500 to 1000 sets.

In the left half of Table XII. all the index-headings are given, under each of which more than 1 per cent of the sets fell, when the method of "_i_ and _o_ in fore-fingers" was adopted; also the respective percentage of the cases that fell under them. In the right half of the table are the corresponding index-headings, together with the percentages of frequency, when the "no slope" method is employed. These are distinguished by Roman numerals. The great advantage of the "_i_ and _o_ fore-finger" method lies in its power of breaking up certain large groups which are very troublesome to deal with by the "no slope" method. According to the latter as many as 9.2 per cent of all the entries fall under the index-heading marked III., but according to the "_i-o_ fore-finger" method these are distributed among the headings 3, 4, and 5. The "all slopes" method has the peculiar merit of breaking up the large group Nos. 11 and VIII. of "all whorls," but its importance is not great on that account, as whorls are distinguishable by their cores, which are less troublesome to observe than their slopes.

The percentage of all the entries that fall under a single index-heading, according to the "_i-o_ fore-finger" method, diminishes with the number of entries at the following rate:--

TABLE XIII.

+----------------------------------------------------------+ | | Total number of entries. | | |--------------------------| | | 100 | 300 | 500 | |-------------------------------|--------|--------|--------| | Percentage of entries falling | | | | | under a single head | 63 | 49.0 | 39.8 | +----------------------------------------------------------+

It may be that every one of the 4{2} x 3{8}, or one hundred and five thousand possible varieties of index-headings, according to the "_i-o_ fore-finger" method, may occur in Nature, but there is much probability that some of them may be so rare that instances of no entry under certain heads would appear in the register, even of an enormous number of persons.

* * * * *

Hitherto we have supposed that prints of the ten fingers have in each case been indexed. The question now to be considered is the gain through dealing in each case with all ten digits, instead of following the easier practice of regarding only a few of them. The following table, drawn up from the hundred cases by the "all slopes" method, will show its amount.

TABLE XIV.--_From 100 Sets._

+----------------------------------------------------------------------+ | | | No. of different index-headings. | | Digits. | No. of |------------------------------------| | | digits. | All | _i_ and _o_ | No slope. | | | | slopes. | fore-finger. | | |-----------------------|---------|---------|--------------|-----------| | Fore and middle of | | | | | | right hand | 2 | 11 | 8 | 7 | | | | | | | | Fore, middle and ring | | | | | | of right hand | 3 | 23 | 16 | 14 | | | | | | | | Fore, middle and ring | | | | | | of both hands | 6 | 65 | 50 | 45 | | | | | | | | All ten digits | 10 | 83 | 76 | 73 | +----------------------------------------------------------------------+

The trouble of printing, reading off, and indexing the ten digits, is practically twice that of dealing with the six fingers; namely, three on each of the hands; the thumb being inconvenient to print from, and having to be printed separately, even for a dabbed impression, while the fingers of either hand can be dabbed down simultaneously.

For a large collection the ten digit method is certainly the best, as it breaks up the big battalions; also in case of one or more fingers having been injured, it gives reserve material to work upon.

* * * * *

We now come to the great difficulty in all classifications; that of transitional cases. What is to be done with those prints which cannot be certainly classed as Arches, Loops, or Whorls, but which lie between some two of them? These occur about once in every forty digits, or once in every four pairs of hands. The roughest way is to put a mark by the side of the entry to indicate doubt, a better one is to make a mark that shall express the nature of the peculiarity; thus a particular eyed pattern (Plate 10, Fig. 16, _n_) may be transitional between a loop and a whorl; under whichever of the two it is entered, the mark might be an _e_ to show that anyhow it is an eye. Then, when it is required to discover whether an index contains a duplicate of a given specimen in which a transitional pattern occurs, the two headings between which the doubt lies have to be searched, and the marked entries will limit the search. Many alternative ways of marking may be successfully used, but I am not yet prepared to propose one as being distinctly the best. When there are two of these marks in the same set, it seldom happens that more than two references have to be made, as it is usual for the ambiguity to be of the same kind in both of the doubtful fingers. If the ambiguities were quite independent, then two marks would require four references, and three marks would require nine. There are a few nondescript prints that would fall under a separate heading, such as Z. Similarly, as regards lost or injured fingers.

I have tried various methods of sub-classification, and find no difficulty in any of them, but general rules seem inadvisable; it being best to treat each large group on its own merits.

One method that I have adopted and described in the _Proc. Royal Soc._, is to sketch in a cursive and symbolic form the patterns of the several fingers in the order in which they appear in the print, confining myself to a limited number of symbols, such as might be used for printer's types. They sufficed fairly for some thousands of the finger marks upon which they were tried, but doubtless they could be improved. A little violence has of course to be used now and then, in fitting some unusual patterns to some one or other of these few symbols. But we are familiar with such processes in ordinary spelling, making the same letter do duty for different sounds, as _a_ in the words _as_, _ale_, _ask_, and _all_. The plan of using symbols has many secondary merits. It facilitates a leisurely revision of first determinations, it affords a pictorial record of the final judgment that is directly comparable with the print itself, and it almost wholly checks blunders between inner and outer slopes. A beginner in finger reading will educate his judgment by habitually using them at first.

The cores give great assistance in breaking up the very large groups of all-loops (see Table XII., Nos. 11 and VIII.); so does an entry of the approximate number of ridges in some selected fingers, that lie between the core and the upper outline of the loop.

* * * * *

The plan I am now using for keeping finger prints in regular order, is this:--In the principal collection, the prints of each person's ten digits are taken on the same large card; the four fingers of either hand being _dabbed_ down simultaneously above, and all the ten digits _rolled_ separately below. (Plate 2, Fig. 3.) Each card has a hole three-eighths of an inch in diameter, punched in the middle near to the bottom edge, and the cards are kept in trays, which they loosely fit, like the card catalogues used in many libraries. Each tray holds easily 500 cards, which are secured by a long stout wire passing like a skewer through the ends of the box and the holes in the cards. The hinder end of the box is sloped, so the cards can be tilted back and easily examined; they can be inserted or removed after withdrawing the wire.

It will be recollected that the leading and therefore the most conspicuous headings in the index refer to the fore, middle, and ring-fingers of the right hand, as entered in column A of the Specimen Register (Table IX.) The variety of these in the "_i_ and _o_ fore-finger" method, of which we are now speaking, cannot exceed thirty-six, there being only four varieties (_a_, _i_, _o_, _w_) in the fore-finger, and three varieties (_a_, _l_, _w_) in each of the other two; so their maximum number is 4 x 3 x 3 = 36. The actual number of such index-headings in 500 cases, and the number of entries that fell under each, was found to be as follows:--

TABLE XV.

_No. of entries in 500 cases, under each of the thirty-six possible index-letters for the fore, middle, and ring-fingers of the right hand by the "i-o fore-finger" method._

+--------------------------------------------------------------+ | _a a a_ | 4 || _i a a_ | 1 || _o a a_ | 1 || _w a a_ | -- | | _l_ | 17 || _l_ | 3 || _l_ | 2 || _l_ | -- | | _w_ | 5 || _w_ | -- || _w_ | 1 || _w_ | 1 | | | || | || | || | | | _a l a_ | 3 || _i l a_ | -- || _o l a_ | 2 || _w l a_ | 1 | | _l_ | 45 || _l_ | 54 || _l_ | 88 || _l_ | 40 | | _w_ | 11 || _w_ | 33 || _w_ | 59 || _w_ | 52 | | | || | || | || | | | _a w a_ | -- || _i w a_ | -- || _o w a_ | -- || _w w a_ | -- | | _l_ | -- || _l_ | 3 || _l_ | -- || _l_ | 10 | | _w_ | -- || _w_ | 11 || _w_ | 6 || _w_ | 47 | +--------------------------------------------------------------+

_a_ = Arch.

_i_ = Inward-sloped Loop on the fore-finger.

_o_ = Outward-sloped Loop on the fore-finger.

_l_ = Loop of either kind on the middle or ring finger.

_w_ = Whorl.

These 500 cases supply no entries at all to eleven of the thirty-six index-headings, less than five entries (or under 1 per cent) to ten others, and the supply is distributed very unevenly among the remaining fifteen. This table makes it easy to calculate beforehand the spaces required for an index of any specified number of prints, whether they be on the pages of a Register, or in compartments, or in drawers of movable cards.