Finger Prints Part 3

I possess a few specimens of hand prints of persons taken when children, and again, after an interval of several years: they show a general accordance in respect to the creases, but not sufficiently close for identification.

The ridges on the feet and toes are less complex than those on the hands and digits, and are less serviceable for present purposes, though equally interesting to physiologists. Having given but little attention to them myself, they will not be again referred to.

The ridges are studded with minute pores which are the open mouths of the ducts of the somewhat deeply-seated glands, whose office is to secrete perspiration: Plate 10, _n_, is a good example of them. The distance between adjacent pores on the same ridge is, roughly speaking, about half that which separates the ridges. The lines of a pattern are such as an artist would draw, if dots had been made on a sheet of paper in positions corresponding to the several pores, and he endeavoured to connect them by evenly flowing curves; it would be difficult to draw a pattern under these conditions, and within definite boundaries, that cannot be matched in a living hand.

The embryological development of the ridges has been studied by many, but more especially by Dr. A. Kollmann,[1] whose careful investigations and bibliography should be consulted by physiologists interested in the subject. He conceives the ridges to be formed through lateral pressures between nascent structures.

[Ill.u.s.tration: PLATE 4.

FIG. 7. SCARS AND CUTS, AND THEIR EFFECTS ON THE RIDGES.

FIG. 8. FORMATION OF INTERs.p.a.cE AND EXAMPLES OF THE ENCLOSED PATTERNS.]

The ridges are said to be first discernible in the fourth month of foetal life, and fully formed by the sixth. In babies and children the delicacy of the ridges is proportionate to the smallness of their stature.

They grow simultaneously with the general growth of the body, and continue to be sharply defined until old age has set in, when an incipient disintegration of the texture of the skin spoils, and may largely obliterate them, as in the finger prints on the t.i.tle-page. They develop most in hands that do a moderate amount of work, and they are strongly developed in the foot, which has the hard work of supporting the weight of the body. They are, as already mentioned, but faintly developed in the hands of ladies, rendered delicate by the continual use of gloves and lack of manual labour, and in idiots of the lowest type who are incapable of labouring at all. When the skin becomes thin, the ridges simultaneously subside in height. They are obliterated by the callosities formed on the hands of labourers and artisans in many trades, by the constant pressure of their peculiar tools. The ridges on the side of the left fore-finger of tailors and seamstresses are often temporarily destroyed by the needle; an instance of this is given in Plate 4, Fig. 7, _b_. Injuries, when they are sufficiently severe to leave permanent scars, destroy the ridges to that extent. If a piece of flesh is sliced off, or if an ulcer has eaten so deeply as to obliterate the perspiratory glands, a white cicatrix, without pores or ridges, is the result (Fig. 7, _a_). Lesser injuries are not permanent. My a.s.sistant happened to burn his finger rather sharply; the daily prints he took of it, ill.u.s.trated the progress of healing in an interesting manner; finally the ridges were wholly restored. A deep clean cut leaves a permanent thin mark across the ridges (Fig. 7, _c_), sometimes without any accompanying puckering; but there is often a displacement of the ridges on both sides of it, exactly like a "fault" in stratified rocks. A cut, or other injury that is not a clean incision, leaves a scar with puckerings on all sides, as in Fig. 7, _a_, making the ridges at that part undecipherable, even if it does not wholly obliterate them.

The latest and best investigations on the evolution of the ridges have been made by Dr. H. Klaatsch.[2] He shows that the earliest appearance in the Mammalia of structures a.n.a.logous to ridges is one in which small eminences occur on the ball of the foot, through which the sweat glands issue in no particular order. The arrangement of the papillae into rows, and the accompanying orderly arrangement of the sweat glands, is a subsequent stage in evolution. The prehensile tail of the Howling Monkey serves as a fifth hand, and the naked concave part of the tail, with which it grasps and holds on to boughs, is furnished with ridges arranged transversely in beautiful order. The numerous drawings of the hands of monkeys by Allix[3] may be referred to with advantage.

The uses of the ridges are primarily, as I suppose, to raise the mouths of the ducts, so that the excretions which they pour out may the more easily be got rid of; and secondarily, in some obscure way, to a.s.sist the sense of touch. They are said to be moulded upon the subcutaneous papillae in such a manner that the ultimate organs of touch, namely, the Pacinian bodies, etc.--into the variety of which it is unnecessary here to enter--are more closely congregated under the bases of the ridges than under the furrows, and it is easy, on those grounds, to make reasonable guesses how the ridges may a.s.sist the sense of touch. They must concentrate pressures, that would otherwise be spread over the surface generally, upon the parts which are most richly supplied with the terminations of nerves. By their means it would become possible to neutralise the otherwise dulling effect of a thick protective epidermis.

Their existence in transverse ridges on the inner surface of the prehensile tails of monkeys admits of easy justification from this point of view. The ridges so disposed cannot prevent the tail from curling, and they must add materially to its sensitiveness. They seem to produce the latter effect on the hands of man, for, as the epidermis thickens under use within moderate limits, so the prominence of the ridges increases.

Supposing the ultimate organs of the sense of touch to be really congregated more thickly under the ridges than under the furrows--on which there has been some question--the power of tactile discrimination would depend very much on the closeness of the ridges. The well-known experiment with the two points of a pair of compa.s.ses, is exactly suited to test the truth of this. It consists in determining the smallest distance apart, of the two points, at which their simultaneous pressure conveys the sensation of a double p.r.i.c.k. Those persons in whom the ridge-interval was short might be expected to perceive the double sensation, while others whose ridge-interval was wide would only perceive a single one, the distance apart of the compa.s.s points, and the parts touched by them, being the same in both cases. I was very glad to avail myself of the kind offer of Mr.

E. B. t.i.tchener to make an adequate course of experiments at Professor Wundt's psycho-physical laboratory at Leipzig, to decide this question. He had the advantage there of being able to operate on fellow-students who were themselves skilled in such lines of investigation, so while his own experience was a considerable safeguard against errors of method, that safety was reinforced by the fact that his experiments were conducted under the watchful eyes of competent and critical friends. The result of the enquiry was decisive. It was proved to demonstration that the fineness or coa.r.s.eness of the ridges in different persons had no effect whatever on the delicacy of their tactile discrimination. Moreover, it made no difference in the results, whether one or both points of the compa.s.s rested on the ridges or in the furrows.

The width of the ridge-interval is certainly no test of the relative power of discrimination of the different parts of the same hand, because, while the ridge-interval is nearly uniform over the whole of the palmar surface, the least distance between the compa.s.s points that gives the sensation of doubleness is more than four times greater when they are applied to some parts of the palm than when they are applied to the bulbs of the fingers.

The ridges may subserve another purpose in the act of touch, namely, that of enabling the character of surfaces to be perceived by the act of rubbing them with the fingers. We all of us perform this, as it were, intuitively. It is interesting to ask a person who is ignorant of the real intention, to shut his eyes and to ascertain as well as he can by the sense of touch alone, the material of which any object is made that is afterwards put into his hands. He will be observed to explore it very carefully by rubbing its surface in many directions, and with many degrees of pressure. The ridges engage themselves with the roughness of the surface, and greatly help in calling forth the required sensation, which is that of a thrill; usually faint, but always to be perceived when the sensation is a.n.a.lysed, and which becomes very distinct when the indentations are at equal distances apart, as in a file or in velvet. A thrill is a.n.a.logous to a musical note, and the characteristics to the sense of touch, of different surfaces when they are rubbed by the fingers, may be compared to different qualities of sound or noise. There are, however, no pure over-tones in the case of touch, as there are in nearly all sounds.

CHAPTER V

PATTERNS: THEIR OUTLINES AND CORES

The patterns on the thumb and fingers were first discussed at length by Purkenje in 1823, in a University Thesis or _Commentatio_. I have translated the part that chiefly concerns us, and appended it to this chapter together with his corresponding ill.u.s.trations. Subsequent writers have adopted his standard types, diminis.h.i.+ng or adding to their number as the case may be, and guided as he had been, by the superficial appearance of the lineations.

In my earlier trials some three years ago, an attempt at cla.s.sification was made upon that same principle, when the experience gained was instructive. It had seemed best to limit them to the prints of a single digit, and the thumb was selected. I collected enough specimens to fill fourteen sheets, containing in the aggregate 504 prints of right thumbs, arranged in six lines and six columns (6 6 14 = 504), and another set of fourteen sheets containing the corresponding left thumbs. Then, for the greater convenience of study these sheets were photographed, and enlargements upon paper to about two and a half times the natural size made from the negatives. The enlargements of the right thumb prints were reversed, in order to make them comparable on equal terms with those of the left. The sheets were then cut up into rectangles about the size of small playing-cards, each of which contained a single print, and the register number in my catalogue was entered on its back, together with the letters L. for left, or R.R. for reversed right, as the case might be.

On trying to sort them according to Purkenje's standards, I failed completely, and many a.n.a.logous plans were attempted without success. Next I endeavoured to sort the patterns into groups so that the central pattern of each group should differ by a unit of "equally discernible difference"

from the central patterns of the adjacent groups, proposing to adopt those central patterns as standards of reference. After tedious re-sortings, some sixty standards were provisionally selected, and the whole laid by for a few days. On returning to the work with a fresh mind, it was painful to find how greatly my judgment had changed in the interim, and how faulty a cla.s.sification that seemed tolerably good a week before, looked then.

Moreover, I suffered the shame and humiliation of discovering that the ident.i.ty of certain duplicates had been overlooked, and that one print had been mistaken for another. Repeated trials of the same kind made it certain that finality would never be reached by the path hitherto pursued.

On considering the causes of these doubts and blunders, different influences were found to produce them, any one of which was sufficient by itself to give rise to serious uncertainty. A complex pattern is capable of suggesting various readings, as the figuring on a wall-paper may suggest a variety of forms and faces to those who have such fancies. The number of illusive renderings of prints taken from the same finger, is greatly increased by such trifles as the relative breadths of their respective lineations and the differences in their depths of tint. The ridges themselves are soft in substance, and of various heights, so that a small difference in the pressure applied, or in the quant.i.ty of ink used, may considerably affect the width of the lines and the darkness of portions of the print. Certain ridges may thereby catch the attention at one time, though not at others, and give a bias to some false conception of the pattern. Again, it seldom happens that different impressions of the same digit are printed from exactly the same part of it, consequently the portion of the pattern that supplies the dominant character will often be quite different in the two prints. Hence the eye is apt to be deceived when it is guided merely by the general appearance. A third cause of error is still more serious; it is that patterns, especially those of a spiral form, may be apparently similar, yet fundamentally unlike, the unaided eye being frequently unable to a.n.a.lyse them and to discern real differences.

Besides all this, the judgment is distracted by the mere size of the pattern, which catches the attention at once, and by other secondary matters such as the number of turns in the whorled patterns, and the relative dimensions of their different parts. The first need to be satisfied, before it could become possible to base the cla.s.sification upon a more sure foundation than that of general appearance, was to establish a well-defined point or points of reference in the patterns. This was done by utilising the centres of the one or two triangular plots (see Plate 4, Fig. 8, ~2~, ~3~, ~4~) which are found in the great majority of patterns, and whose existence was pointed out by Purkenje, but not their more remote cause, which is as follows:

The ridges, as was shown in the diagram (Plate 3) of the palm of the hand, run athwart the fingers in rudely parallel lines up to the last joint, and if it were not for the finger-nail, would apparently continue parallel up to the extreme finger-tip. But the presence of the nail disturbs their parallelism and squeezes them downwards on both sides of the finger. (See Fig. 8, ~2~.) Consequently, the ridges that run close to the tip are greatly arched, those that successively follow are gradually less arched until, in some cases, all signs of the arch disappear at about the level of the first joint (Fig. 8, ~1~). Usually, however, this gradual transition from an arch to a straight line fails to be carried out, causing a break in the orderly sequence, and a consequent inters.p.a.ce (Fig.

8, ~2~). The topmost boundary of the inters.p.a.ce is formed by the lowermost arch, and its lowermost boundary by the topmost straight ridge. But an equally large number of ducts exist within the inters.p.a.ce, as are to be found in adjacent areas of equal size, whose mouths require to be supported and connected. This is effected by the interpolation of an independent system of ridges arranged in loops (Fig. 8, ~3~; also Plate 5, Fig. 9, _a_, _f_), or in scrolls (Fig. 8, ~4~; also Fig. 9, _g_, _h_), and this interpolated system forms the "pattern." Now the existence of an inters.p.a.ce implies the divergence of two previously adjacent ridges (Fig.

8, ~2~), in order to embrace it. Just in front of the place where the divergence begins, and before the sweep of the pattern is reached, there are usually one or more very short cross-ridges. Their effect is to complete the enclosure of the minute triangular plot in question. Where there is a plot on both sides of the finger, the line that connects them (Fig. 8, ~4~) serves as a base line whereby the pattern may be oriented, and the position of any point roughly charted. Where there is a plot on only one side of the finger (Fig. 8, ~3~), the pattern has almost necessarily an axis, which serves for orientation, and the pattern can still be charted, though on a different principle, by dropping a perpendicular from the plot on to the axis, in the way there shown.

These plots form corner-stones to my system of outlining and subsequent cla.s.sification; it is therefore extremely important that a sufficient area of the finger should be printed to include them. This can always be done by slightly _rolling_ the finger (p. 39), the result being, in the language of map-makers, a cylindrical projection of the finger (see Plate 5, Fig. 9, _a-h_). Large as these impressions look, they are of the natural size, taken from ordinary thumbs.

[Ill.u.s.tration: PLATE 5.

FIG. 9. EXAMPLES OF OUTLINED PATTERNS (The Specimens are rolled impressions of natural size).]

_The outlines._--The next step is to give a clear and definite shape to the pattern by drawing its outline (Fig. 9). Take a fine pen, pencil, or paint brush, and follow in succession each of the two diverging ridges that start from either plot. The course of each ridge must be followed with scrupulous conscientiousness, marking it with a clean line as far as it can be traced. If the ridge bifurcates, always follow the branch that trends towards the middle of the pattern. If it stops short, let the outline stop short also, and recommence on a fresh ridge, choosing that which to the best of the judgment prolongs the course of the one that stopped. These outlines have an extraordinary effect in making finger markings intelligible to an untrained eye. What seemed before to be a vague and bewildering maze of lineations over which the glance wandered distractedly, seeking in vain for a point on which to fix itself, now suddenly a.s.sumes the shape of a sharply-defined figure. Whatever difficulties may arise in cla.s.sifying these figures, they are as nothing compared to those experienced in attempting to cla.s.sify unoutlined patterns, the outlines giving a precision to their general features which was wanting before.

After a pattern has been treated in this way, there is no further occasion to pore minutely into the finger print, in order to cla.s.sify it correctly, for the bold firm curves of the outline are even more distinct than the largest capital letters in the t.i.tle-page of a book.

A fair idea of the way in which the patterns are distributed, is given by Plate 6. Eight persons were taken in the order in which they happened to present themselves, and Plate 6 shows the result. For greater clearness, colour has been employed to distinguish between the ridges that are supplied from the inner and outer sides of the hand respectively. The words right and left _must be avoided_ in speaking of patterns, for the two hands are symmetrically disposed, only in a reversed sense. The right hand does not look like a left hand, but like the reflection of a left hand in a looking-gla.s.s, and _vice versa_. The phrases we shall employ will be the _Inner_ and the _Outer_; or thumb-side and little-finger side (terms which were unfortunately misplaced in my memoir in the _Phil.

Trans._ 1891).

There need be no difficulty in remembering the meaning of these terms, if we bear in mind that the great toes are undoubtedly innermost; that if we walked on all fours as children do, and as our remote ancestors probably did, the thumbs also would be innermost, as is the case when the two hands are impressed side by side on paper. Inner and outer are better than thumb-side and little-finger side, because the latter cannot be applied to the thumbs and little fingers themselves. The anatomical words radial and ulnar referring to the two bones of the fore-arm, are not in popular use, and they might be similarly inappropriate, for it would sound oddly to speak of the radial side of the radius.

[Ill.u.s.tration: PLATE 6.

FIG. 10. OUTLINES OF THE PATTERNS OF THE DIGITS OF EIGHT PERSONS, TAKEN AT RANDOM.]

The two plots just described will therefore be henceforth designated as the Inner and the Outer plots respectively, and symbolised by the letters I and O.

The system of ridges in Fig. 10 that comes from the inner side "I" are coloured blue; those from the outer "O" are coloured red. The employment of colour instead of variously stippled surfaces is of conspicuous advantage to the great majority of persons, though unhappily nearly useless to about one man in every twenty-five, who is const.i.tutionally colour-blind.

It may be convenient when marking finger prints with letters for reference, to use those that look alike, both in a direct and in a reversed aspect, as they may require to be read either way. The print is a reversed picture of the pattern upon the digit that made it. The pattern on one hand is, as already said, a reversed picture of a similar pattern as it shows on the other. In the various processes by which prints are multiplied, the patterns may be reversed and re-reversed. Thus, if a finger is impressed on a lithographic stone, the impressions from that stone are reversals of the impression made by the same finger upon paper.

If made on transfer paper and thence transferred to stone, there is a re-reversal. There are even more varied possibilities when photography is employed. It is worth recollecting that there are twelve capital letters in the English alphabet which, if printed in block type, are unaffected by being reversed. They are A.H.I.M.O.T.U.V.W.X.Y.Z. Some symbols do the same, such as, * + - = :. These and the letters H.O.I.X. have the further peculiarity of appearing unaltered when upside down.

_Lenses._--As a rule, only a small magnifying power is needed for drawing outlines, sufficient to allow the eye to be brought within six inches of the paper, for it is only at that short distance that the _minutiae_ of a full-sized finger print begin to be clearly discerned. Persons with normal sight, during their childhood and boy- or girlhood, are able to read as closely as this without using a lens, the range in adjustment of the focus of the eye being then large. But as age advances the range contracts, and an elderly person with otherwise normal eyesight requires gla.s.ses to read a book even at twelve inches from his eye. I now require much optical aid; when reading a book, spectacles of 12-inch focus are necessary; and when studying a finger print, 12-inch eye-gla.s.ses in addition, the double power enabling me to see clearly at a distance of only six inches. Perhaps the most convenient focus for a lens in ordinary use is 3 inches. It should be mounted at the end of a long arm that can easily be pushed in any direction, sideways, backwards, forwards, and up or down. It is undesirable to use a higher power than this unless it is necessary, because the field of view becomes narrowed to an inconvenient degree, and the nearer the head is to the paper, the darker is the shadow that it casts; there is also insufficient room for the use of a pencil.

Every now and then a closer inspection is wanted; for which purpose a doublet of 1/2-inch focus, standing on three slim legs, answers well.

For studying the markings on the fingers themselves, a small folding lens, sold at opticians' shops under the name of a "linen tester," is very convenient. It is so called because it was originally constructed for the purpose of counting the number of threads in a given s.p.a.ce, in a sample of linen. It is equally well adapted for counting the number of ridges in a given s.p.a.ce.

Whoever desires to occupy himself with finger prints, ought to give much time and practice to drawing outlines of different impressions of the same digits. His own ten fingers, and those of a few friends, will furnish the necessary variety of material on which to work. He should not rest satisfied until he has gained an a.s.surance that all patterns possess definite figures, which may be latent but are potentially present, and that the ridges form something more than a nondescript congeries of ramifications and twists. He should continue to practise until he finds that the same ridges have been so nearly followed in duplicate impressions, that even in difficult cases his work will rarely vary more than a single ridge-interval.

When the triangular plot happens not to be visible, owing to the print failing to include it, which is often the case when the finger is not rolled, as is well shown in the prints of my own ten digits on the t.i.tle-page, the trend of the ridges so far as they are seen, usually enables a practised eye to roughly estimate its true position. By means of this guidance an approximate, but fairly correct, outline can be drawn.

When the habit of judging patterns by their outlines has become familiar, the eye will trace them for itself without caring to draw them, and will prefer an unoutlined pattern to work upon, but even then it is essential now and then to follow the outline with a fine point, say that of a penknife or a dry pen.