A sealing-wax impression is the simplest and best kind of cast, and the finger need not be burnt in making it. The plan is to make a considerable pool of flaming sealing-wax, stirring it well with the still unmelted piece of the stick, while it is burning. Then blow out the flame and wait a little, until the upper layer has cooled. Sealing-wax that has been well aflame takes a long time to harden thoroughly after it has parted with nearly all its heat. By selecting the proper moment after blowing out the flame, the wax will be cool enough for the finger to press it without discomfort, and it will still be sufficiently soft to take a sharp impression. Dentist’s wax, which is far less brittle, is easily worked, and takes impressions that are nearly as sharp as those of sealing-wax; it has to be well heated and kneaded, then plunged for a moment in cold water to chill the surface, and immediately impressed. Gutta-percha can also be used. The most delicate of all impressions is that left upon a thick clot of varnish, which has been exposed to the air long enough for a thin film to have formed over it. The impression is transient, but lingers sufficiently to be easily photographed. It happened, oddly enough, that a few days after I had noticed this effect, and had been experimenting upon it, I heard an interesting memoir “On the Minute Structure of Striped Muscle, with special allusion to a newmethod of investigation by means of ‘Impressions’ stamped in Collodion,” submitted to the Royal Society by Dr. John Berry Haycraft, in which an analogous method was used to obtain impressions of delicate microscopic structures.
Photographsare valuable in themselves, and the negatives serve for subsequentenlargements. They are unquestionably accurate, and the labour of making them being mechanical, may be delegated. If the print be in printer’s ink on white paper, the process is straightforward, first of obtaining a negative and afterwards photo-prints from it. The importance of the paper or card used to receive the finger print being quite white, has already been pointed out. An imprint on white crockery-ware is beautifully clear. Some of the photographs may be advantageously printed by the ferro-prussiate process. The paper used for it does not curl when dry, its texture is good for writing on, and the blue colour of the print makes handwriting clearly legible, whether it be in ink or in pencil.
Prints on glass have great merits for use as lantern slides, but it must be recollected that they may take some days to dry, and that when dry the ink can be only too easily detached from them by water, which insinuates itself between the dry ink and the glass. Of course they could be varnished, if the trouble and cost were no objection, and so preserved. The negative print left on an inked slab, after the finger has touched it, is sometimes very clear, that on smoked glass better, and on smoked mica the clearest of all.These have merely to be placed in the enlarging camera, where the negative image thrown on argento-bromide paper will yield a positive print. (Seep. 90.)
I have made, by hand, many enlargements with a prism (camera lucida), but it is difficult to enlarge more than five times by means of it. So much shade is cast by the head that the prism can hardly be used at a less distance than 3 inches from the print, or one quarter the distance (12 inches) at which a book is usually read, while the paper on which the drawing is made cannot well be more than 15 inches below the prism; so it makes an enlargement of4 × 15⁄12or five-fold. This is a very convenient method of analysing a pattern, since the lines follow only the axes of the ridges, as inPlate 3, Fig. 5. The prism and attached apparatus may be kept permanently mounted, ready for use at any time, without the trouble of any adjustment.
An enlarging pantagraph has also been of frequent use to me, in which the cross-wires of a low-power microscope took the place of the pointer. It has many merits, but its action was not equally free in all directions; the enlarged traces were consequently jagged, and required subsequent smoothing.
All hand-made enlargements are tedious to produce, as the total length of lineations to be followed is considerable. In a single finger print made by dabbing down the finger, their actual length amounts to about 18 inches; therefore in a five-fold enlargement of the entire print the pencil has to be carefully directed over five times that distance, or more than 7 feet.
Large copies of tracings made on transparent paper, either by the Camera Lucida or by the Pantagraph, are easily printed by the ferro-prussiate photographic process mentioned above, in the same way that plans are copied by engineers.
THE RIDGES AND THEIR USES
The palmar surface of the hands and the soles of the feet, both in men and monkeys, are covered with minute ridges that bear a superficial resemblance to those made on sand by wind or flowing water. They form systems which run in bold sweeps, though the courses of the individual ridges are less regular. Each ridge (Plate 3, Fig. 5) is characterised by numerous minute peculiarities, calledMinutiæin this book, here dividing into two, and there uniting with another (a,b), or it may divide and almost immediately reunite, enclosing a small circular or elliptical space (c); at other times its beginning or end is markedly independent (d,e); lastly, the ridge may be so short as to form a small island (f).
Whenever an interspace is left between the boundaries of different systems of ridges, it is filled by a small system of its own, which will have some characteristic shape, and be called apatternin this book.
PLATE 3.
Fig. 5.
Characteristic peculiarities in Ridges(about 8 times the natural size).
Fig. 6.
Systems of Ridges, and the Creases in the Palm.
There are three particularly well-marked systems of ridges in the palm of the hand marked inPlate 3, Fig. 6,1, as Th, AB, and BC. The system Th is that which runs over the ball of the thumb and adjacent parts of the palm. It is bounded by the lineawhich starts from the middle of the palm close to the wrist, and sweeps thence round the ball of the thumb to the edge of the palm on the side of the thumb, which it reaches about half an inch, more or less, below the base of the fore-finger. The system AB is bounded towards the thumb by the above linea, and towards the little finger by the lineb; the latter starts from about the middle of the little-finger side of the palm, and emerges on the opposite side just below the fore-finger. Consequently, every ridge that wholly crosses the palm is found in AB. The system BC is bounded thumbwards by the lineb, until that line arrives at a point immediately below the axis of the fore-finger; there the boundary of BC leaves the lineb, and skirts the base of the fore-finger until it reaches the interval which separates the fore and middle fingers. The upper boundary of BC is the linec, which leaves the little-finger side of the palm at a small distance below the base of the little finger, and terminates between the fore and middle fingers. Other systems are found betweencand the middle, ring, and little fingers; they are somewhat more variable than those just described, as will be seen by comparing the five different palms shown in Fig. 6.
An interesting example of the interpolation of a small and independent system occurs frequently in the middle of one or other of the systems AB or BC, at the place where the space covered by the systemsof ridges begins to broaden out very rapidly. There are two ways in which the necessary supply of ridges makes its appearance, the one is by a series of successive embranchments (Fig. 6,1), the other is by the insertion of an independent system, as shown in4,5. Another example of an interpolated system, but of rarer occurrence, is found in the system Th, on the ball of the thumb, as seen in2.
Far more definite in position, and complex in lineation, are the small independent systems which appear on the bulbs of the thumb and fingers. They are more instructive to study, more easy to classify, and will alone be discussed in this book.
In the diagram of the hand, Fig. 6,1, the three chief cheiromantic creases are indicated by dots, but are not numbered. They are made (1) by the flexure of the thumb, (2) of the four fingers simultaneously, and (3) of the middle, ring, and little fingers simultaneously, while the fore-finger remains extended. There is no exact accordance between the courses of the creases and those of the adjacent ridges, less still do the former agree with the boundaries of the systems. The accordance is closest between the crease (1) and the ridges in Th; nevertheless that crease does not agree with the linea, but usually lies considerably within it. The crease (2) cuts the ridges on either side, at an angle of about 30 degrees. The crease (3) is usually parallel to the ridges between which it runs, but is often far from accordant with the linec. The creases at the various joints of thethumb and fingers cut the ridges at small angles, say, very roughly, of 15 degrees.
The supposition is therefore untenable that the courses of the ridges are wholly determined by the flexures. It appears, however, that the courses of the ridges and those of the lines of flexure may be in part, but in part only, due to the action of the same causes.
The fact of the creases of the hand being strongly marked in the newly-born child, has been considered by some to testify to the archaic and therefore important character of their origin. The crumpled condition of the hand of the infant, during some months before its birth, seems to me, however, quite sufficient to account for the creases.
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 ofthem. 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.
PLATE 4.
Fig. 7.
SCARS and CUTS, and their Effects on the Ridges.
Fig. 8.
FORMATION OF INTERSPACE and Examples of the Enclosed Patterns.
The ridges are said to be first discernible in the fourth month of fœtal 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 title-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 ofsupporting 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 inPlate 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 assistant happened to burn his finger rather sharply; the daily prints he took of it, illustrated 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 analogous 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 papillæ 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 assist the sense of touch. They are said to be moulded upon the subcutaneous papillæ 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 assist 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 compasses, 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 prick. 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 compass 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. Titchener 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 coarseness 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 compass 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 compass 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 aperson 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 analysed, and which becomes very distinct when the indentations are at equal distances apart, as in a file or in velvet. A thrill is analogous 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.
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 orCommentatio. I have translated the part that chiefly concerns us, and appended it to this chapter together with his corresponding illustrations. Subsequent writers have adopted his standard types, diminishing 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 classification 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 abouttwo 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 analogous 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 classification that seemed tolerably good a week before, looked then. Moreover, I suffered the shame and humiliation of discovering that the identity 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 quantity 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 analyse 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 classification 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 (seePlate 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 interspace (Fig. 8,2). The topmost boundary of the interspace is formed by the lowermost arch, and its lowermost boundary by the topmost straight ridge. But an equally large number ofducts exist within the interspace, 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; alsoPlate 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 interspace 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 classification; it is therefore extremely important that a sufficient area of the finger should be printed to include them. This can always be done by slightlyrollingthe finger (p. 39), the result being, in the language of map-makers, a cylindrical projection of the finger (seePlate 5, Fig. 9,a-h). Large as these impressions look, they are of the natural size, taken from ordinary thumbs.
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 assumes the shape of a sharply-defined figure. Whatever difficulties may arise in classifying these figures, they are as nothing compared to those experienced in attempting to classify 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 classify it correctly, for the bold firm curves of the outline are even more distinct than the largest capital letters in the title-page of a book.
A fair idea of the way in which the patterns are distributed, is given byPlate 6. Eight persons were taken in the order in which they happened to present themselves, andPlate 6shows 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 leftmust be avoidedin 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-glass, andvice versa. The phrases we shall employ will be theInnerand theOuter; or thumb-side and little-finger side (terms which were unfortunately misplaced in my memoir in thePhil. 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.
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 constitutionally 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 areA.H.I.M.O.T.U.V.W.X.Y.Z.Some symbols do the same,such as, * + - = :. These and the lettersH.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 theminutiæof 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 glasses 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-glasses 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 ½-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 space, in a sample of linen. It is equally well adapted for counting the number of ridges in a given space.
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 assurance 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 title-page, the trend of the ridges so far asthey 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.
In selecting standard forms of patterns for the convenience of description, we must be content to disregard a great many of the more obvious characteristics. For instance, the size of generally similar patterns in Fig. 10 will be found to vary greatly, but the words large, medium, or small may be applied to any pattern, so there is no necessity to draw a standard outline for each size. Similarly as regards the inwards or outwards slope of patterns, it is needless to print here a separate standard outline for either slope, and equally unnecessary to print outlines in duplicate, with reversed titles, for the right and left hands respectively. The phrase “a simple spiral” conveys a well-defined general idea, but there are four concrete forms of it (see bottom row ofPlate 11, Fig. 17,oj,jo,ij,ji) which admit of being verbally distinguished. Again the internal proportions of any pattern, say those of simple spirals, may vary greatly without affecting the fact of their being simple spirals. They may be wide or narrow at their mouths, they may be twisted up into a point (Plate 8, Fig. 14,52), or they may run in broad curls of uniform width (Fig. 14,51,54). Perhaps the best general rule in selecting standard outlines, is to limit them to such as cannot be turned into any other by viewing them in an altered aspect, as upside down or from the back, or by magnifying or deforming them, whether it be through stretching, shrinking, or puckering any part of them. Subject to this general rule and to further and more particular descriptions, the sets (Plates 7and8, Figs. 11, 12, 13) will be found to give considerable help in naming the usual patterns.
PLATE 7.
Fig. 11.ARCHES.
Fig. 12.LOOPS.
PLATE 8.
Fig. 13.WHORLS.
Fig. 14.CORES to LOOPS.
It will be observed that they are grouped under the three principal heads of Arches, Loops, and Whorls, and that under each of these heads some analogous patterns as4,5,7,8, etc., are introduced and underlined with the word “see” so and so, and thus noted as really belonging to one of the other heads. This is done to indicate the character of the transitional cases that unite respectively the Arches with the Loops, the Arches with the Whorls, and the Loops with the Whorls. More will follow in respect to these. The “tented arch” (3) is extremely rare on the thumb; I do not remember ever to have seen it there, consequently it did not appear in the plate of patterns in thePhil. Trans.which referred to thumbs. On the other hand, the “banded duplex spiral” (30) is common in the thumb, but rare elsewhere. There are some compound patterns, especially the “spiral in loop” (21) and the “circlet in loop” (22), which are as much loops as whorls; but are reckoned as whorls.The “twinned loop” (16) is of more frequent occurrence than would be supposed from the examination ofdabbedimpressions, as the only part of the outer loop then in view resembles outside arches; it is due to a double separation of the ridges (Plate 4, Fig. 8), and a consequent double interspace. The “crested loop” (13) may sometimes be regarded as an incipient form of a “duplex spiral” (29).
The reader may also refer toPlate 16, which contains what is there called the C set of standard patterns. They were arranged and used for a special purpose, as described inChapter XI.They refer to impressions of the right hand.
As a variety of Cores, differing in shape and size, may be found within each of the outlines, it is advisable to describe them separately.Plate 8, Fig. 14 shows a series of the cores of loops, in which the innermost lineations may be either straight or curved back; in the one case they are here called rods (31to35); in the other (36to42), staples. The first of the ridges that envelops the core, whether the core be a rod, many rods, or a staple, is also shown and named (43to48). None of the descriptions are intended to apply to more than thevery endof the core, say, from the tip downwards to a distance equal to two average ridge-intervals in length. If more of the core be taken into account, the many varieties in their lower parts begin to make description confusing. In respect to the “parted” staples and envelopes, and those that are single-eyed, the description may further mention the side on which theparting or the eye occurs, whether it be the Inner or the Outer.
At the bottom of Fig. 14,49-54, is given a series of rings, spirals, and plaits, in which nearly all the clearly distinguishable varieties are included, no regard being paid to the direction of the twist or to the number of turns.49is a set of concentric circles,50of ellipses: they are rarely so in a strict sense throughout the pattern, usually breaking away into a more or less spiriform arrangement as in51. A curious optical effect is connected with the circular forms, which becomes almost annoying when many specimens are examined in succession. They seem to be cones standing bodily out from the paper. This singular appearance becomes still more marked when they are viewed with only one eye; no stereoscopic guidance then correcting the illusion of their being contour lines.
Another curious effect is seen in53, which has the appearance of a plait or overlap; two systems of ridges that roll together, end bluntly, the end of the one system running right into a hollow curve of the other, and there stopping short; it seems, at the first glance, to run beneath it, as if it were a plait. This mode of ending forms a singular contrast to that shown in51and52, where the ridges twist themselves into a point.54is a deep spiral, sometimes having a large core filled with upright and nearly parallel lines; occasionally they are bulbous, and resemble the commoner “monkey” type, see35.
When the direction of twist is described, thelanguage must be unambiguous: the following are the rules I adopt. The course of the ridge is always followedtowardsthecentreof the pattern, and not away from it. Again, the direction of its course when so followed is specified at the place where it attains itshighestpoint, or that nearest to the finger-tip; its course at that point must needs be horizontal, and therefore directed either towards the inner or the outer side.
The amount of twist has a strong tendency to coincide with either one, two, three, four, or more half-turns, and not to stop short in intermediate positions. Here are indications of some unknown fundamental law, analogous apparently to that which causes Loops to be by far the commonest pattern.
The classification into Arches, Loops, and Whorls is based on the degree of curvature of the ridges, and enables almost any pattern to be sorted under one or other of those three heads. There are a few ambiguous patterns, and others which are nondescript, but the former are uncommon and the latter rare; as these exceptions give little real inconvenience, the classification works easily and well.
Arches are formed when the ridges run from one side to the other of the bulb of the digit without making any backward turn or twist. Loops, when there is a single backward turn, but no twist. Whorls, when there is a turn through at least one complete circle; they are also considered to include all duplex spirals.
PLATE 9.
Fig. 15.
TRANSITIONAL PATTERNS—Arches and Loops(enlarged three times).
PLATE 10.
FIG. 16.
TRANSITIONAL PATTERNS—Loops and Whorls(enlarged three times).
The chief theoretical objection to this threefold system of classification lies in the existence of certain compound patterns, by far the most common of which are Whorls enclosed within Loops (Plates 7,8, Fig. 12,15,18,19, and Fig. 13,20-23). They are as much Loops as Whorls, and properly ought to be relegated to a fourth class. I have not done so, but called them Whorls, for a practical reason which is cogent. In an imperfect impression, such as is made by merely dabbing the inked finger upon paper, the enveloping loop is often too incompletely printed to enable its existence to be surely ascertained, especially when the enclosed whorl is so large (Fig. 13,23) that there are only one or two enveloping ridges to represent the loop. On the other hand, the whorled character of the core can hardly fail to be recognised. The practical difficulties lie almost wholly in rightly classifying a few transitional forms, diagrammatically and roughly expressed in Fig. 11,4,5, and Fig. 12,8,18,19, with the words “see” so and so written below, and of which actual examples are given on an enlarged scale inPlates 9and10, Figs. 15 and 16. Here Fig. 15,ais an undoubted arch, andcan undoubted nascent loop; butbis transitional between them, though nearer to a loop than an arch,dmay be thought transitional in the same way, but it has an incipient curl which becomes marked ine, while it has grown into a decided whorl inf;dshould also be compared withj, which is in some sense a stage towardsk.gis a nascent tented-arch, fully developed ini, where the pattern as a whole has aslight slope, but is otherwise fairly symmetrical. Inhthere is some want of symmetry, and a tendency to the formation of a loop on the right side (refer back toPlate 7, Fig. 11,4, and Fig. 12,12); it is a transitional case between a tented arch and a loop, with most resemblance to the latter.Plate 10, Fig. 16 illustrates eyed patterns; herelandmare parts of decided loops;p,q, andrare decided whorls, butnis transitional, inclining towards a loop, andois transitional, inclining towards a whorl.sis a nascent form of an invaded loop, and is nearly related tol;tanduare decidedly invaded loops.
The Arch-Loop-Whorl, or, more briefly, the A. L. W. system of classification, while in some degree artificial, is very serviceable for preliminary statistics, such as are needed to obtain a broad view of the distribution of the various patterns. A minute subdivision under numerous heads would necessitate a proportional and somewhat overwhelming amount of statistical labour. Fifty-four different standard varieties are by no means an extravagant number, but to treat fifty-four as thoroughly as three would require eighteen times as much material and labour. Effort is economised by obtaining broad results from a discussion of the A. L. W. classes, afterwards verifying or extending them by special inquiries into a few of the further subdivisions.
PLATE 11.
Fig. 17.
ORIGIN OF SUPPLY OF RIDGES TO PATTERNS OF PRINTS OF RIGHT HAND.
Of the two letters in the left upper corner of each compartment, the first refers to the source of upper boundary of the pattern,the second to the lower boundary. For patterns on the prints of left hands,IiandOomust be interchanged.
Fig. 18.
Ambiguities in prints of the Minutiæ.
The divergent ridges that bound any simple pattern admit of nine, and only nine, distinct variations in the first part of their course. The bounding ridge that has attained the summit of any such pattern must have arrived either from the Inner plot (I), the Outer plot (O), or from both. Similarly as regards the bounding ridge that lies at the lowest point of the pattern. Any one of the three former events may occur in connection with any of the three latter events, so they afford in all 3 × 3, or nine possible combinations. It is convenient to distinguish them by easily intelligible symbols. Thus, letisignify a bounding line which starts from the point I, whether it proceeds to the summit or to the base of the pattern; letobe a line that similarly proceeds from O, and letube a line that unites the two plots I and O, either by summit or by base. Again, let two symbols be used, of which the first shall always refer to the summit, and the second to the base of the pattern. Then the nine possible cases are—uu,ui,uo;iu,ii,io;ou,oi,oo. The case of the arches is peculiar, but they may be fairly classed under the symboluu.
This easy method of classification has much power. For example, the four possible kinds of simple spirals (see the 1st, 2nd, and the 5th and 6th diagrams in the lowest row ofPlate 11, Fig. 17) are wholly determined by the lettersoj,jo,ij,jirespectively. The two forms of duplex spirals are similarly determined byoiandio(see 4th and 5th diagrams in the upper row of Fig. 17), the two slopes of loops byooandii(3rd and 4th in the lower row). It also shows very distinctly the sources whence the streams of ridges proceed that feed the pattern, which itself affords another basis for classification.The resource against uncertainty in respect to ambiguous or difficult patterns is to compile a dictionary of them, with the heads under which it is advisable that they should severally be classed. It would load these pages too heavily to give such a dictionary here. Moreover, it ought to be revised by many experienced eyes, and the time is hardly ripe for this; when it is, it would be no difficult task, out of the large number of prints of separate fingers which for instance I possess (some 15,000), to make an adequate selection, to enlarge them photographically, and finally to print the results in pairs, the one untouched, the other outlined and classified.
It may be asked why ridges are followed and not furrows, the furrow being the real boundary between two systems. The reply is, that the ridges are the easiest to trace; and, as the error through following the ridges cannot exceed one-half of a ridge-interval, I have been content to disregard it. I began by tracing furrows, but preferred the ridges after trial.
Measurements.—It has been already shown that when both plots are present (Plate 4, Fig. 8,4), they form the termini of a base line, from which any part of the pattern may be triangulated, as surveyors would say. Also, that when only one plot exists (3), and the pattern has an axis (which it necessarily has in all ordinaryiiandoocases), a perpendicular can be let fall upon that axis, whose intersection with it will serve as a second point of reference. But our methods must not be too refined. The centres of the plots are not determinable with real exactness,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.