1. “Personal Identification.”Journal Royal Inst.25th May 1888, andNature, 28th June 1888.2. “Patterns in Thumb and Finger Marks.”Phil. Trans. Royal Society, vol. clxxxii. (1891) b. pp. 1-23. [This almost wholly referred to thumb marks.]3. “Method of Indexing Finger Marks.”Proc. Royal Society, vol. xlix. (1891).4. “Identification by Finger Tips.”Nineteenth Century, August 1891.
1. “Personal Identification.”Journal Royal Inst.25th May 1888, andNature, 28th June 1888.
2. “Patterns in Thumb and Finger Marks.”Phil. Trans. Royal Society, vol. clxxxii. (1891) b. pp. 1-23. [This almost wholly referred to thumb marks.]
3. “Method of Indexing Finger Marks.”Proc. Royal Society, vol. xlix. (1891).
4. “Identification by Finger Tips.”Nineteenth Century, August 1891.
This first and introductory chapter contains a brief and orderly summary of the contents of those that follow.
The second chapter treats of the previous employment of finger prints among various nations, which has been almost wholly confined to making daubs, without paying any regard to the delicate lineations with which this book is alone concerned. Their object was partly superstitious and partly ceremonial; superstitious, so far as a personal contact betweenthe finger and the document was supposed to be of mysterious efficacy: ceremonial, as a formal act whose due performance in the presence of others could be attested. A few scattered instances are mentioned of persons who had made finger prints with enough care to show their lineations, and who had studied them; some few of these had used them as signatures. Attention is especially drawn to Sir William Herschel, who brought the method of finger prints into regular official employment when he was “Collector” or chief administrator of the Hooghly district in Bengal, and my large indebtedness to him is expressed in this chapter and in other places.
In the third chapter various methods of making good prints from the fingers are described at length, and more especially that which I have now adopted on a somewhat large scale, at my anthropometric laboratory, which, through the kindness of the authorities of South Kensington, is at present lodged in the galleries of their Science Collections. There, the ten digits of both hands of all the persons who come to be measured, are impressed with clearness and rapidity, and a very large collection of prints is steadily accumulating, each set being, as we shall see, a sign-manual that differentiates the person who made it, throughout the whole of his life, from all the rest of mankind.
Descriptions are also given of various methods of enlarging a finger print to a convenient size, when it is desired to examine it closely. Photography is the readiest of all; on the other hand the prism (as ina camera lucida) has merits of its own, and so has an enlarging pantagraph, when it is furnished with a small microscope and cross wires to serve as a pointer.
In the fourth chapter the character and purpose of the ridges, whose lineations appear in the finger print, are discussed. They have been the topic of a considerable amount of careful physiological study in late years, by writers who have investigated their development in early periods of unborn life, as well as their evolutionary history. They are perfectly defined in the monkeys, but appear in a much less advanced stage in other mammalia. Their courses run somewhat independently of the lines of flexure. They are studded with pores, which are the open mouths of ducts proceeding from the somewhat deeply-seated glands which secrete perspiration, so one of their functions is to facilitate the riddance of that excretion. The ridges increase in height as the skin is thickened by hard usage, until callosities begin to be formed, which may altogether hide them. But the way in which they assist the touch and may tend to neutralise the dulling effect of a thick protective skin, is still somewhat obscure. They certainly seem to help in the discrimination of the character of surfaces that are variously rubbed between the fingers.
These preliminary topics having been disposed of, we are free in the fifth chapter to enter upon the direct course of our inquiry, beginning with a discussion of the various patterns formed by the lineations. It will be shown how systems of parallel ridges sweep in bold curves across the palmar surfaceof the hand, and how, whenever the boundaries of two systems diverge, the interspace is filled up by a compact little system of its own, variously curved or whorled, having a fictitious resemblance to an eddy between two currents. An interspace of this kind is found in the bulb of each finger. The ridges run in parallel lines across the finger, up to its last joint, beyond which the insertion of the finger-nail causes a compression of the ridges on either side; their intermediate courses are in consequence so much broadened out that they commonly separate, and form two systems with an interspace between them. The independent patterns that appear in this interspace upon the bulbs of the fingers, are those with which this book is chiefly concerned.
At first sight, the maze formed by the minute lineations is bewildering, but it is shown that every interspace can be surely outlined, and when this is done, the character of the pattern it encloses, starts conspicuously into view. Examples are given to show how the outlining is performed, and others in which the outlines alone are taken into consideration. The cores of the patterns are also characteristic, and are described separately. It is they alone that have attracted the notice of previous inquirers. The outlines fall for the most part into nine distinct genera, defined by the relative directions of the divergent ridges that enclose them. The upper pair (those that run towards the finger-tip) may unite, or one or other of them may surmount the other, thus making three possibilities. There are three similarpossibilities in respect to the lower pair; so, as any one of the first group may be combined with any one of the second, there are 3 × 3, or nine possibilities in all. The practice of somewhat rolling the finger when printing from it, is necessary in order to impress enough of its surface to ensure that the points at which the boundaries of the pattern begin to diverge, shall be always included.
Plates are given of the principal varieties of patterns, having regard only to their more fundamental differences, and names are attached for the convenience of description; specimens are also given of the outlines of the patterns in all the ten digits of eight different persons, taken at hazard, to afford a first idea of the character of the material to be dealt with. Another and less minute system of classification under three heads is then described, which is very useful for rough preliminary purposes, and of which frequent use is made further on. It is into Arches, Loops, and Whorls. In the Arches, there is no pattern strictly speaking, for there is no interspace; the need for it being avoided by a successive and regular broadening out of the ridges as they cross the bulb of the finger. In Loops, the interspace is filled with a system of ridges that bends back upon itself, and in which no one ridge turns through a complete circle. Whorls contain all cases in which at least one ridge turns through a complete circle, and they include certain double patterns which have a whorled appearance. The transitional cases are few; they are fully described, pictured, andclassified. One great advantage of the rude A. L. W. system is that it can be applied, with little risk of error, to impressions that are smudged or imperfect; it is therefore very useful so far as it goes. Thus it can be easily applied to my own finger prints on the title-page, made as they are from digits that are creased and roughened by seventy years of life, and whose impressions have been closely clipped in order to fit them into a limited space.
A third method of classification is determined by the origin of the ridges which supply the interspace, whether it be from the thumb side or the little-finger side; in other words, from the Inner or the Outer side.
Lastly, a translation from the Latin is given of the famous Thesis orCommentatioof Purkenje, delivered at the University of Breslau in 1823, together with his illustrations. It is a very rare pamphlet, and has the great merit of having first drawn attention to the patterns and attempted to classify them.
In the sixth chapter we reach the question of Persistence: whether or no the patterns are so durable as to afford a sure basis for identification. The answer was different from what had been expected. So far as the proportions of the patterns go, they arenotabsolutely fixed, even in the adult, inasmuch as they change with the shape of the finger. If the finger is plumped out or emaciated, or variously deformed by usage, gout, or age, the proportions of the pattern will vary also. Two prints of the same finger, one taken before and the other after an interval of many years,cannot be expected to be as closely alike as two prints similarly made from the same woodcut. They are far from satisfying the shrewd test of the stereoscope, which shows if there has been an alteration even of a letter in two otherwise duplicate pages of print. The measurements vary at different periods, even in the adult, just as much if not more than his height, span, and the lengths of his several limbs. On the other hand, the numerous bifurcations, origins, islands, and enclosures in the ridges that compose the pattern, are proved to bealmost beyond change. A comparison is made between the pattern on a finger, and one on a piece of lace; the latter may be stretched or shrunk as a whole, but the threads of which it is made retain their respective peculiarities. The evidence on which these conclusions are founded is considerable, and almost wholly derived from the collections made by Sir W. Herschel, who most kindly placed them at my disposal. They refer to one or more fingers, and in a few instances to the whole hand, of fifteen different persons. The intervals before and after which the prints were taken, amount in some cases to thirty years. Some of them reach from babyhood to boyhood, some from childhood to youth, some from youth to advanced middle age, one from middle life to incipient old age. These four stages nearly include the whole of the ordinary life of man. I have compared altogether some 700 points of reference in these couplets of impressions, and only found a single instance of discordance, in which a ridge that wascleft in a child became united in later years. Photographic enlargements are given in illustration, which include between them a total of 157 pairs of points of reference, all bearing distinctive numerals to facilitate comparison and to prove their unchangeableness. Reference is made to another illustrated publication of mine, which raises the total number of points compared to 389, all of which were successful, with the single exception above mentioned. The fact of an almost complete persistence in the peculiarities of the ridges from birth to death, may now be considered as determined. They existed before birth, and they persist after death, until effaced by decomposition.
In the seventh chapter an attempt is made to appraise the evidential value of finger prints by the common laws of Probability, paying great heed not to treat variations that are really correlated, as if they were independent. An artifice is used by which the number of portions is determined, into which a print may be divided, in each of which the purely local conditions introduce so much uncertainty, that a guess derived from a knowledge of the outside conditions is as likely as not to be wrong. A square of six ridge-intervals in the side was shown by three different sets of experiments to be larger than required; one of four ridge-intervals in the side was too small, but one of five ridge-intervals appeared to be closely correct. A six-ridge interval square was, however, at first adopted, in order to gain assurance that the error should be on the safe side. Asan ordinary finger print contains about twenty-four of these squares, the uncertainty in respect to the entire contents of the patterndue to this cause alone, is expressed by a fraction of which the numerator is 1, and the denominator is 2 multiplied into itself twenty-four times, which amounts to a number so large that it requires eight figures to express it.
A further attempt was made to roughly appraise the neglected uncertainties relating to the outside conditions, but large as they are, they seem much inferior in their joint effect to the magnitude of that just discussed.
Next it was found possible, by the use of another artifice, to obtain some idea of the evidential value of identity when two prints agree in all but one, two, three, or any other number of particulars. This was done by using the five ridge-interval squares, of which thirty-five may be considered to go into a single finger print, being about the same as the number of the bifurcations, origins, and other points of comparison. The accidental similarity in their numbers enables us to treat them roughly as equivalent. On this basis the well-known method of binomial calculation is easily applied, with the general result that, notwithstanding a failure of evidence in a few points, as to the identity of two sets of prints, each, say, of three fingers, amply enough evidence would be supplied by the remainder to prevent any doubt that the two sets of prints were made by the same person. When a close correspondence exists in respect to all the ten digits, the thoroughnessof the differentiation of each man from all the rest of the human species is multiplied to an extent far beyond the capacity of human imagination. There can be no doubt that the evidential value of identity afforded by prints of two or three of the fingers, is so great as to render it superfluous to seek confirmation from other sources.
The eighth chapter deals with the frequency with which the several kinds of patterns appear on the different digits of the same person, severally and in connection. The subject is a curious one, and the inquiry establishes unexpected relationships and distinctions between different fingers and between the two hands, to whose origin there is at present no clue. The relationships are themselves connected in the following way;—calling any two digits on one of the hands by the letters A and B respectively, and the digit on the other hand, that corresponds to B, by the symbol B1, then the kinship between A and B1 is identical, in a statistical sense, with the kinship between A and B.
The chief novelty in this chapter is an attempt to classify nearness of relationship upon a centesimal scale, in which the number of correspondences due to mere chance counts as 0°, and complete identity as 100°. It seems reasonable to adopt the scale with only slight reservation, when the average numbers of the Arches, Loops, and Whorls are respectively the same in the two kinds of digit which are compared together; but when they differ greatly, there are no means free from objection, of determining the 100°division of the scale; so the results, if noted at all, are subject to grave doubt.
Applying this scale, it appears that digits on opposite hands, which bear the same name, are more nearly related together than digits bearing different names, in about the proportion of three to two. It seems also, that of all the digits, none are so nearly related as the middle finger to the two adjacent ones.
In the ninth chapter, various methods of indexing are discussed and proposed, by which a set of finger prints may be so described by a few letters, that it can be easily searched for and found in any large collection, just as the name of a person is found in a directory. The procedure adopted, is to apply the Arch-Loop-Whorl classification to all ten digits, describing each digit in the order in which it is taken, by the lettera,l, orw, as the case may be, and arranging the results in alphabetical sequence. The downward direction of the slopes of loops on the fore-fingers is also taken into account, whether it be towards the Inner or the Outer side, thus replacing L on the fore-finger by eitherioro.
Many alternative methods are examined, including both the recognition and the non-recognition of all sloped patterns. Also the gain in differentiation, when all the ten digits are catalogued, instead of only a few of them. There is so much correlation between the different fingers, and so much peculiarity in each, that theoretical notions of the value of different methods of classification are of little worth; it is only by actual trial that the best can be determined.Whatever plan of index be adopted, many patterns must fall under some few headings and few or no patterns under others, the former class resembling in that respect the Smiths, Browns, and other common names that occur in directories. The general value of the index much depends on the facility with which these frequent forms can be broken up by sub-classification, the rarer forms being easily dealt with. This branch of the subject has, however, been but lightly touched, under the belief that experience with larger collections than my own, was necessary before it could be treated thoroughly; means are, however, indicated for breaking up the large battalions, which have answered well thus far, and seem to admit of considerable extension. Thus, the number of ridges in a loop (which is by far the commonest pattern) on any particular finger, at the part of the impression where the ridges are cut by the axis of the loop, is a fairly definite and effective datum as well as a simple one; so also is the character of its inmost lineation, or core.
In the tenth chapter we come to a practical result of the inquiry, namely, its possible use as a means of differentiating a man from his fellows. In civil as well as in criminal cases, the need of some such system is shown to be greatly felt in many of our dependencies; where the features of natives are distinguished with difficulty; where there is but little variety of surnames; where there are strong motives for prevarication, especially connected with land-tenure and pensions, and a proverbial prevalence of unveracity.
It is also shown that the value to honest men ofsure means of identifying themselves is not so small among civilised nations even in peace time, as to be disregarded, certainly not in times of war and of strict passports. But the value to honest men is always great of being able to identify offenders, whether they be merely deserters or formerly convicted criminals, and the method of finger prints is shown to be applicable to that purpose. For aid in searching the registers of a criminal intelligence bureau, its proper rank is probably a secondary one; the primary being some form of the already established Bertillon anthropometric method. Whatever power the latter gives of successfully searching registers, that power would be multiplied many hundredfold by the inclusion of finger prints, because their peculiarities are entirely unconnected with other personal characteristics, as we shall see further on. A brief account is given in this chapter of the Bertillon system, and an attempt is made on a small scale to verify its performance, by analysing five hundred sets of measures made at my own laboratory. These, combined with the quoted experiences in attempting to identify deserters in the United States, allow a high value to this method, though not so high as has been claimed for it, and show the importance of supplementary means. But whenever two suspected duplicates of measurements, bodily marks, photographs and finger prints have to be compared, the lineations of the finger prints would give an incomparably more trustworthy answer to the question, whether or no the suspicion of their referring to the same person was justified, than allthe rest put together. Besides this, while measurements and photographs are serviceable only for adults, and even then under restrictions, the finger prints are available throughout life. It seems difficult to believe, now that their variety and persistence have been proved, the means of classifying them worked out, and the method of rapidly obtaining clear finger prints largely practised at my laboratory and elsewhere, that our criminal administration can long neglect the use of such a powerful auxiliary. It requires no higher skill and judgment to make, register, and hunt out finger prints, than is to be found in abundance among ordinary clerks. Of course some practice is required before facility can be gained in reading and recognising them, but not a few persons of whom I have knowledge, have interested themselves in doing so, and found no difficulty.
The eleventh chapter treats of Heredity, and affirmatively answers the question whether patterns are transmissible by descent. The inquiry proved more troublesome than was expected, on account of the great variety in patterns and the consequent rarity with which the same pattern, other than the common Loop, can be expected to appear in relatives. The available data having been attacked both by the Arch-Loop-Whorl method, and by a much more elaborate system of classification—described and figured as the C system, the resemblances between children of either sex, of the same parents (or more briefly “fraternal” resemblances, as they are here called, for wantof a better term), have been tabulated and discussed. A batch of twins have also been analysed. Then cases have been treated in which both parents had the same pattern on corresponding fingers; this pattern was compared with the pattern on the corresponding finger of the child. In these and other ways, results were obtained, all testifying to the conspicuous effect of heredity, and giving results that can be measured on the centesimal scale already described. But though the qualitative results are clear, the quantitative are as yet not well defined, and that part of the inquiry must lie over until a future time, when I shall have more data and when certain foreseen improvements in the method of work may perhaps be carried out. There is a decided appearance, first observed by Mr. F. Howard Collins, of whom I shall again have to speak, of the influence of the mother being stronger than that of the father, in transmitting these patterns.
In the twelfth chapter we come to a branch of the subject of which I had great expectations, that have been falsified, namely, their use in indicating Race and Temperament. I thought that any hereditary peculiarities would almost of necessity vary in different races, and that so fundamental and enduring a feature as the finger markings must in some way be correlated with temperament.
The races I have chiefly examined are English, most of whom were of the upper and middle classes; the others chiefly from London board schools; Welsh, from the purest Welsh-speaking districts of SouthWales; Jews from the large London schools, and Negroes from the territories of the Royal Niger Company. I have also a collection of Basque prints taken at Cambo, some twenty miles inland from Biarritz, which, although small, is large enough to warrant a provisional conclusion. As a first and only an approximately correct description, the English, Welsh, Jews, Negroes, and Basques, may all be spoken of as identical in the character of their finger prints; the same familiar patterns appearing in all of them with much the same degrees of frequency, the differences between groups of different races being not larger than those that occasionally occur between groups of the same race. The Jews have, however, a decidedly larger proportion of Whorled patterns than other races, and I should have been tempted to make an assertion about a peculiarity in the Negroes, had not one of their groups differed greatly from the rest. The task of examination has been laborious thus far, but it would be much more so to arrive with correctness at a second and closer approximation to the truth. It is doubtful at present whether it is worth while to pursue the subject, except in the case of the Hill tribes of India and a few other peculiarly diverse races, for the chance of discovering some characteristic and perhaps a more monkey-like pattern.
Considerable collections of prints of persons belonging to different classes have been analysed, such as students in science, and students in arts; farm labourers; men of much culture; and the lowestidiots in the London district (who are all sent to Darenth Asylum), but I do not, still as a first approximation, find any decided difference between their finger prints. The ridges of artists are certainly not more delicate and close than those of men of quite another stamp.
In Chapter XIII. the question is discussed and answered affirmatively, of the right of the nine fundamentally differing patterns to be considered as different genera; also of their more characteristic varieties to rank as different genera, or species, as the case may be. The chief test applied, respected the frequency with which the various Loops that occurred on the thumbs, were found to differ, in successive degrees of difference, from the central form of all of them; it was found to accord with the requirements of the well-known law of Frequency of Error, proving the existence of a central type, from which the departures were, in common phraseology, accidental. Now all the evidence in the last chapter concurs in showing that no sensible amount of correlation exists between any of the patterns on the one hand, and any of the bodily faculties or characteristics on the other. It would be absurd therefore to assert that in the struggle for existence, a person with, say, a loop on his right middle finger has a better chance of survival, or a better chance of early marriage, than one with an arch. Consequently genera and species are here seen to be formed without the slightest aid from either Natural or Sexual Selection, and these fingerpatterns are apparently the only peculiarity in which Panmixia, or the effect of promiscuous marriages, admits of being studied on a large scale. The result of Panmixia in finger markings, corroborates the arguments I have used inNatural Inheritanceand elsewhere, to show that “organic stability” is the primary factor by which the distinctions between genera are maintained; consequently, the progress of evolution is not a smooth and uniform progression, but one that proceeds by jerks, through successive “sports” (as they are called), some of them implying considerable organic changes, and each in its turn being favoured by Natural Selection.
The same word “variation” has been indiscriminately applied to two very different conceptions, which ought to be clearly distinguished; the one is that of the “sports” just alluded to, which are changes in the position of organic stability, and may, through the aid of Natural Selection, become fresh steps in the onward course of evolution; the other is that of the Variations proper, which are merely strained conditions of a stable form of organisation, and not in any way an overthrow of them. Sports do not blend freely together; variations proper do so. Natural Selection acts upon variations proper, just as it does upon sports, by preserving the best to become parents, and eliminating the worst, but its action upon mere variations can, as I conceive, be of no permanent value to evolution, because there is a constant tendency in the offspring to “regress” towards the parental type. The amount and resultsof this tendency have been fully established inNatural Inheritance. It is there shown, that after a certain departure from the central typical form has been reached in any race, a further departure becomes impossible without the aid of these sports. In the successive generations of such a population, the average tendency of filial regression towards the racial centre must at length counterbalance the effects of filial dispersion; consequently the best of the produce cannot advance beyond the level already attained by the parents, the rest falling short of it in various degrees.
In concluding these introductory remarks, I have to perform the grateful duty of acknowledging my indebtedness to Mr. F. Howard Collins, who materially helped me during the past year. He undertook the numerous and tedious tabulations upon which the chapters on Heredity, and on Races and Classes, are founded, and he thoroughly revised nearly the whole of my MS., to the great advantage of the reader of this book.
PREVIOUS USE OF FINGER PRINTS
The employment of impressions of the hand or fingers to serve as sign-manuals will probably be found in every nation of importance, but the significance attached to them differs. It ranges from a mere superstition that personal contact is important, up to the conviction of which this book will furnish assurance, that when they are properly made, they are incomparably the most sure and unchanging of all forms of signature. The existence of the superstitious basis is easily noted in children and the uneducated; it occupies a prominent place in the witchcrafts of barbarians. The modern witness who swears on the Bible, is made to hold it and afterwards to kiss it; he who signs a document, touches a seal or wafer, and declares that “this is my act and deed.” Students of the primitive customs of mankind find abundant instances of the belief, that personal contact communicates some mysterious essence from the thing touched to the person who touches it, andvice versa; but it is unnecessary here to enter further into these elementary human reasonings,which are fully described and discussed by various well-known writers.
The next grade of significance attached to an impression resembles that which commends itself to the mind of a hunter who is practised in tracking. He notices whether a footprint he happens to light upon, is larger or smaller, broader or narrower, or otherwise differs from the average, in any special peculiarity; he thence draws his inferences as to the individual who made it. So, when a chief presses his hand smeared with blood or grime, upon a clean surface, a mark is left in some degree characteristic of him. It may be that of a broad stumpy hand, or of a long thin one; it may be large or small; it may even show lines corresponding to the principal creases of the palm. Such hand prints have been made and repeated in many semi-civilised nations, and have even been impressed in vermilion on their State documents, as formerly by the sovereign of Japan. Though mere smudges, they serve in a slight degree to individualise the signer, while they are more or less clothed with the superstitious attributes of personal contact. So far as I can learn, no higher form of finger printing than this has ever existed, in regular and well-understood use, in any barbarous or semi-civilised nation. The ridges dealt with in this book could not be seen at all in such rude prints, much less could they be utilised as strictly distinctive features. It is possible that when impressions of the fingers have been made in wax, and used as seals to documents, they may sometimes have been subjectedto minute scrutiny; but no account has yet reached me of trials in any of their courts of law, about disputed signatures, in which the identity of the party who was said to have signed with his finger print, had been established or disproved by comparing it with a print made by him then and there. The reader need be troubled with only a few examples, taken out of a considerable collection of extracts from books and letters, in which prints, or rather daubs of the above kind, are mentioned.
A good instance of their small real value may be seen in theTrans. China Branch of the Royal Asiatic Society, Part 1, 1847, published at Hong-Kong, which contains a paper on “Land Tenure in China,” by T. Meadows Taylor, with a deed concerning a sale of land, in facsimile, and its translation: this ends, “The mother and the son, the sellers, have in the presence of all the parties, received the price of the land in full, amounting to sixty-four taels and five mace, in perfect dollars weighed in scales.Impression of the finger of the mother, of the maiden name of Chin.” The impression, as it appears in the woodcut, is roundish in outline, and was therefore made by the tip and not the bulb of the finger. Its surface is somewhat mottled, but there is no trace of any ridges.
The native clerks of Bengal give the name oftipsahito the mark impressed by illiterate persons who, refusing to make either aXor their caste-mark, dip their finger into the ink-pot and touch the document. The tipsahi is not supposed to individualise the signer, it is merely a personal ceremony performed in the presence of witnesses.
PLATE 1.
Fig. 1.
Chinese Coin, Tang Dynasty, about 618A.D.,with nail mark of the Empress Wen-teh, figured in relief.
Fig. 2.
Order on a Camp Sutler, by the officer of a surveying party in New Mexico. 1882.
Many impressions of fingers are found on ancient pottery, as on Roman tiles; indeed the Latin wordpalmatusis said to mean an impression in soft clay, such as a mark upon a wall, stamped by a blow with the palm. Nail-marks are used ornamentally by potters of various nations. They exist on Assyrian bricks as signatures; for instance, in the Assyrian room of the British Museum, on the west side of the case C 43, one of these bricks contains a notice of sale and is prefaced by words that were translated for me thus: “Nail-mark of Nabu-sum-usur, the seller of the field, (used) like his seal.” A somewhat amusing incident affected the design of the Chinese money during the great Tang dynasty, about 618A.D.A new and important issue of coinage was to be introduced, and the Secretary of the Censors himself moulded the design in wax, and humbly submitted it to the Empress Wen-teh for approval. She, through maladroitness, dug the end of her enormously long finger-nail into its face, marking it deeply as with a carpenter’s gouge. The poor Secretary of the Censors, Ngeu-yang-siun, who deserves honour from professional courtiers, suppressing such sentiments as he must have felt when his work was mauled, accepted the nail-mark of the Empress as an interesting supplement to the design; he changed it into a crescent in relief, and the new coins were stamped accordingly. (SeeCoins and Medals, edited by Stanley Lane Poole, 1885, p.221.) A drawing of one of these is given inPlate 1, Fig. 1.
The European practitioners of palmistry and cheiromancy do not seem to have paid particular attention to the ridges with which we are concerned. A correspondent of the American JournalScience, viii. 166, states, however, that the Chinese class the striæ at the ends of the fingers into “pots” when arranged in a coil, and into “hooks.” They are also regarded by the cheiromantists in Japan. A curious account has reached me of negroes in the United States who, laying great stress on the possession of finger prints in wax or dough for witchcraft purposes, are also said to examine their striæ.
Leaving Purkenje to be spoken of in a later chapter, because he deals chiefly with classification, the first well-known person who appears to have studied the lineations of the ridges as a means of identification, was Bewick, who made an impression of his own thumb on a block of wood and engraved it, as well as an impression of a finger. They were used as fanciful designs for his illustrated books. Occasional instances of careful study may also be noted, such as that of Mr. Fauld (Nature, xxii. p. 605, Oct. 28, 1880), who seems to have taken much pains, and that of Mr. Tabor, the eminent photographer of San Francisco, who, noticing the lineations of a print that he had accidentally made with his own inked finger upon a blotting-paper, experimented further, and finally proposed the method of finger prints for the registration of Chinese, whose identificationhas always been a difficulty, and was giving a great deal of trouble at that particular time; but his proposal dropped through. Again Mr. Gilbert Thompson, an American geologist, when on Government duty in 1882 in the wild parts of New Mexico, paid the members of his party by order of the camp sutler. To guard against forgery he signed his name across the impression made by his finger upon the order, after first pressing it on his office pad. He was good enough to send me the duplicate of one of these cheques made out in favour of a man who bore the ominous name of “Lying Bob” (Plate 1, Fig. 2). The impression took the place of the scroll work on an ordinary cheque; it was in violet aniline ink, and looked decidedly pretty. From time to time sporadic instances like these are met with, but none are comparable in importance to the regular and official employment made of finger prints by Sir William Herschel, during more than a quarter of a century in Bengal. I was exceedingly obliged to him for much valuable information when first commencing this study, and have been almost wholly indebted to his kindness for the materials used in this book for proving the persistence of the lineations throughout life.
Sir William Herschel has presented me with one of the two original “Contracts” in Bengali, dated 1858, which suggested to his mind the idea of using this method of identification. It was so difficult to obtain credence to the signatures of the natives, that he thought he would use the signature of the handitself, chiefly with the intention of frightening the man who made it from afterwards denying his formal act; however, the impression proved so good that Sir W. Herschel became convinced that the same method might be further utilised. He finally introduced the use of finger prints in several departments at Hooghly in 1877, after seventeen years’ experience of the value of the evidence they afforded. A too brief account of his work was given by him inNature, xxiii. p. 23 (Nov. 25, 1880). He mentions there that he had been taking finger marks as sign-manuals for more than twenty years, and had introduced them for practical purposes in several ways in India with marked benefit. They rendered attempts to repudiate signatures quite hopeless. Finger prints were taken of Pensioners to prevent their personation by others after their death; they were used in the office for Registration of Deeds, and at a gaol where each prisoner had to sign with his finger. By comparing the prints of persons then living, with their prints taken twenty years previously, he considered he had proved that the lapse of at least that period made no change sufficient to affect the utility of the plan. He informs me that he submitted, in 1877, a report in semi-official form to the Inspector-General of Gaols, asking to be allowed to extend the process; but no result followed. In 1881, at the request of the Governor of the gaol at Greenwich (Sydney), he sent a description of the method, but no further steps appear to have been taken there.
If the use of finger prints ever becomes of generalimportance, Sir William Herschel must be regarded as the first who devised a feasible method for regular use, and afterwards officially adopted it. His method of printing for those purposes will be found in the next chapter.
METHODS OF PRINTING
It will be the aim of this chapter to show how to make really good and permanent impressions of the fingers. It is very easy to do so when the principles of the art are understood and practised, but difficult otherwise.
One example of the ease of making good, but not permanent impressions, is found, and should be tried, by pressing the bulb of a finger against well-polished glass, or against the highly-polished blade of a razor. The finger must bevery slightlyoiled, as by passing it through the hair; if it be moist, dry it with a handkerchief before the oiling. Then press the bulb of the finger on the glass or razor, as the case may be, and a beautiful impression will be left. The hardness of the glass or steel prevents its surface from rising into the furrows under the pressure of the ridges, while the layer of oil which covers the bottom of the furrows is too thin to reach down to the glass or steel; consequently the ridges alone are printed. There is no capillary or other action to spread the oil, so the impressionremains distinct. A merely moist and not oily finger leaves a similar mark, but it soon evaporates.
This simple method is often convenient for quickly noting the character of a finger pattern. The impression may be made on a window-pane, a watch-glass, or even an eye-glass, if nothing better is at hand. The impression is not seen to its fullest advantage except by means of a single small source of bright light. The glass or steel has to be so inclined as justnotto reflect the light into the eye. That part of the light which falls on the oily impression is not so sharply reflected from it as from the surface of the glass or steel. Consequently some stray beams of the light which is scattered from the oil, reach the eye, while all of the light reflected from the highly-polished glass or steel passes in another direction and is unseen. The result is a brilliantly luminous impression on a dark background. The impression ceases to be visible when the glass or steel is not well polished, and itself scatters the light, like the oil.
There are two diametrically opposed methods of printing, each being the complement of the other. The method used in ordinary printing, is to ink the projecting surfaces only, leaving the depressed parts clean. The other method, used in printing from engraved plates, is to ink the whole surface, and then to clean the ink from the projecting parts, leaving the depressions only filled with it. Either of these two courses can be adopted in taking finger prints, but not the two together, for when they arecombined in equal degrees the result must be a plain black blot.
The following explanations will be almost entirely confined to the first method, namely, that of ordinary printing, as the second method has so far not given equally good results.
The ink used may be either printer’s ink or water colour, but for producing the best work, rapidly and on a large scale, the method of printer’s ink seems in every respect preferable. However, water colour suffices for some purposes, and as there is so much convenience in a pad, drenched with dye, such as is commonly used for hand stamps, and which is always ready for use, many may prefer it. The processes with printer’s ink will be described first.
The relief formed by the ridges is low. In the fingers of very young children, and of some ladies whose hands are rarely submitted to rough usage, the ridges are exceptionally faint; their crests hardly rise above the furrows, yet it is the crests only that are to be inked. Consequently the layer of ink on the slab or pad on which the finger is pressed for the purpose of blackening it, must bevery thin. Its thickness must be less than half the elevation of the ridges, for when the finger is pressed down, the crests displace the ink immediately below them, and drives it upwards into the furrows which would otherwise be choked with it.
It is no violent misuse of metaphor to compare the ridges to the crests of mountain ranges, and the depth of the blackening that they ought to receive,to that of the newly-fallen snow upon the mountaintops in the early autumn, when it powders them from above downwards to a sharply-defined level. The most desirable blackening of the fingers corresponds to a snowfall which covers all the higher passes, but descends no lower.
With a finger so inked it is scarcely possible to fail in making a good imprint; the heaviest pressure cannot spoil it. The first desideratum is, then, to cover the slab by means of which the finger is to be blackened, with an extremely thin layer of ink.
This cannot be accomplished with printer’s ink unless the slab is very clean, the ink somewhat fluid, and the roller that is used to spread it, in good condition. When a plate of glass is used for the slab, it is easy, by holding the inked slab between the eye and the light, to judge of the correct amount of inking. It should appear by no means black, but of a somewhat light brown.
The thickness of ink transferred by the finger to the paper is much less than that which lay upon the slab. The ink adheres to the slab as well as to the finger; when they are separated, only a portion of the ink is removed by the finger. Again, when the inked finger is pressed on the paper, only a portion of the ink that was on the finger is transferred to the paper. Owing to this double reduction, it seldom happens that a clear impression is at the same time black. An ideally perfect material for blackening would lie loosely on the slab like dust, it would cling very lightly to the finger, but adhere firmly to the paper.
The last preliminary to be noticed is the slowness with which the printer’s ink hardens on the slab, and the rapidity with which it dries on paper. While serviceable for hours in the former case, in the latter it will be dry in a very few seconds. The drying or hardening of this oily ink has nothing whatever to do with the loss of moisture in the ordinary sense of the word, that is to say, of the loss of the contained water: it is wholly due to oxidisation of the oil. An extremely thin oxidised film soon forms on the surface of the layer on the slab, and this shields the lower-lying portions of the layer from the air, and retards further oxidisation. But paper is very unlike a polished slab; it is a fine felt, full of minute interstices. When a printed period (.) is placed under the microscope it looks like a drop of tar in the middle of a clean bird’s-nest. The ink is minutely divided among the interstices of the paper, and a large surface being thereby exposed to the air, it oxidises at once, while a print from the finger upon glass will not dry for two or three days. One effect of oxidisation is to give a granulated appearance to the ink on rollers which have been allowed to get dirty. This granulation leaves clots on the slab which are fatal to good work: whenever they are seen, the roller must be cleaned at once.
The best ink for finger printing is not the best for ordinary printing. It is important to a commercial printer that his ink should dry rapidly on the paper, and he does not want a particularly thin layer of it; consequently, he prefers ink that contains variousdrying materials, such as litharge, which easily part with their oxygen. In finger prints this rapid drying is unnecessary, and the drying materials do harm by making the ink too stiff. The most serviceable ink for our purpose is made of any pure “drying” oil (or oil that oxidises rapidly), mixed with lampblack and very little else. I get mine in small collapsible tubes, each holding about a quarter of an ounce, from Messrs. Reeve & Sons, 113 Cheapside, London, W.C. Some thousands of fingers may be printed from the contents of one of these little tubes.
Let us now pass on to descriptions of printing apparatus. First, of that in regular use at my anthropometric laboratory at South Kensington, which has acted perfectly for three years; then of a similar but small apparatus convenient to carry about or send abroad, and of temporary arrangements in case any part of it may fail. Then lithographic printing will be noticed. In all these cases some kind of printer’s ink has to be used. Next, smoke prints will be described, which at times are very serviceable; after this the methods of water colours and aniline dyes; then casts of various kinds; last of all, enlargements.
Laboratory apparatus.—Mine consists of: 1, slab; 2, roller; 3, bottle of benzole (paraffin, turpentine, or solution of washing soda); 4, a funnel, with blotting-paper to act as a filter; 5, printer’s ink; 6, rags and duster; 7, a small glass dish; 8, cards to print on.
TheSlabis a sheet of polished copper, 10½ inches by 7, and about1⁄16inch thick, mounted on a solid board ¾ inch thick, with projecting ears for ease ofhandling. The whole weighs 2½ lbs. Each day it is cleaned with the benzole and left bright. [A slab of more than double the length and less than half the width might, as my assistant thinks, answer better.]
TheRolleris an ordinary small-sized printer’s roller, 6 inches long and 3 in diameter, obtained from Messrs. Harrild, 25 Farringdon Street, London. Mine remained in good condition for quite a year and a half. When it is worn the maker exchanges it for a new one at a trifling cost. A good roller is of the highest importance; it affords the only means of spreading ink evenly and thinly, and with quickness and precision, over a large surface. The ingenuity of printers during more than four centuries in all civilised nations, has been directed to invent the most suitable composition for rollers, with the result that particular mixtures of glue, treacle, etc., are now in general use, the proportions between the ingredients differing according to the temperature at which the roller is intended to be used. The roller, like the slab, is cleansed with benzole every day (a very rapid process) and then put out of the reach of dust. Its clean surface is smooth and shining.
TheBenzoleis kept in a pint bottle. Sometimes paraffin or turpentine has been used instead; washing soda does not smell, but it dissolves the ink more slowly. They are otherwise nearly equally effective in cleansing the rollers and fingers. When dirty, the benzole can be rudely filtered and used again.
TheFunnelholds blotting-paper for filtering the benzole. Where much printing is going on, andconsequent washing of hands, it is worth while to use a filter, as it saves a little daily expense, though benzole is very cheap, and a few drops of it will clean a large surface.
TheInkhas already been spoken of. The more fluid it is the better, so long as it does not “run.” A thick ink cannot be so thinned by adding turpentine, etc., as to make it equal to ink that was originally fluid. The variety of oils used in making ink, and of the added materials, is endless. For our purpose, any oil that dries and does not spread, such as boiled or burnt linseed oil, mixed with lampblack, is almost all that is wanted. The burnt oil is the thicker of the two, and dries the faster. Unfortunately the two terms, burnt and boiled linseed oil, have no definite meaning in the trade, boiling or burning not being the simple processes these words express, but including an admixture of drying materials, which differ with each manufacturer; moreover, there are two, if not three, fundamentally distinct qualities of linseed, in respect to the oil extracted from it. The ink used in the laboratory and described above, answers all requirements. Many other inks have suited less well; less even than that which can be made, in a very homely way, with a little soot off a plate that had been smoked over a candle, mixed with such boiled linseed oil as can be bought at unpretentious oil and colour shops, its only fault being a tendency to run.
Rags, and a comparatively clean duster, are wanted for cleaning the slab and roller, without scratching them.
The smallGlass Dishholds the benzole, into which the inked fingers are dipped before wiping them with the duster. Soap and water complete the preliminary cleansing.
Cards, lying flat, and being more easily manipulated than paper, are now used at the laboratory for receiving the impressions. They are of rather large size, 11½ × 5 inches, to enable the prints of the ten digits to be taken on the same card in two rather different ways (seePlate 2, Fig. 3), and to afford space for writing notes. The cards must have a smooth and yet slightly absorbent surface. If too highly glazed they cease to absorb, and more ink will remain on the fingers and less be transferred from them to the paper. A little trial soon determines the best specimen from among a few likely alternatives. “Correspondence cards” are suitable for taking prints of not more than three fingers, and are occasionally employed in the laboratory. Paper books and pads were tried, but their surfaces are inferior to cards in flatness, and their use is now abandoned.
The cards should beverywhite, because, if a photographic enlargement should at any time be desired, a slight tint on the card will be an impediment to making a photograph that shall be as sharp in its lines as an engraving, it being recollected that the cleanest prints are brown, and therefore not many shades darker than the tints of ordinary cards.
The method of printing at the laboratory is to squeeze a drop or so of ink on to the slab, and to work it thoroughly with the roller until a thin and evenlayer is spread, just as is done by printers, from one of whom a beginner might well purchase a lesson. The thickness of the layer of ink is tested from time to time by taking a print of a finger, and comparing its clearness and blackness with that of a standard print, hung up for the purpose close at hand. If too much ink has been put on the slab, some of it must be cleaned off, and the slab rolled afresh with what remains on it and on the roller. But this fault should seldom be committed; little ink should be put on at first, and more added little by little, until the required result is attained.
The right hand of the subject, which should be quite passive, is taken by the operator, and the bulbs of his four fingers laid flat on the inked slab and pressed gently but firmly on it by the flattened hand of the operator. Then the inked fingers are laid flat upon the upper part of the right-hand side of the card (Plate 2, Fig. 3), and pressed down gently and firmly, just as before, by the flattened hand of the operator. This completes the process for one set of prints of the four fingers of the right hand. Then the bulb of the thumb is slightlyrolledon the inked slab, and again on the lower part of the card, which gives a more extended but not quite so sharp an impression. Each of the four fingers of the same hand, in succession, is similarly rolled and impressed. This completes the process for the second set of prints of the digits of the right hand. Then the left hand is treated in the same way.
The result is indicated by the diagram, whichshows on what parts of the card the impressions fall. Thus each of the four fingers is impressed twice, once above with a simple dab, and once below with a rolled impression, but each thumb is only impressed once; the thumbs being more troublesome to print from than fingers. Besides, the cards would have to be made even larger than they are, if two impressions of each thumb had to be included. It takes from two and a half to three minutes to obtain the eighteen impressions that are made on each card.
Thepocket apparatusis similar to one originally made and used by Sir William J. Herschel (seePlate 3, Fig. 4, in which the roller and its bearings are drawn of the same size as those I use). A small cylinder of hard wood, or of brass tube, say 1¾ inch long, and ½ or ¾ inch in diameter, has a pin firmly driven into each end to serve as an axle. A piece of tightly-fitting india-rubber tubing is drawn over the cylinder. The cylinder, thus coated with a soft smooth compressible material, turns on its axle in two brackets, each secured by screws, as shown inPlate 2, Fig. 4, to a board (say 6 × 2½ × ¼ inch) that serves as handle. This makes a very fair and durable roller; it can be used in the heat and damp of the tropics, and is none the worse for a wetting, but it is by no means so good for delicate work as a cylinder covered with roller composition. These are not at all difficult to make; I have cast them for myself. The mould is a piece of brass tube, polished inside. A thick disc, with a central hole for the lower pin of the cylinder, fits smoothly into the lower end of the mould, and aring with a thin bar across it, fits over the other end, the upper pin of the cylinder entering a hole in the middle of the bar; thus the cylinder is firmly held in the right position. After slightly oiling the inside of the mould, warming it, inserting the disc and cylinder, and fitting on the ring, the melted composition is poured in on either side of the bar. As it contracts on cooling, rather more must be poured in than at first appears necessary. Finally the roller is pushed out of the mould by a wooden ramrod, applied to the bottom of the disc. The composition must be melted like glue, in a vessel surrounded by hot water, which should never be allowed to boil; otherwise it will be spoilt. Harrild’s best composition is more than twice the cost of that ordinarily used, and is expensive for large rollers, but for these miniature ones the cost is unimportant. The mould with which my first roller was made, was an old pewter squirt with the nozzle cut off; its piston served the double purpose of disc and ramrod.
TheSlabis a piece of thick plate glass, of the same length and width as the handle to the roller, so they pack up easily together; its edges are ground to save the fingers and roller alike from being cut. (Porcelain takes the ink better than glass, but is not to be commonly found in the shops, of a convenient shape and size; a glazed tile makes a capital slab.) A collapsible tube of printer’s ink, a few rags, and a phial of washing soda, complete the equipment (benzole may spoil india-rubber). When using the apparatus, spread a newspaper on the table to preventaccident, have other pieces of newspaper ready to clean the roller, and to remove any surplus of ink from it by the simple process of rolling it on the paper. Take care that the washing soda is in such a position that it cannot be upset and ruin the polish of the table. With these precautions, the apparatus may be used with cleanliness even in a drawing-room. The roller is of course laid on its back when not in use.
My assistant has taken good prints of the three first fingers of the right hands of more than 300 school children, say 1000 fingers, in a few hours during the same day, by this apparatus. Hawksley, 357 Oxford Street, W., sells a neatly fitted-up box with all the necessary apparatus.
Rougher arrangements.—A small ball made by tying chamois leather round soft rags, may be used in the absence of a roller. The fingers are inked from the ball, over which the ink has been evenly distributed, by dabbing it many times against a slab or plate. This method gives good results, but is slow; it would be intolerably tedious to employ it on a large scale, on all ten digits of many persons.
It is often desirable to obtain finger prints from persons at a distance, who could not be expected to trouble themselves to acquire the art of printing for the purpose of making a single finger print. On these occasions I send folding-cases to them, each consisting of two pieces of thin copper sheeting, fastened side by side to a slip of pasteboard, by bending the edges of the copper over it. The pasteboard is half cut through at the back, along the space betweenthe copper sheets, so that it can be folded like a reply post-card, the copper sheets being thus brought face to face, but prevented from touching by the margin of an interposed card, out of which the middle has been cut away. The two pieces of copper being inked and folded up, may then be sent by post. On arrival the ink is fresh, and the folders can be used as ordinary inked slabs. (See also Smoke Printing, page 47.)
The fluidity of even a very thin layer of ink seems to be retained for an indefinite time if the air is excluded to prevent oxidisation. I made experiments, and found that if pieces of glass (photographic quarter plates) be inked, and placed face to face, separated only by narrow paper margins, and then wrapped up without other precaution, they will remain good for a year and a half.
A slight film of oxidisation on the surface of the ink is a merit, not a harm; it is cleaner to work with and gives a blacker print, because the ink clings less tenaciously to the finger, consequently more of it is transferred to the paper.
If a blackened plate becomes dry, and is re-inked without first being cleaned, the new ink will rob the old of some of its oxygen and it will become dry in a day or even less.
Lithography.—Prints may be made on “transfer-paper,” and thence transferred to stone. It is better not to impress the fingers directly upon the stone, as the print from the stone would be reversed as compared with the original impression, and mistakes arelikely to arise in consequence. The print is re-reversed, or put right, by impressing the fingers on transfer-paper. It might sometimes be desirable to obtain rapidly a large number of impressions of the finger prints of a suspected person. In this case lithography would be easier, quicker, and cheaper than photography.
Water Colours and Dyes.—The pads most commonly used with office stamps are made of variously prepared gelatine, covered with fine silk to protect the surface, and saturated with an aniline dye. If the surface be touched, the finger is inked, and if the circumstances are all favourable, a good print may be made, but there is much liability to blot. The pad remains ready for use during many days without any attention, fresh ink being added at long intervals. The advantage of a dye over an ordinary water colour is, that it percolates the silk without any of its colour being kept back; while a solution of lampblack or Indian ink, consisting of particles of soot suspended in water, leaves all its black particles behind when it is carefully filtered; only clear water then passes through.
A serviceable pad may be made out of a few thicknesses of cloth or felt with fine silk or cambric stretched over it. The ink should be of a slowly drying sort, made, possibly, of ordinary ink, with the admixture of brown sugar, honey, glycerine or the like, to bring it to a proper consistence.
Mr. Gilbert Thompson’s results by this process have already been mentioned. A similar process was employed for the Bengal finger prints by Sir W.Herschel, who sent me the following account: “As to the printing of the fingers themselves, no doubt practice makes perfect. But I took no pains with my native officials, some dozen or so of whom learnt to do it quite well enough for all practical purposes from Bengali written instructions, and using nothing but a kind of lampblack ink made by the native orderly for use with the office seal.” A batch of these impressions, which he was so good as to send me, are all clear, and in most cases very good indeed. It would be easier to employ this method in a very damp climate than in England, where a very thin layer of lampblack is apt to dry too quickly on the fingers.
Printing as from Engraved Plates.—Professor Ray Lankester kindly sent me his method of taking prints with water colours. “You take a watery brushful or two of the paint and rub it over the hands, rubbing one hand against the other until they feel sticky. Athinpaper (tissue is best) placed on an oval cushion the shape of the hand, should be ready, and the hand pressed not too firmly on to it. I enclose a rough sample, done without a cushion. You require a cushion for the hollow of the hand, and the paint must be rubbed by the two hands until they feel sticky, not watery.” This is the process of printing from engravings, the ink being removed from the ridges, and lying in the furrows. Blood can be used in the same way.
The following is extracted from an article by Dr. Louis Robinson in theNineteenth Century, May 1892, p. 303:—
“I found that direct prints of the infant’s feet on paper would answer much better [than photography]. After trying various methods I found that the best results could be got by covering the foot by means of a soft stencil brush with a composition of lampblack, soap, syrup, and blue-black ink; wiping it gently from heel to toe with a smoothly-folded silk handkerchief to remove the superfluous pigment, and then applying a moderately flexible paper, supported on a soft pad, direct to the foot.”
A curious method with paper and ordinary writing ink, lately contrived by Dr. Forgeot, is analogous to lithography. He has described in one of the many interesting pamphlets published by the “Laboratoire d’Anthropologie Criminelle” of Lyon (Stenheil, 2 Rue Casimir-Delavigne, Paris), his new process of rendering visible the previously invisible details of such faint finger prints as thieves may have left on anything they have handled, the object being to show how evidence may sometimes be obtained for their identification. It is well known that pressure of the hand on the polished surface of glass or metal leaves a latent image very difficult to destroy, and which may be rendered visible by suitable applications, but few probably have suspected that this may be the case, to a considerable degree, with ordinary paper. Dr. Forgeot has shown that if a slightly greasy hand, such for example as a hand that has just been passed through the hair, be pressed on clean paper, and if common ink be afterwards brushed lightly over the paper, it will refuse to lie thickly on the greasy parts, and that the result will be a very fair picture of the minute markings on the fingers. He has even usedthese productions as negatives, and printed good photographs from them. He has also sent me a photographic print made from a piece of glass which had been exposed to the vapour of hydrofluoric acid, after having been touched by a greasy hand. I have made many trials of his method with considerable success. It affords a way of obtaining serviceable impressions in the absence of better means. Dr. Forgeot’s pamphlet describes other methods of a generally similar kind, which he has found to be less good than the above.
Smoke Printing.—When other apparatus is not at hand, a method of obtaining very clear impressions is to smoke a plate over a lighted candle, to press the finger on the blackened surface, and then on an adhesive one. The following details must, however, be borne in mind: the plate must not be smoked too much, for the same reason that a slab must not be inked too much; and the adhesive surface must be only slightly damped, not wetted, or the impression will be blurred. A crockery plate is better than glass or metal, as the soot does not adhere to it so tightly, and it is less liable to crack. Professor Bowditch finds mica (which is sold at photographic stores in small sheets) to be the best material. Certainly the smoke comes wholly off the mica on to the parts of the finger that touch it, and a beautiful negative is left behind, which can be utilised in the camera better than glass that has been similarly treated; but it does not serve so well for a plate that is intended to be kept ready for use in a pocket-book, its softnessrendering it too liable to be scratched. I prefer to keep a slip of very thin copper sheeting in my pocket-book, with which, and with the gummed back of a postage stamp, or even the gummed fringe to a sheet of stamps, impressions can easily be taken. The thin copper quickly cools, and a wax match supplies enough smoke. The folders spoken of (p. 42) may be smoked instead of being inked, and are in some cases preferable to carry in the pocket or to send by post, being so easy to smoke afresh. Luggage labels that are thickly gummed at the back furnish a good adhesive surface. The fault of gummed paper lies in the difficulty of damping it without its curling up. The gummed paper sold by stationers is usually thinner than luggage labels, and still more difficult to keep flat. Paste rubbed in a very thin layer over a card makes a surface that holds soot firmly, and one that will not stick to other surfaces if accidentally moistened. Glue, isinglass, size, and mucilage, are all suitable. It was my fortune as a boy to receive rudimentary lessons in drawing from a humble and rather grotesque master. He confided to me the discovery, which he claimed as his own, that pencil drawings could be fixed by licking them; and as I write these words, the image of his broad swab-like tongue performing the operation, and of his proud eyes gleaming over the drawing he was operating on, come vividly to remembrance. This reminiscence led me to try whether licking a piece of paper would give it a sufficiently adhesive surface. It did so. Nay, it led me a step further, for I took two pieces of paper and licked both.The dry side of the one was held over the candle as an equivalent to a plate for collecting soot, being saved by the moisture at the back from igniting (it had to be licked two or three times during the process), and the impression was made on the other bit of paper. An ingenious person determined to succeed in obtaining the record of a finger impression, can hardly fail altogether under any ordinary circumstances.
Physiologists who are familiar with the revolving cylinder covered with highly-glazed paper, which is smoked, and then used for the purpose of recording the delicate movements of a tracer, will have noticed the beauty of the impression sometimes left by a finger that had accidentally touched it. They are also well versed in the art of varnishing such impressions to preserve them in a durable form.
A cake of blacklead (plumbago), such as is sold for blackening grates, when rubbed on paper leaves a powdery surface that readily blackens the fingers, and shows the ridges distinctly. A small part of the black comes off when the fingers are pressed on sticky paper, but I find it difficult to ensure good prints. The cakes are convenient to carry and cleanly to handle. Whitening, and still more, whitening mixed with size, may be used in the same way, but it gathers in the furrows, not on the ridges.
Castsgive undoubtedly the most exact representation of the ridges, but they are difficult and unsatisfactory to examine, puzzling the eye by showing too conspicuously the variation of their heights,whereas we only want to know their courses. Again, as casts must be of a uniform colour, the finer lines are indistinctly seen except in a particular light. Lastly, they are both cumbrous to preserve and easily broken.