Dactyloscopy: The Forensic Science of Fingerprint Identification
What are fingerprints?
Fingerprints are reproductions of the patterns formed by the papillary ridges located on the palm side of the fingers and thumbs.
The established facts show that the chance against one finger producing a print identical with that of another finger, whether on the same hand or on the hand of another person, is so astronomical in number that for all practical human purposes it is reasonable to conclude that such a chance will never materialise. It has been computed that, theoretically, two identical prints would be found only once during a period longer than that which astronomers estimate is needed for the sun to grow cold.
The History of Fingerprinting
Three great Englishmen — Sir William Herschel (1833 - 1917), Sir Francis Galton (1822 - 1911), and Sir Edward Henry (1859 - 1931) — were outstanding in their contributions to the science of fingerprint identification.
It was Herschel who proved that the groupings of the papillary ridges (they are formed in the first few months of foetal life) remain constant from birth to death. This he did by taking test prints at intervals, ranging over a long period, of his own fingers and those of other people. The result of these tests established the reliability of fingerprints as a means of human identification.
Galton did much pioneer research work, chiefly from data supplied by Herschel, but it was Henry who produced a workable system. In 1901 his system was officially adopted and the same year saw the inception of the Fingerprint Bureau at Scotland Yard.
Henry's system displaced Bertillon's anthropometric method of identification by means of bodily measurements, and its superiority soon became recognised.
The new system of the registration of habitual criminals was implemented by directions to the governors of prisons to take and forward to Scotland Yard the fingerprints of prisoners convicted and sentenced to one month's imprisonment or more. Later the scope of registration was extended to include persons sentenced to imprisonment at lower courts for lesser offences.
How fingerprints are taken
For official use fingerprints are recorded on a special form with spaces for impressions of all the digits. The spaces are numbered from one to ten. The prints taken in these spaces are called rolled impressions because they are obtained by rolling each finger from side to side on an inked plate and then repeating the same process with the inked finger during the taking of the impressions on the form.
The reason for the rolling action is to obtain the largest picture possible of the area of the skin ridges which lies between the edges of the nail and the flexure of the distal (first) joint of the finger, thus ensuring the inclusion of all data necessary for classification purposes. Below the rolled impressions are taken what are known as plain impressions. These are obtained by inking the fingers of each hand simultaneously and pressing them with the fingers held together in the spaces provided on the form. The thumbs held side by side are treated in a similar manner.
The purpose of taking the plain impressions is to ensure that the rolled impressions have been taken in the correct order. A misplaced impression could result in an incorrect classification formula. After all the impressions have been taken the prisoner signs the form and immediately after the signature a print of one of his fingers is taken as a check that the prints are his. New methods of taking prints are being sought, but, at present, the method described is universal.
Fingerprints are divided into four main groups of patterns: arches, loops, whorls, and compounds. There are variations of each pattern.
The arch1, as the name implies, the ridges are arranged in an archlike fashion; a variation of this type is the tented arch2.
The loop3 is the most common type of print. The point indicated by the arrow is called the delta. The ridges lying between the delta which cut a direct line to the core are counted. As the number varies in different prints it provides useful data for classification.
Whorls4 have a circular arrangement of the ridges. There are two deltas in all whorls; one only in loops. Whorls are sub-divided by tracing to the right the course of the lower limb of the left-hand delta and noting whether it passes inside the right-hand delta, meets it, or drops below it. The cores of some whorls are more or less elongated.
Compounds5 possess features of other patterns. There are two deltas and sometimes more. They are classified in the same manner as whorls. Extra deltas lying between the two outer deltas are ignored.
When two prints are of the same pattern it does not follow that they originated from the same finger. Identity or non-identity is determined by comparing the order in which the ridge characteristics appear in each print. Characteristics comprise such features as ending ridges, forking ridges, and ridges forming lakes and islands.
When the expert finds a number of these appearing in the same order in each print he knows that both prints were made by the same finger and that the remaining characteristics will coincide. Ridge characteristic data remain constant unless disturbed by a deep-seated injury that leaves a permanent scar. Superficial damage to the epidermis is more or less transient and subsequently leaves little or no trace of such injury.
The four main types of pattern form a basis for the primary classification of fingerprints. For this purpose the patterns are placed in two categories. Whorls and compounds are given a numerical value according to the number of the digit on which they occur. Arches and loops have no value numerically. By this arrangement 1024 primary groups are arrived at.
Further subdivisions are obtained by using the delta ridge tracing of whorls and compounds and by counting the ridge lines in loops. Many permutations result from the intercombination of the ridge tracing and ridge counting formulas. The presence of arches and radial loops provides additional data in some sub-groups.
Filing and searching
The place a set of prints will occupy in a collection depends firstly on the primary classification number. This may be any one of the 1024 groups referred to. Reference is then made to the secondary classification derived from the methods used for subdividing patterns. If a tertiary classification appears in the formula it must be consulted next, and finally the set of prints is arranged according to the number of ridges in loops, if they are present, on the little fingers and thumbs.
If a person has been previously convicted and providing he has given his correct name when he is again fingerprinted, the location of his prints in the collection is a matter of a few minutes. Should he give an alias the task is much more difficult.
Developments in computers and technology has certainly sped matters up.