FINGER - PRINT PHOTOGRAPHY (Pr., Photographie d marque de doigt ; Ger., Fingerspur Photographie) The photography and systematic registration of finger-prints has become of immense import ance in the detection of crime. The methods employed may be divided into two classes: (1) those used when the individual whose finger prints are to be examined or registered is pre sent to give more or less voluntary assistance ; and (2) those in which the finger-prints have been involuntarily left behind by their originator. The procedure adopted at New Scotland Yard under the first circumstances is very simple. The best black printers' ink is spread in a thin, even film on flat tin-plate or copper by means of a roller. The finger to be recorded is pressed down lightly and steadily on the film of ink, taking care not to move it sideways at all. After a few seconds the finger is raised and pressed down on a smooth white card or paper, when a sharp, dear impression should result. This is known as a " plain " impression. Another kind is obtained by placing the bulb of the finger on the inked slab, facing to the left, and turning it gently over until it faces to the right. The finger is then pressed gently on the paper, roll ing it from left to right as before, thus making what is called a " rolled " impression. With an unwilling subject, trouble may arise from the finger being deliberately moved.
For experimental work, a good substitute for the above method is allow a drop of printing ink to fall on a smooth card or glass, and to spread it out with a finger. Finger-prints on white paper may be photographed on a photo-mechan ical plate, giving a short exposure to secure contrast and developing with hydroquinone. Prints from the negatives may be made on gas light paper. Another and quicker way is to have the inked finger pressed on a dean sheet of glass iustead of on paper. Several thicknesses of gummed paper in strips are then stuck on the glass at the sides, and it is placed in a printing frame. In the dark-room, a slow, ordinary plate, backed, is laid film side downwards on the glass, the gummed strips serving to protect the film from the still wet ink. Having inserted the back and fastened the frame, an exposure of about half a second is given at two feet distance from a fish-tail gas burner. This may be done by turning down the gas to an almost invisible blue point, holding up the frame, and then turning the gas up and off as rapidly as possible. On development with hydroquinone a good sharp negative should be secured. The frame must, of course, be kept still. The prints here shown were obtained from negatives made in this way ; A, B, C, and D are finger-prints, E and r being thumb-prints. A and B are from
the same finger at different times ; the lines will be found to tally, and the mark of a slight cut at x is plainly seen in both.
If, however, the finger-prints have to be searched for patiently and carefully, and are eventually found in awkward places, on un Another class of finger-prints which call for great care, since the record is so easily disturbed and lost, are those in dust. If on a dark sur face, these are readily photographed with a direct front lighting. When they occur on a window-pane or other colourless glass, a dark background should be placed at the back, a short distance away, and the light should come obliquely from behind. In rare cases semi invisible finger-prints may sometimes be ren dered conspicuous by chemical treatment, if there is any idea of the occupation of their suspected producer. Thus, for example, a tanner fresh from work might reasonably be expected to leave traces of tannin in his finger prints, which, under favourable conditions, could be rendered black by treatment with a solution of a ferric salt.
It is found that a solution of silver nitrate, of say 6 per cent. strength, will occasionally cause the appearance of an unsuspected finger favourable surfaces, and in all probability faint and nearly invisible, the work becomes much more difficult. When trouble arises owing to any unusual colour of the ground on which the finger-print occurs, an orthochromatic plate must be used in conjunction with a suitable colour filter, to secure contrast, each case being treated, of course, according to its particular requirements.
The majority of involuntary finger-prints are greasy ones. To photograph these, if on a light surface, such as china, enamelled furniture, etc., they should be dusted lightly with dry, finely powdered plumbago or graphite ; a fiat, broad camel-hair brush is charged with a little of the powder, and, holding this a short distance above the finger-print, the handle is tapped gently with the forefinger of the disengaged hand. Or, instead, the hand holding the brush may be nudged or shaken. The surplus powder is care fully blown away with a small bellows ; on no account should the breath be used. Greasy finger-prints on a dark surface—old oak furni ture or black ironwork, for instance—may be treated in the same manner, using, however, dry, finely powdered whitelead instead of the plumbago.
Finger-prints in blood on a dark surface should be placed where there is no extraneous light, as in a cellar or dark room, and lit from the front with magnesium ribbon or the electric arc, of course screening the direct rays from the lens of the camera.
print on ordinary paper, or will intensify a semi invisible one.