Finger Prints

by Francis Galton

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 be very 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 various drying 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.