The judgment by the naked eye as to the colors or shades of two inks in the same paper or document is very likely to be erroneous for the reason that when a lighter ink is more heavily massed than a darker one the effect on the eye is as if it were the darker.  Under a microscope or magnifying glass the field is more restricted, the finer lines are broadened, and one has larger areas of ink to compare with less surface of strongly contrasted white paper.  Then, again, an ink without noticeable bluish tinge to the naked eye may appear quite blue under the glass where the films of ink are broadened and thinned and their characters better observed.

In order to judge whether two marks have been made by the same ink, they should be viewed by reflected light to note the color, luster and thickness of the ink film.  Many inks blot or “run” on badly sized paper—­i.e., the lines are accompanied by a paler border which renders their edges less well defined.

Even on well-sized papers this class of inks usually exhibits only a stained line of no appreciable thickness where the fluid has touched the paper.

The copying and glossy inks, which often contain a considerable quantity of gum, do not “run” or blot even on partially sized paper, and show under the glass a convexity on the surface of the line and an appreciable thickness of the film.

It does not always follow when an ink has made a blur on one part of the paper and not on another that the paper has been tampered with.  A drop of water accidentally let fall on the blank page will frequently affect the sizing in that place, and, besides, all papers are not evenly sized in every part.

The inks rich in gum, or those concentrated by evaporation from standing in an open inkstand, give a more lustrous and thicker stroke.  Some inks penetrate deeper into the paper than others, and some produce chemical effects upon the sizing and even upon the paper itself, so that the characters can easily be recognized on the underside of the sheet.  In some old documents the ink has been known to so far destroy the fiber of the paper that a slight agitation of the sheet would shake out as dust much of the part which it covered, thus leaving an imperfect stencil plate of the original writing.

Distilled water is very useful in many cases to ascertain whether paper has been scratched and partially sized or treated with resin.  If it has not been altered by chemical agents, the partial sizing and the resinous matter used give to the paper a peculiar appearance.  Sizing takes away from the whiteness of the paper, and, thinned by the scratching or washing, it absorbs much more quickly even when it has been partially sized.

A simple mode of operation is to place a document or paper suspected of being a forgery, on a sheet of paper or better still, on a piece of glass; then moisten little by little with a paint brush all parts of it, paying close attention to the behavior of the liquid as it comes in contact with the paper.