Science Finds a Weapon to Fight the Forger

Evening Post, Volume CXV, Issue 11, 14 January 1933, Page 16

Science Finds a Weapon to Fight the Forger

, Science nas.nowput another powerful instrument into the hand of the expert to foil the forger. Not a new weapon, but an old one used in a new way. .It used to bo called "black light." Now it brings to light things which the forger would like to keep dark. Mr. Ellbridge Walker in the "Scientific American" tells about it:— "This instrument is the quartz-mer-cury vapour-arc, which radiates a high percentage of ultra-violet rays. These rays are not a recent discovery; neither is the mercury vapour arc an invention of yesterday. But new uses aro constantly being found for the valuable rays. "The ultra-violet rays are outside of the field of visibility on the violet end of the spectrum. The light generated is not all invisible ultra-violet, and it is necessary to filter out most of the visible light. In the examination of documents this filtering is done by means of a glass screen made on a base of nickel oxide. It is a very dark redviolet colour appearing black, and the early- experimenters called ultra-violet rays "black light." The secret of their power is the fact that although the rays themselves are invisible, they are capable of generating visible light when* they strike certain substances. Fortunately, a number vt these substances are used in documents. Moreover: "There are three «sain divisions in a fluorescence study of a document. First: certain kinds of materials can be distinguished from each other, although by ordinary observation in daylight they are identical. Second: certain things that are invisible even under the microscope are made visible by filtered ultra-violet light. ■ Third, and perhaps the most valuable, is the possibility of photographing the effect of the rays on document ; materials. The sensitised photographic plate records things that cannot be seen by the eye. ; "Fluorescence of a document depends upon the paper, ink, or any chemical or other materials in the object examined. It is sometimes a silvery glow, while other materials, which under ordinary light seem to be white, wilLap-

pear a surprising dark brown or bluish colour. Chemical erasures which show no trace in ordinary light often appear as a dark blot. "Papers made of different basic materials or sized with different compositions can be positively distinguished from each other by the fluorescence .test. Therefore, a document coEslsting of several sheets with a substituted page on different paper can thus be shown to be irregular. Paper made of pure cotton rags gives a clear white fluorescence, while that made of pure linen rags is a distinct bluish colour under the rays. Paper containing chemical wood-pulp appears as a dark grayish brown. Mechanical wood-pulp paper appears almost black. Vegetable or animal size and various coating materials have their own individual reactions. "Alterations in documents may sometimes be shown with startling clearness. Such a result is often of. the greatest importance. The document is not harmed or changed in any way, no matter how fragile or how valuable it maybe." Heretofore there has been no way to "restore bleached or erased writing made with anilin ink, but the residue of many of these inks fluoresees in a pronounced way, so that they can be photographed and made visible, although not a trace can be discovered on the altered document. Certain erased writings can easily be deciphered by examining the back of the paper under the rays. To quote again: "Sometimes the field in .which an erasure has been ma"de is shown as a distinctly disturbed area even when the details of the original writing cannot be deciphered. It can thus be shown that an alteration has been made, and compels an explanation by the persons who would profit by the alteration. "The quartz mercury vapour are is only an additional tool in the hands of the expert, and does not supplant other scientific instruments or processes. It does not replace knowledge, intelligence, or reasoning, and it is of value only when used by a skilled and experienced observer."