The expert may possibly be called upon to give evidence as to the existence of a "_trompe-l'oeil_;" as was the case in the trial of _M. de Preigne_, which took place at Montpelier in 1852. A "_trompe-l'oeil_" consists of two sheets of paper, glued together at the edges, but having the upper sheet shorter than the other which therefore extends below it. This species of fraud is executed by writing unimportant matter on the uppermost sheet, and then obtaining the desired signature, care being taken that it is written on the portion of the paper projecting below. The signature having been procured, it is only necessary to detach the two sheets in order to obtain a blank paper containing the signature, over which whatever is desired can be inserted. The trial referred to above, was in reference to a receipt for 3,000 francs. The expert, upon placing pieces of moistened paper upon the suspected document, noticed that they adhered to certain points, and that these formed a border around the paper but passing _above_ the signature. The fraudulency of the act was thus established, and so recognized by the court, although the accused was acquitted by the jury. Numerous means have been proposed, in order to render the falsification of documents a matter of difficulty. The most reliable of these is the use of "Grimpe's safety-paper," containing microscopic figures, the reproduction of which is impossible. Unfortunately, up to the present, the government has adopted methods less sure. EXAMINATION OF WRITINGS IN CASES WHERE A SYMPATHETIC INK HAS BEEN USED. Sympathetic inks are those which, although invisible at the time of writing, become apparent by the application of certain agents. They are of two classes: those which are rendered visible by the mere application of heat, such as chloride of cobalt, or the juice of onions; and those which are brought out only by the action of a reagent. The inks of the second class most frequently used are solutions of acetates of lead, and other metals which give a colored sulphide when treated with sulphuretted hydrogen. Characters written with a solution of ferrocyanide of potassium acquire a blue color, if washed with a solution of perchloride of iron. It is scarcely necessary to add that the latter solution can be used as the ink, and the ferrocyanide as the developer. When the presence of characters written with a sympathetic ink is suspected, the document is examined as follows: 1. The paper is at first warmed: if the ink used is of the first class, the characters will now become legible; otherwise the examination is continued as below.