Thus, a formidable group of weapons are available against the forger. To be effective, however, the significance of the facts they bring to light must be understood. Decisive proof that an object is not of the period or by the hand to which it is attributed comes only through the discovery of facts which are not only inconsistent with the attribution but cannot be explained except by assuming that the attribution is wrong. For instance, the body of a work may contain a substance unknown at its ostensible date. Modern nails inside a piece of wood sculpture said to be of the fourteenth century; cobalt, unknown as a pigment until the early nineteenth century, in a painting attributed to Velázquez; and titanium white, a twentieth-century invention, in a portrait labelled fifteenth-century Florentine—these are all good evidence that the object is not what it is held out to be. Moreover, the facts discovered always have to be controlled by reference to established standards. Structure revealed by microscopic examination must be compared with that of known substances; chemical and spectrographic analysis has to be checked by reference to a codified series of earlier tests; crackle on a surface can only be labelled as false if the nature of genuine crackle is known; and the reading of whatever is discovered by X-ray, ultra-violet, or infra-red rays calls for comparison with verified results of previous examinations. The facts yielded by one method of investigation may by themselves not be sufficient evidence of forgery; if, however, they can be joined with the results of other methods, all pointing in the same direction, a strong case can be built up. As in a court of law, this, rather than production of a single dramatic and decisive piece of evidence, is what usually happens. It might be thought that the combination of expert and scientist would leave the forger with his occupation gone. On the contrary, he continues to flourish. In the face of the expert, he discards the clumsy copy and the inept certificate, utilizing the improved methods of photography and reproduction and the increasing flood of learned works, to help save him from anachronisms and inherent contradictions in his work. The scientist he meets either by concentrating in fields where scientific methods of inquiry are relatively helpless, or by himself going to school with the scientist. The results of recent scientific work have put at his disposal much knowledge of what to do, what to avoid, and how to baffle certain types of investigation. Moreover, he has even taken over certain procedures worked out by scientists, such as those for hastening the effect of time, and has applied them to his own problems; cases are known of forgeries having been submitted, through innocent hands, for scientific investigation, to find out whether they will survive the ordeal, and if not, what are the mistakes to be avoided in the future. * * * * * * * *