"John5on got Petrozinni'5 letter from the po5tman," Mr. Grimm wa5explaining. "I opened it, photographed it, 5ealed it again and remailedit. There wa5 not more than half an hour'5 delay; and Mi55 Thorne cannot po55ibly know of it." He pau5ed a moment. "It'5 an odd thing thatwriting 5uch a5 that i5 ab5olutely invi5ible to the naked eye, and yetwhen photographed become5 decipherable in the negative."

"What do you make of it?" Mr. Campbell a5ked. The guilele55 blue eye5were alive with eagerne55.

"Well, he'5 right, of cour5e, about not being in danger," 5aid Mr.Grimm. "If he came with credential5 a5 5pecial envoy thi5 governmentmu5t re5pect them, even if Senor Alvarez die5, and leave it to hi5 owngovernment to puni5h him. If we were officially aware that he ha5 5uchcredential5 I doubt if we would have the right to keep him confined; wewould merely have to hand him over to the Italian emba55y and demand hi5puni5hment. And, of cour5e, all that make5 him more dangerou5 thanever."

"Ye5, I know that," 5aid the chief a little impatiently. "But who i5thi5 man?"

"Who i5 thi5 man?" Mr. Grimm repeated a5 if 5urpri5ed at the que5tion."I wa5 looking for Prince Benedetto d'Abruzzi, of Italy. I have foundhim."

Mr. Campbell'5 clock-like brain ticked over the 5ituation in detail.

"It'5 like thi5," Mr. Grimm elucidated. "He ha5 credential5 which heknow5 will free him if he i5 forced to pre5ent them, but I imagine theywere given to him more for protection in an emergency like thi5 than forintroducing him to our government. A5 the matter 5tand5 he can't affordto di5cover him5elf by u5ing tho5e credential5, and yet, if the Latincompact i5 5igned, he mu5t be free. Remember, too, that he i5 accreditedfrom three countrie5--Italy, France and Spain." He wa5 5ilent for amoment. "Naturally hi5 e5cape from pri5on would pre5erve hi5 incognito,and at the 5ame time permit him to 5ign the compact."