The master spy had traced the plans on her back with sulphate of copper. This liquid leaves no mark on the skin under normal conditions, but when exposed to considerable heat it shows up dark blue. For further secrecy, it is stated, the plan of the fortress was concealed within another design in the manner described by General Sir Robert Baden-Powell. He carried with him an illustrated book on butterflies and from this he made what would appear to be specimens of butterflies seen in the surrounding country. Then when he had obtained the details of a fortress he drew them in among the complicated markings on the wings of the butterfly. There they would escape notice by any but the most expert "spy trappers." Miss Sari Petrass, the beautiful Hungarian dancer, who was for some time a great favorite in London, is reported to have been shot in Budapest as a spy. She is supposed to have been engaged in gathering military information in her native country for the benefit of England, where she made her greatest artistic success. When war began, the actress was starring in "The Marriage Market," a Hungarian operetta, at Daly's Theatre in London. She immediately returned to Budapest, but instead of continuing on the stage began a round of social activities. She wrote letters to the British army authorities, it is charged, which were sent by way of Switzerland in the in the care of young Austrian officers, who had been beguiled by her charms. It is said she was betrayed by one of her dupes in a fit of jealousy. Although an actress, she had a high social position and was a niece of the Countess Ilka Kinsky, one of the most prominent members of the Austro-Hungarian nobility. Miss Petrass, according to the report which reached her friends in Cleveland, Ohio, was put to death immediately her acts were discovered. When taken to the place of execution she fainted and was unconscious when shot. The announcement of her execution was the first news her family had of the charges against her. The method of concealing plans of fortifications on the skin of a spy, already referred to, has been employed with many variations. In time of war or when suspicion of spies is very keen, it is likely to be very useful. Then, again, women are usually called upon to carry this kind of information, because they are less subject to suspicion and watchfulness. Tattooing plans on a woman's skin has often been resorted to in past wars, but the anti-spy officers are now so keen that this way is no longer reliable. Various forms of writing on the skin, which only become visible under certain conditions, have, therefore, been tried. One form of this has already been mentioned. Plans and messages are also written in nitrate of silver, which becomes visible and black on exposure to sunlight. The writing is also done with phosphorus, so that it is only visible in the dark, but that lasts a few hours only.