As a matter of fact, it was through this very slip on his part that I had "spotted" van Rosen. We had suddenly lost sight of him a year or two before, and try as we would--for we knew that wherever he might be he would be dangerous--we could not locate him. The accident which led to his discovery was curious. I had been spending a few days in North London, and one morning stopped at the railway bookstall to buy a paper. As I approached the stall a tall, gentlemanly looking man, who had been chatting with the clerk, turned away and entered the train. Something about him struck me as strangely familiar. "Who is that gentleman?" I asked the man at the stall. "Oh," he replied, "that's Mr Huggon-Rose. He used to be a solicitor in the City, but he retired and has been living here for a year or two." But I was not quite satisfied; some odd memory, in which I felt sure Mr Huggon-Rose was concerned, haunted me all the way to town, and I could not shake it off. I had only seen van Rosen once, though I had had a good deal of experience, and it was not surprising that I failed to recognise him. When I got up to town I consulted the _Law List_, but could find no trace of Mr Huggon-Rose. Then I became more suspicious, and before many days were over I had succeeded in definitely identifying Mr Huggon-Rose with one of the Kaiser's cleverest spies. Thus the mystery of van Rosen's disappearance was solved by his own slip. If, when I looked up the _Law List_, I had found Mr Huggon-Rose's name and address duly set out, I should probably have thought I had been deceived by some chance resemblance and wasted no more time on the matter. How true it is that trifles make the sum of life! The position now was plainly serious: van Rosen's presence in London boded no good. The man was as unscrupulous as he was clever. The British contra-espionage department knew him well and had been greatly chagrined at losing sight of him. Afterwards, in consequence of my report to Hecq, he had been kept under close observation; but we had never been able to secure sufficient proof to justify his arrest, strong though our suspicions were. He had evidently been walking very warily since the outbreak of war. Unfortunately he had adopted that easiest of all cloaks of the German spy, and had become a naturalised Englishman just before war broke out. But the adage "Once a German always a German" applied with special force to van Rosen. After speaking with Madame Gabrielle I had a long chat with Hecq over the private wire, and together we mapped out a pretty comprehensive plan of campaign, in which both Aubert and Madame Gabrielle had very definite parts to play.