"Merton, having been rejected for active service on grounds of health, was commissioned in 1916 as a lieutenant in the Royal Naval Volunteer Reserve in the secret service, the first scientist to be so appointed. His success in identifying the secret ink carried by German spies in their clothing, and inventing a new means of secret writing, won a mention in dispatches."
The book also notes that, being personally wealthy, Merton served "without pay". The secret writing involved writing with a silver 'pen' on glass. The writing could then be 'developed' chemically to become visible. As for the discovery of the German secret ink, the agency managed to find a Belgian woman with a hatred of Germans, and convince her to screw her way into the confidences of a German spymaster. He ultimately used the woman to transport secret messages, via her underwear, into France. But she took her panties to Merton, and the spy ring was unmasked and destroyed. In related news, the service also discovered that semen was a good invisible ink, since it passed one of the common tests for invisible writing, though they "had to remove the discoverer from the office immediately as his colleagues were making life intolerable by accusations of masturbation." There were other problems: "'We thought', wrote Stagg, 'we had solved a great problem.' But 'our man in Copenhagen ... evidently stocked it in a bottle -- for his letters stank to high heaven, and we had to tell him that a fresh operation was necessary for each letter.'"
The other interesting story part of the history is the bureaucratic fight over control of the secret service. Founded in 1909 as a new separate agency, it had hardly got off the ground before WWI broke out. The War Office and the Admiralty immediately wanted to grab it back under their wings; somewhat ironic since part of the reason to have a separate secret service was to have a Chinese Wall of sorts between nasty spies (at this point, generally foreign nationals spilling info for money -- on the one hand not entirely trustworthy, and on the other, having a legitimate need for their own identities to remain secret) and the military chain of command.