“I know the notion will seem to you jarring and even comic; but that’s because you are English. It sounds to you like saying the Archbishop of Canterbury’s daughter will be married in St George’s, Hanover Square, to a crossing-sweeper on ticket-of-leave. You don’t do justice to the climbing and aspiring power of our more remarkable citizens. You see a good-looking grey-haired man in evening-dress with a sort of authority about him, you know he is a pillar of the State, and you fancy he had a father. You are in error. You do not realize that a comparatively few years ago he may have been in a tenement or (quite likely) in a jail. You don’t allow for our national buoyancy and uplift. Many of our most influential citizens have not only risen recently, but risen comparatively late in life. Todd’s daughter was fully eighteen when her father first made his pile; so there isn’t really anything impossible in her having a hanger-on in low life; or even in her hanging on to him, as I think she must be doing, to judge by the lantern business. If so, the hand that held the lantern may not be unconnected with the hand that held the gun. This case, sir, will make a noise.”
“Well,” said the priest patiently, “and what did you do next?”
“I reckon you’ll be shocked,” replied Greywood Usher, “as I know you don’t cotton to the march of science in these matters. I am given a good deal of discretion here, and perhaps take a little more than I’m given; and I thought it was an excellent opportunity to test that Psychometric Machine I told you about. Now, in my opinion, that machine can’t lie.”
“No machine can lie,” said Father Brown; “nor can it tell the truth.”
“It did in this case, as I’ll show you,” went on Usher positively. “I sat the man in the ill-fitting clothes in a comfortable chair, and simply wrote words on a blackboard; and the machine simply recorded the variations of his pulse; and I simply observed his manner. The trick is to introduce some word connected with the supposed crime in a list of words connected with something quite different, yet a list in which it occurs quite naturally. Thus I wrote ‘heron’ and ‘eagle’ and ‘owl’, and when I wrote ‘falcon’ he was tremendously agitated; and when I began to make an ‘r’ at the end of the word, that machine just bounded. Who else in this republic has any reason to jump at the name of a newly-arrived Englishman like Falconroy except the man who’s shot him? Isn’t that better evidence than a lot of gabble from witnesses--if the evidence of a reliable machine?”