"Why should people, whose lives are blameless, think of racks and thumbscrews when they see a simple machine like this?" he asked suddenly, taking up David's apparently unnoted exclamation. Not waiting for an answer, he went on, as if with a lecture to which they had been invited to listen. "So far as I know this machine is the first of its kind to reach this country. It is an ingenious development of certain laws psychologists have been using for some time in their experiments, and is based on a theory that is, roughly, something like this: "A thought is a part of the body that gives it birth. Thinking is not confined to the brain. Like the assimilation of food, it involves man's entire physical nature. In cases of exaggerated thought or emotion--intense grief, fear, joy--the physical effects are obvious. The scientist, however, claims that the physical result from a mental cause is not confined to these extreme cases. A thought, the presence of which is not perceptible in gesture, facial expression, or the slightest visible emotion, is, nevertheless, communicated physically to every part of the body. Throw a stone into a pool of water. If the stone is large, the waves caused by it can be seen until they spend themselves on the shore; if it is small, the resulting ripples become invisible long before that. The point is, the ripple exists, whether we see it or not, just as does the wave, until it has run its course. "A thought, in its physical effect, is like the stone thrown into a pool. If it is a big, exaggerated thought, the agitation produced is outwardly visible. If it is small, more subtle, less sensational, its physical effects are invisible, although, theoretically, reaching in ripples to the extremities of the body. Hence, the psychologist's problem is: to detect and measure these invisible, intangible ripples of the mind. "This machine, my 'ghost-hunter,' solves the problem. A Russian scientist discovered that an electric current passing through the body is affected by any abnormal physical, or nervous, activity there encountered. Thought is a form of electric impulse and would, therefore, modify any other electric force crossing its path. Hence, Tarchanoff's law. Its practical application means, the literal measurement of our mental ripples. And this is done by the psychometer." "How?" asked David. "It's very simple. You hold these electrodes in your hands. An electric current is turned on and passes through you. While you are thus charged with electricity, I throw the stone, the thought, into your mind. The degree, or quality, of disturbance caused by this thought modifies the electric current, the varying agitation of which is made visible by the movements of an electric finger across this mirror. From there it is recorded on the sheet of paper in this cylinder." "What a horrible contrivance!" exclaimed Una. "I see how it works," mused David, "except for one thing. How do you introduce the thought you want to measure?"