For consider the course of the reaction experiment in the light of our typical formula of action! The observer sits down with the _intention of moving_ when he has perceived the stimulus; and he has an _idea of the result_ of his movement, namely, the performance of a reaction experiment. The stimulus is presented; he _perceives the object_ of movement; and slips his finger from the key. He thus _perceives the movement_ itself, and also, by the movement, _realises in perception his idea of result_. He has performed a complete impulsive action, but an action which, on the mental side, has been thinned out to a manageable degree of simplicity. The mental accompaniment is there; but the intention to move bears upon a single finger, the idea of result is just the idea of completing the experiment, the perception of object is the perception of a simple stimulus, the movement itself is a slight local displacement of a single member; nothing is left out, although the action is reduced to a skeleton. It has thus been made manageable; the mental accompaniments of the movement are not so complex that they baffle observation; and the technique of the experiment is an outline which can be filled in and further complicated in all manner of ways. We may hope that that Greenwich assistant found further employment; but we can hardly, as psychologists, regret that he timed his transits later than he should! § 56. =Sensory and Motor Reaction.=—Suppose that you are performing the simple reaction experiment, and that you tell your observers beforehand to react as soon as they perceive the stimulus. You soon find that this instruction is differently interpreted. One observer will prepare to react as soon as _he perceives the stimulus_; and another, _to react as soon as_ he perceives the stimulus. The difference of emphasis may be brought out by a homely illustration. When the lights are turned on in the evening, it is not uncommon, even in the best regulated families, for a clothes-moth to start up from some corner. You say ‘There’s a _moth_!’ and clap your hands to kill it. But it escapes; and henceforth you do not trouble to identify it; you clap your hands at anything mothlike that flits across the field of vision; you are set or disposed for the movement. So in the two forms of the simple reaction: some observers tend naturally to make sure of the stimulus, before they move, and others tend naturally to move, as soon as any stimulus has appeared.