outset. To distinguish from one another the various causes of the delay of the signal, the conditions of experiment are variously modified. In one set of experiments, the observer does not know beforehand whether he is to experience a flash of light, or a sound, or some sensation of touch, nor how intense the sensation will be, nor when it will come; but he knows that he is to be on the lookout for one of the three kinds of sensation. He waits, with attention all aroused. In this case, it always takes him longer to signal than if he knew beforehand the kind and the strength of the coming sensation. Moreover, his attention now makes him uneasy; the coming sensation is expected, with signs of excitement, and is often received with a start. Here the feeling of effort that accompanies attention affects by its strength the character of the impression received. Moreover, in many of these experiments there appear phenomena that show that attention alters our perception of time, not merely as to length, but also as to sequence; so that under circumstances, an impression that really precedes another can appear in consciousness as succeeding it. Yet more: attention sometimes serves to combine two sets of simultaneous impressions, and to make them seem as if proceeding from one source.
So much for the influence of attention alone. But what is attention? We reply, evidently an active process. When impressions are modified by attention, they are actively modified. And if you ask about the nature of this active process, the reply is, attention, in its most elementary forms, is the same
