If psychologists are asked, what the business of psychology is, they generally make some such answer as follows, if they belong to the empirical school: that this science has to investigate the facts of consciousness, its combinations and relations, so that it may ultimately discover the laws which govern these relations and combinations. Now although this definition seems quite perfect, it is really to some extent a vicious circle. For if we ask further, what is this consciousness which psychology investigates? the answer will be, "It consists of the sum total of facts of which we are conscious." In spite of this, our definition is the simplest, and therefore for the present it will be well for us to keep to it. All objects of experience have this peculiarity, namely, that we cannot really define them but only point to them, and if they are of a complex nature analyse them into their separate qualities. Such an analysis we call a description. We will therefore best be able to answer more accurately the question as to the nature of psychology by describing as exactly as possible all the separate qualities of that consciousness, the content of which psychological investigation has to deal with. For this purpose let us make use of a little instrument to help us--an instrument well known to all who have studied music, i.e. the metronome. It is really nothing more than a clockwork with an upright standing pendulum, on which a sliding weight is attached, so that beats may follow each other at equal intervals in greater or less rapidity. If the weight is fixed at the upper end of the pendulum, the beats follow each other at an interval of two seconds; if at the lower end, the interval is shortened to about a third of a second. Between these limits every different length of beat can be produced. We can, however, increase these limits considerably by taking off the sliding weight altogether. Now the lower limit falls to a quarter of a second. Similarly we can obtain any longer time we choose with a sufficient degree of accuracy, if we have some one to help us. Instead of letting the pendulum swing of its own accord, the assistant moves it backwards and forwards with his hand, measuring off the longer interval fixed upon, by means of a watch, that marks the seconds. This instrument is not only very useful for teaching singing and music, but it is also a psychological apparatus of the simplest kind. In psychology, as we shall see, we can use it for so many purposes that we are almost justified in saying that with its help we can demonstrate the most important part of the psychology of consciousness. In order to be able to do this the instrument must satisfy one requirement, which every instrument does not possess. The strength of the beats must be sufficiently uniform, so that even to the most attentive listener differences in the intensity of the successive beats may not be noticed. To test an instrument in this respect, we proceed thus.