A third experiment concerns our judgment of time intervals. A wheel, driven by clock work, carries a pointer which clicks against con tact strips arranged along the circumference. The instrument is shown in Fig. 5. It is pos sible to give two clicks and to let the observer arrest the dock after an interval which seems to him to be equal to the interval that separates the clicks; or to sound three clicks — two con tinuous intervals — and to let the observer judge of the equality or inequality of the times en closed by them; or, finally, to sound four clicks — two intervals with a pause between — and to see how the observer's judgment of equality or inequality of the times vanes with variation of this intervening pause. A newer and more elaborate instrument is shown in Fig. 6.
A fourth experiment, known as the "simple reaction experiment? is important for the psy chological analysis of attention and action. A simple stimulus (a flash of light or the sound of a ball falling on a wooden plate) is given to the observer, and he replies to it by making some simple movement, which has been deter mined beforehand. Instruments are so ar ranged that the giving of the stimulus starts an electric clock, and the responsive movement stops it: so that the time elapsing between stimulus and answer to stimulus can be exactly measured. It is clear that the "reactions ex periment reproduces for us, in an elementary form, the consciousness of voluntary action. We see or hear something and act upon it. Since the experiment can be repeated as often as necessary it is possible, by means of it, to subject the action consciousness to minute and refined introspective examination. We find, further, that the time of reaction varies accord ing as the attention is directed predominantly to the stimulus or to the reaction movement, according as the stimulus attended to is to be apprehended in its qualitative or intensive character, according as the observer is fresh or fatigued, etc. The apparatus employed are shown in Fig. 7. The of the ball upon the plate in F starts the electric clock H, whose unit is 1/1000 second. Pressure of the finger upon the key U stops the clock; this pressure constitutes the movement of reaction. C is a hammer, whose time of fall is constant; it serves to check the accuracy of the clock (chrono scope). The other instruments figured are sub sidiary. Fig. 8 shows an elaborate apparatus for giving visual stimuli in reaction work.
The reaction experiment may be adapted to more complex consciousness. Thus the ob server may be told that he will hear either a noise or a tone; he is not to move until he is sure of the stimulus e action). Or he may 'e told either a noise or a tone, and that to noise with the right and to tone wi hand (choice reaction). Or again, the st may be a spoken word, to which the ob reacts by speaking a second word which he associated to the stimulus. Instruments ha been devised which meet the requirements of these and of many other variations of the ex perimental procedure.
The four experiments thus far discussed are typical of the problems with which experimental psychology set out. In a well-equipped modern laboratory the apparatus fall into three main groups: demonstration instruments, with which experiments may be performed upon the lec turer's desk; class instruments, which are put into the hands of undergraduate students for training and drill-work; and research instru ments, the materials of investigation by grad uate students and by the instructing staff. As an illustration of the demonstration group we may cite Wundt's apparatus for negative after images of vision (Fig. 9). A large circle of colored glass, placed directly behind a ground glass plate and illuminated by a powerful lamp, is fixated by the members of the class for some 30 seconds. At the end of this time a screen is dropped between colored glass and lamp. The ground glass surface then appears, as a light gray; and upon this gray background the after image of the colored stimulus develops. As an illustration of the second group we may men tion an apparatus for the determination of the minimum visible of distance (Fig. 10). Two black points appear upon a white cardboard sur face. The one point is fixed; it is painted upon the back surface of a sheet of glass which cov ers the cardboard. The other, painted upon the cardboard background, can be moved to or from the fixed point by means of a micrometer screw (reading to 1/200 mm.) placed at the side of the frame. The problem is to determine the last separation of the points which (under given conditions of experimentation) can still be perceived by the observer. Finally, as an illustration of a recent research instrument, we may refer to the drop-tachistoscope for range of visual attention shown in Fig. 11 (see Fig. 4).