In the case of momentary, and rapidly changing impressions, it is very brief; when, on the other hand, we concentrate on particular objects, it is longer. But, even when the attention is thus concentrated on objects of a constant character, a periodic interruption, due to the alternating relaxation and renewed concentration, always appears. This may be [p. 213] easily observed, even in the ordinary action of attention. But here, too, we gain more detailed information through experiments. If we allow a weak, continuous impression to act on a sense-organ and remove so far as possible all other stimuli, it will be observed when the attention is concentrated upon it that at certain, generally irregular, intervals the impression becomes for a short time indistinct, or even appears. to fade out entirely, only to appear again the next moment. This wavering begins, when the impressions are very weak, after 3-6"; when they are somewhat stronger, after 18-24".
These variations are readily distinguished from changes in the intensity of the impression itself, as may be easily demonstrated when, in the course of the experiment, the stimulus is purposely weakened or interrupted. There are essentially two characteristics that distinguish the subjective variations from those due to the changes in the stimulus. First, so long as the impression merely passes back and forth from the obscure field of consciousness to the inner fixation-point, there is always an idea of its continuance, just as there was in the experiments with momentary impressions an indefinite and obscure idea of the components which were not apperceived. Secondly, the oscillations of attention are attended by characteristic feelings and sensations which are entirely absent when the changes are objective. The characteristic feelings are those of expectation and activity, which regularly increase with the concentration of attention and decrease with its relaxation.
These will be discussed more fully later. The sensations come from the sense-organ affected, or at least emanate indirectly from it. They consist in sensations of tension in the tympanum, or in those of accommodation and convergence, etc. These two series of characteristics distinguish the concepts of the clearness and distinctness of psychical contents from that of the intensity of their sensational elements. A strong impression may be obscure and a weak one clear. The only causal relation between these two different concepts is to be found in the fact that in general the stronger impressions force themselves more upon the apperception. Whether or not they are really more clearly apperceived, depends on the other conditions present at the moment. The same is true of the advantages which those parts of a visual impression have that fall within the region of clearest vision.
As a rule, the fixated [p. 214] objects are also the ones apperceived.
But, in the experiments with momentary impressions described above, it can be shown that this interconnection may be broken up. This happens when we voluntarily concentrate our attention on a point in the eccentric regions of the field of vision. The object which is obscurely seen then becomes the one which is clearly ideated.
6b. In the same way that momentary spacial impressions are used to determine the scope of attention, we may use those which succeed one another in time, as a measure for the scope of consciousness. In this case we start with the assumption that a series of impressions can be united in a single unitary idea only when they are all together in consciousness, at least for one moment. If we listen to a series of hammer-strokes, it is obvious that while the present sound is apperceived, those immediately preceding it are still in the field of consciousness.
Their clearness diminishes, however, just in proportion to their distance in time from the apperceived impression, and those lying beyond a certain limit disappear from consciousness entirely. If we can determine this limit, we shall have a direct measure for the scope of consciousness under the special conditions given in the experiment. As a means for the determination of this limit we may use the ability to compare temporal ideas that follow one another immediately. So long as such an idea is present in consciousness as a single unitary whole, we can compare a succeeding idea with it and decide whether the two are alike or not. On the other hand, such a comparison is absolutely impossible when the preceding temporal series is not a unitary whole for consciousness, that is, when a part of its constituents have passed into unconsciousness before the end is reached. If, then, we present two series of strokes, such as can be produced, for example, by a metronome, one immediately after the other, marking of each series by a signal at its beginning, as, for example, with a bell stroke, we can judge directly from the impression, so long as they can be grasped as single units in consciousness, whether the, two series are alike or not. Of course, in such experiments counting of the strokes must be strictly avoided. judgments it may be noticed that the impression produced by the affective elements of the temporal before (p. 156). Every stroke in the second series is preceded [p. 215] by a feeling of expectation corresponding to the analogous stroke of the first series, so that every stroke too many or too few produces a feeling of disappointment attending the disturbance of the expectation.