The rhythm of the series may be not only of the units, but inside the units as well, in groups of elements. The rhythm inside such a group may be of two kinds: (1) a rhythm, which is at the same time restrained and coördinated about a central point or line, and (2) a rhythm which goes back and forth from edge to edge, and has a tendency to overstep its limits, thereby carrying on the series with more activity. The former is more connected with odd-number groups, and the latter with even, although modifications in their arrangement may reverse the effect. Since the eye moves more slowly and intricately over a curve than over a straight line,[86] it may be that that is the reason why an arched series is taken as the unit of a series, rather than the vertical supports (as it invariably is in some unfinished experiments not recorded here), whereas in a series of lintels the horizontal line of the lintels requires less muscular adjustment to perceive it than the vertical support, therefore the latter are taken as the units. In any case, the unit of the series which attracts the most attention and interest, for whatever reason, is taken as the principal unit, and may vary in details, while the alternate must be invariable, except in certain cases where it meets other demands. There may be rhythm in a series, and at the same time symmetry with respect to a middle point. In such a case a balance must be obtained between the two different motor responses. A series of analogies between the rhythm of sounds and of visual objects, will illustrate more forcibly the similar demands of both. (1) Auditory Rhythm: Periodicity is necessary. Accentuation may be supplied by the subject, but there must be fixed temporal intervals, and if the temporal conditions are not fulfilled, no impression of rhythm is possible.[87] Visual Repetition: Alternate spaces must be of invariable size, or the series is broken up. Different degrees of interest may attach to the principal unit, or the subject may group them in different ways, but the alternate spaces must be uniform, or there is no feeling of rhythmic repetition. (2) Auditory Rhythm: Sounds must recur within a certain rate. When succession falls below, or rises above a certain rate, no impression of rhythm results.[88] A certain voluminousness is necessary for very slow measures, to make the separate elements connect themselves in a series.[89] Visual Repetition: Objects must recur at certain proportionate distances from each other, to connect themselves into a series. Larger objects may be at a greater distance from each other than smaller ones, and still form a series. (3) Auditory Rhythm: "Perception of rhythm is an impression, an immediate affection of consciousness, depending on a particular kind of sensory experience. It is never a construction or reflective perception that certain relations of intensity, duration, etc., do obtain."[90]