I IV J. 53 mm. J. 53 mm. S. 45 S. 47 U. 52 U. 50 R. 35 R. 37 W. 23 W. 43 V. 35 V. 34 [Illustration: Fig. 28] If, for the sake of comparison, the average be taken of the favorite arrangements of the four blank cards, and they compared with the interspacing of the oblique line design, it will be seen they approach each other closely, except in the case of W. These experiments would seem to show that an empty space, or one completely covered with decoration, is taken in its entirety when repeated in the series. But when decorated, especially toward the centre, the _design_, instead of the whole including space, is taken as the repeated unit, and for this reason the different units must approach each other to make a satisfactory series. To what extent does change in _level_ and _plane_ affect the units of a series? To test this, a series of diamond shapes was hung on the same level and at equal distances, and the subjects enjoyed them as a repeated series. [Illustration: Fig. 29] Then another row was hung above them, and halfway between. [Illustration: Fig. 30] The subjects grouped them either in twos or threes, thus transforming them into one series of similar group-units of triplets and pairs. They were asked if they could take them up and down, one after the other without grouping, as they would have done when on the same level. With a little practice two of them succeeded, but they found the series tiresome when taken in this way, and deprived of much of its pleasure. The series was then changed by hanging a smaller diamond between the others, at the same level. [Illustration: Fig. 31 A] [Illustration: Fig. 31 B] This was enjoyed even more than the other, as an alternating series, but when the smaller diamond was hung between but on a higher level although it could still be included if _grouped_ in some way with one or two of the larger diamonds, it baffled all attempts to include it as an alternate minor unit in the other series. The two series separated, and one ran along above the other, or else a definite grouping took place, so that the large and small diamonds made one group-unit which was repeated. But to combine two different elements as major and minor units of one series, when the two were on different _levels_, was generally declared impossible. Provided units stay on the same level, however, a difference in _plane_ does not prevent their being in one series, provided the plane varies regularly, and the variation is not too great. The variation in plane of a few inches, used with these shapes, did not prevent their being taken as one series, although it much facilitated their being taken as two, if desired.