(169) Change of interval immediately modifies the character of a color sequence. This is readily shown by having an under-mask, with a long, continuous slit, and an over-mask whose perforations are arranged in several rows, each row giving different spaces between the perforations. In the case of the girl’s clothing, the same sequence produces quite a different effect, if two perforations of the over-mask are brought nearer to select a lighter yellow-green dress, while the ends of the sequence remain unchanged. To move the middle perforation near the other end, selects a darker bluish green dress, on which the trimming will be less contrasted, while the hat appears brighter than before, because of greater contrast. (170) The variations of color sequence which can thus be studied out by simple masks are almost endless; yet upon a measured system the character of each effect is easily described, and, if need be, preserved by a written record. +Invention of color groups.+ (171) Experiments with variable masks for the selection of color intervals, such as have been described, soon stimulate the imagination, so that it conceives sequences through any part of the color solid. The color image becomes a permanent mental adjunct. Five middle colors, tempered with white and black, permit us to devise the greatest variety of sequences, some light, others dark, some combining small difference of chroma with large difference of hue, others uniting large intervals of chroma with small intervals of hue, and so on through a well-nigh inexhaustible series. (172) As this constructive imagination gains power, the solid and its charts may be laid aside. _We can now think color consecutively._ Each color suggests its place in the system, and may be taken as a point of departure for the invention of groups to carry out a desired relation. (173) This selective mental process is helped by the score described in the last chapter; and the quantity of each color chosen for the group is easily indicated by a variable circle, drawn round the various points on the diagram. Thus, in the case of the child’s clothes, a large circle around G 6/5 gives the area of that color as compared with smaller circles around Y 8/7 and BG 4/3, representing the area of the straw and the trimming. (174) When the plotting of color groups has become instinctive from long practice, it opens a wide field of color study. Take as illustration the wings of butterflies or the many varieties of pansies. These fascinating color schemes can be written with indications of area that record their differences by a simple diagram. In the same way, rugs, tapestries, mosaics,--whatever attracts by its beauty and harmony of color,--can be recorded and studied in measured terms; and the mental process of estimating hues, values, chromas, and areas by established scales must lead the color sense to finer and finer perceptions.