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 place them in such relation to each other that when rotated the radial edges exposed on the face toward the audience will not "catch the wind." With small disks on the color wheel this is not important, and if there is no whole graduated disk on the arbor behind the slitted disks there is no advantage, but in using the larger disks it is well to put the graduated disk behind the others for this purpose, as at best it is quite laborious to keep up speed when using several of the large disks, even with the best possible conditions. With the thin paper disks of the color top this is an important matter. It will be noticed that the method of joining the disks for use on the Color Top is the reverse of that to be observed with the disks of the Color Wheel as shown in Fig. 5.

Fig. 7 shows the same two color disks placed in front of a large white disk having its edge graduated to one hundred parts, so that the relative proportions of two Or more colors to be combined can be determined accurately.

As the smaller disks offer so much less resistance in rotation than the larger ones they are most desirable in private experiments or before a small class, and the largest disks of the Normal School Wheel are necessary only when more than three expressions of color are required to be shown at the same time. In making experiments before an audience those persons in front should if possible be at least ten feet from the color wheel.