The fourth dimension

154 THE FOURTH DIMENSION

come in the motion of the whole block through the plane.

With regard to these squares severally, however. different names must be used, determined by their relations in the block.

Thus, in fig. 93, when the cube first rests against the plane the null cube is in contact by its pink face ; as the block passes through we get an ochre section of the null cube, but this is better called a yellow section, as it is made by a plane perpendicular to the yellow line, When

Fig. 94,

The cube swung round yellow line, with red line running from left to right, and white line running down.

the null cube has passed through the plane, as it is leaving it, we get again a pink face.

The same series of changes take place with the cube appearances which follow on those of the null cube. In this motion the yellow cube follows on the null cube, and the square marked yellow in 2 in the plane will be first “yellow pink face,” then “ yellow yellow section,” then “yellow pink face.”

In fig. 94, in which the cube is turned about the yellow line, we have a eertain difficulty, for the plane being will

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