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 the null cube has passed through the plane, as it is leaving it, we get again a pink face.
Fig. 94.
The cube swung round yellow line, with red line running from left to right, and white line running down.
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 certain difficulty, for the plane being will find that the position his squares are to be placed in will lie below that which they first occupied. They will come where the support was on which he stood his first set of squares. He will get over this difficulty by moving his support.
Then, since the cubes come upon his plane by the light yellow face, he will have, taking the null cube as before for an example, null, light yellow face; null, red section, because the section is perpendicular to the red line; and finally, as the null cube leaves the plane, null, light yellow face. Then, in this case red following on null, he will have the same series of views of the red as he had of the null cube.
Fig. 95.
There is another set of considerations which we will briefly allude to.