eyond the upper margin of the board. Under these conditions
the upper margin of the board appears darker to the vision, by contrast,
than the lower part. It should also be understood that, in general, the
nearer the object the lighter it is, so that as the upper edge of the
board is farthest from the eye the heavy shading there will at least
give the appearance of distance to that edge.

But suppose that instead of having the surface of the board flat, it
should be concaved, as in Fig. 130, it is obvious that the hollow, or
the concaved, portion of the board must intensify the shadows or the
darkness at the upper edge. This explains why the heavy shading in Fig.
126 is at that upper margin.

FLAT EFFECTS.--If the board is flat it may be shaded, as shown in Fig.
131, in which the lines are all of the same thickness, and are spaced
farther and farther apart at regularly increasing intervals.

[Illustration: _Fig. 133._]

[Illustration: _Fig. 134._]

THE DIRECTION OF LIGHT.--Now, in drawing, we must observe another thing.
Not only does the light always come from above, but it comes also from
the left side. I show in Fig. 132 two squares, one within the other. All
the lines are of the same thickness. Can you determine by means of such
a drawing what the inner square represents? Is it a block, or raised
surface, or is it a depression?

RAISED SURFACES.--Fig. 133 shows it in the form of a block, simply by
thickening the lower and the right-hand lines.

DEPRESSED SURFACES.--If, by chance, you should make the upper and the
left-hand lines heavy, as in Fig. 134, it would, undoubtedly, appear
depressed, and would need no further explanation.

FULL SHADING,--But, in order to furnish an additional example of the
effect of shading, suppose we shade the surface of the large square, as
shown in Fig. 135, and you will at once see that not only is the effect
emphasized, but it all the more clearly expresses what you want to show.
In like manner, in Fig. 136, we shade only the space within the inner
square, and it is only too obvious how shadows give us surface
conformation.

[Illustration: _Fig. 135._]

[Illustration: _Fig. 136._]

ILLUSTRATING CUBE SHADING.--In Fig. 137 I show merely nine lines joined
together, all lines being of equal thickness.

As thus drawn it may represent, for instance, a cube, or it may show
simply a square base (A) with two sides (B, B) of equal dimensions.

SHADING EFFECTS.--Now, to examine it