Part 7 (2/2)

[Ill.u.s.tration: _Fig. 129._]

[Ill.u.s.tration: _Fig. 130._]

[Ill.u.s.tration: _Fig. 131._]

CONCAVE SURFACES.--In Fig. 126 the shading lines commence at the upper margin, and are heaviest there, the lines gradually growing thinner and farther apart.

CONVEX SURFACES.--In Fig. 127 the shading is very light along the upper margin, and heavy at the lower margin. The first shaded figure, therefore, represents a concaved surface, and the second figure a convex surface. But why? Simply for the reason that in drawings, as well as in nature, light is projected downwardly, hence when a beam of light moves past the margin of an object, the contrast at the upper part, where the light is most intense, is strongest.

The shading of the S-shaped surface (Fig. 128) is a compound of Figs.

126 and 127.

[Ill.u.s.tration: _Fig. 132._]

SHADOWS FROM A SOLID BODY.--We can understand this better by examining Fig. 129, which shows a vertical board, and a beam of light (A) pa.s.sing downwardly beyond 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 s.p.a.ced farther and farther apart at regularly increasing intervals.

[Ill.u.s.tration: _Fig. 133._]

[Ill.u.s.tration: _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 s.p.a.ce within the inner square, and it is only too obvious how shadows give us surface conformation.

[Ill.u.s.tration: _Fig. 135._]

[Ill.u.s.tration: _Fig. 136._]

ILl.u.s.tRATING 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.

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