The Draughtsman's Handbook of Plan and Map Drawing / Including instructions for the preparation of engineering, architectural, and mechanical drawings.

by George G. André

Fig. 61.

Fig. 62.

Application of Shade Lines.

—As we have already explained, the light is supposed to fall upon the objects in a drawing in parallel rays from the upper left-hand corner for elevations, and from the lower left-hand corner for plans. To determine whether or not a given line should be a shade line, we have only to ascertain whether or not the light, introduced in such a manner, falls upon that edge of the object which the line represents. All those parts of a body upon which the rays of light fall directly, are said to be in light; all those parts upon which the rays of light do not fall directly, are said to be in shade; and those parts of a surface which are deprived of light by another body intercepting the rays, are said to be in shadow. These definitions should be borne in mind. Lines representing the boundaries of surfaces in light should be fine lines, and lines representing the boundaries of surfaces in shade should be thick or shade lines. Let it be required, for example, to determine the shade lines of the cube shown in elevation in [Fig. 61]. The extreme rays of light falling upon the cube meet the edges in b and c; hence the surfaces a b, a c, are in light, and the surfaces d b, d c, are in shade. The foregoing rule will thus make a b and a c fine lines, and d b and d c shade lines. If the cube were turned so that a b should be at right angles to the rays of light, the extreme rays would fall on the edges a and b, and the middle ray which now falls on a would fall on the middle of the line a b. The rays immediately beyond those which are arrested by the edges a and b, may be considered to pass along in contact with the surfaces a c and b d; and these surfaces must, therefore, be regarded as in light. Thus we shall have in this case the lines a b, a c, and b d, fine lines, and the line c d a shade line. It is the practice of some draughtsmen to make a c and b d in such cases a medium line, and the practice has propriety to recommend it. The foregoing explanations of the shade lines in the elevation of the cube, render any further remarks concerning those in the plan, [Fig. 62], unnecessary. In practice, whether or not a surface is in light may be determined by placing the set square of 45° against it.

Fig. 63.

Fig. 64.