SHADES AND SHADOWS, 1. The drawings of which an architect makes use can be divided into two general kinds: those for designing the building and illustrating to the client its scheme and appearance; and "working drawings" which, as their name implies, are the draw ings from which the building is erected. The first class includes "studies," "preliminary sketches," and "rendered drawings." Working drawings consist of dimensioned drawings at various scales, and full-sized details.
2. It is in the drawings Of the first kind that "shades and shadows" are employed, their use being an aid to a more truthful and realistic representation of the building or object illustrated. All architectural drawings are conventional; that is to say, they are made according to certain rules, but are not pictures in the sense that a painter represents a building. The source of light casting the shadows in an architectural representation of a building is sup posed to be, as in the "picture" of a building, the sun, but the direction of its rays is fixed and the laws of light observed in nature are also somewhat modified. The purpose of the architect's draw ing is to explain the building, therefore the laws of light in nature are followed only to the extent in which they help this explanation, and are, therefore, not necessarily to be followed qonsistently or completely. The fixed direction of the sun's rays is a further aid to the purpose of an architectural drawing in that it gives all the drawings a certain uniformity.
3. Definitions. A clear understanding of the following terms is necessary to insure an understanding of the explanations which 4. Shade: When a body is subjected to rays of light, that portion which is turned away.from the source of light and which, therefore, does not receive any of the rays, is said to be in shade. See Fig. 1.
5. Shadow: When a surface is in light and an object is placed between it and the source of light, intercepting thereby some of the rays, that Tortion of the surface from which light is thus excluded is said to be in shadow.
6. In actual practice distinction is seldom made between these terms "shade" and "shadow," and "shadow" is generally used for that part of an object from which light is excluded.
7. Umbra: That portion of space from which light is excluded is called the umbra or invisible shadow.
(a) The umbra of a point in space is evidently a line.
(b) The umbra of a line is in general a plane.
(c) The umbra of a plane is in general a solid, (d) It is also evident, from Fig. 1, that the shadow of an object upon another object is the intersection of the umbra of the first object with the surface of the second object. For example, in Fig. 1, the shadow of the given sphere on the surface in light is the intersection of its umbra (in this case a cylinder) with the given surface producing an ellipse as the shadow of the sphere.
8. Ray of light: The sun is the supposed source of light in " shades and shadows," and the rays are propo gated from it in straight lines and in all directions. Therefore, the ray of light can be represented graph ically by a straight fine. Since the sun is at an in finite distance, it can be safely assumed that the rays of light are 411 par allel.
9. Plane of light: A plane of light is any plane containing a ray of light, that is, in the sense of the ray lying in the plane.
10. Shade line: The line of separation between the portion of an object in light and the portion in shade is called the shade line. • 11. It is. evident, from Fig. 1, that this shade line is the boundary of the shade. It is made up of the points of tangency of rays of light tangent to the object.
12. It is also evident that the shadow of the object is the space enclosed by the shadow of the shade line, In Fig. 1, the shade the given sphere is a great circle of the sphere. The shadow of this great circle on the given plane is an ellipse. The portion within the ellipse is the shadow, of the sphere.