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This historic book may have numerous typos, missing text, images, or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1902. Not illustrated. Excerpt: ... CHAPTER X. SHADOWS IN PERSPECTIVE. 187. -- Shadow is the obstruction of light. If any object is lighted from a single source, the rays of light...
This historic book may have numerous typos, missing text, images, or index. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. 1902. Not illustrated. Excerpt: ... CHAPTER X. SHADOWS IN PERSPECTIVE. 187. -- Shadow is the obstruction of light. If any object is lighted from a single source, the rays of light which fall upon it will be interrupted and a portion of space extending indefinitely behind the object will be deprived of light. This darkened space is called the invisible shadow of the object and will take the form of a cone or cylinder, the elements of which are the rays of light tangent to the object. 188. --The intersection of this invisible shadow with any plane is called the visible shadow of the object upon the plane. When the term shadow alone is used it usually means the visible shadow. In Fig. 81, 0 represents an object in space lighted by solar rays coming in the direction indicated by the arrow. The sun being at so great a distance, no appreciable error will be made if its rays are assumed to be parallel. Thus the invisible shadow of the object will take the form of a cylinder (C). The intersection of this cylinder with any plane (P) will be seen as the visible shadow (as) of the object upon the plane. 189. --If the object is reduced in size until it becomes a single point, its invisible shadow will become the single ray of light which has passed through the point. Thus the shadow of any point upon a plane is where the ray of light through the point pierces the plane. 190. --The invisible shadow of a straight line will be a plane made up of all the rays of light which pass through the line. Thus the shadow of a straight line upon a plane, being the intersection of two planes, will be a straight line. In Fig. 82, a'b' represents the shadow of the line ab upon the plane P. The shadow of any point (d) in the line, will be where the ray of light through the point crosses the shadow of the line. 191. --...
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