## Theory Of Orthographic Projection

but for the moment 'we will conline our attention to those marked a-b, c-d and e.

a-I is the vertical ¡Jane, and c-d the horizontal plane; e is a special or auxiliary vertical plane perpendicular to the other two, which are known as co-ordinate planes. Now, if we stand exactly in front of the prism, looking in the direction of the arrow II, we shall see the side marked 1, 2, 3, 4, but though we know that it is inclined, having tho solid before us, we cannot see the inclination in this position. What we do see is the exact height measured perpendicularly between the upper and lower edges, also the exact length between the ends, and to obtain this view upon the plane, projectors marked p are imagined to shoot forth from each extremity of an edge until they intersect the plane. If these points are joined by straight lines, as at A, an " elevation " of the object is obtained upon the vertical plane. In like manner, imagine other projectors shooting forth from the base until they impinge upon the horizontal plane ; their points, joined up as at P, produce the " plan," or view, we should see if looking straight down from above the object. With these two views we can obtain the correct height and length of the prism, but not tho real length of its inclined sides; to obtain these we must use the auxiliary plane which is arranged parallel with the end of the prism. Three projectors shot forth 011 to this plane as shown will give the shape of the end. This view, E, is termed an end elevation. If now we unfold the ¡Janes into one surface, as indicated by the dotted lines, we get the three views in their relative positions above and below the ground line G- L. Fig. 2 shows the complete plane with the views or projections in correct position, as they appear upon a sheet of drawing paper, and it will be clear that not only can we obtain all the dimensions from these three views, but also the height of the object above the ground (shown at A) and its distance from the vertical plane (shown at E or P). It should be noted that only what is visible upon the suiface of the object we are looking