35. Problem III. To find the shad= o\v of a given plane on a given plane.

Plane surfaces are bounded by" straight or curved lines. Find the shadows of the bounding lines by the method shown in Problem II. The resulting figure will be the required shadow.

36. In Fig. 17, the plane abc is so situated that its shadow falls wholly upon Y. The shadows of its bounding lines, ab, bc, ca have been found by Problem II.

That portion of the shadow hidden by the plane in elevation is cross-hatched along the edge of the shadow only. This method of indicat-incr actual shadows which are hidden by the object is to be followed in working out the problems of the examination plates.

37. Fig. 18 shows the construction of the shadow of a plane on the co-ordinate plane to which the given plane is parallel. (In this case the vertical plane.) It is to be observed that the shadow is equal in size and shape to the given plane.

Fig. 19 shows that, in case of a circle parallel to one of the co-ordinate planes, it is only necessary to find the shadow of the center of the circle and with that point as a center construct a circle of the same radius as that of the given circle.

FIG-I7.

FIG-18.

FIG-19.

NOTE:

39. Any point, line, or plane lying in a surface is considered to be its own shadow on that surface.

40. A surf ace parallel to a ray of light is considered to be in shade.

41. In the above problems the points, lines and planes have been given in vortical and horizontal projection. The methods for finding their shadows are, in general, equally true when the points, lines and planes are given by vertical and profile projection or horizontal and profile projection.