. Perspective for art students . L. of a directly ascending plane. What we do is, we create a line on the ground-planewider the line AB. To do this we find the of AB,that is, we continue AB backwards till we obtain now we drop a vertical vanishing line from downto cut through the horizon, we obtain a point in theground-plane, that is, on the horizon (for the horizon ispart, if only the back edge, of the ground-plane). Hencethe of trace is under of AB. What we do next differs slightly according to the wayin which A is related to the ground-plane. Suppose in Pictur
. Perspective for art students . L. of a directly ascending plane. What we do is, we create a line on the ground-planewider the line AB. To do this we find the of AB,that is, we continue AB backwards till we obtain now we drop a vertical vanishing line from downto cut through the horizon, we obtain a point in theground-plane, that is, on the horizon (for the horizon ispart, if only the back edge, of the ground-plane). Hencethe of trace is under of AB. What we do next differs slightly according to the wayin which A is related to the ground-plane. Suppose in Picture-lines continued 121 Fig. 83 A is kuown to be on the , and in Fig. 84 itsdistance above it is known. The difference is seen when we draw the trace of theintersection of our obliging third plane; for if A is onthe ground, we must draw through it. What we do inboth cases is to draw this trace, and then a line over andunder it. In Fig. 83 we draw a line from of tracethrough A till it cuts the ground-line in point 4. Then. Fig. 84.—Finding the of an obliquely ascending plane. from 4 we run up or down a vertical line, which will beparallel to our vertical through and oftrace. We saw that the of trace was exactly underthe of AB. If now we draw a line from throughB and A forward till it cuts the new vertical line throughpoint 4, we find point 5. 5 is just as immediately under4 as the of the trace is under The line to 5 is certainly in our inclined plane, because itincludes point A, which we know to be in that plane. 122 Perspective and, what is more important, it includes , which isalso in the inclined plane. Now, point 5 is known to bein the picture-plane, because it is a point on a linethrough point 4, and 4 is a point on our original picture-line, Hence if 5 be in the picture-plane, draw to through 5, and you have the of aninclined plane containing AB. In Fig. 84, where A is so
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