The ground line yz (Fig. 30) of the picture must now be divided into as many equal parts as is the ground line yz (Fig. 31) of the plan. Each of the new points (Fig. 30) is now joined to the point of sight s. The resulting lines will be the perspective views of the lines ab, cd, &c., of Fig. 3 1. They are analogous in all respects to the lines representing the sides of the book in Figs. 26 and 27. Let us now consider the diagonals of the squares traced on the ground-plan, Fig. 31. I have not drawn them, for it is only necessary to call attention to the fact that they are at angles of 450 with the ground line yz. Being at c half right-angles 5 with the ground line they become in all respects analogous to the sides of our book in its second position (Fig. 28) ; so the point of their perspective intersection is the distance point. This point, as we have seen, is on the horizon line, which is a horizontal drawn through the point of sight. We must now decide on the distance between the point of sight s and the distance point d. This distance is, it will be remembered, the same as the distance from the eye to the canvas. If this distance be chosen too short we shall be dealing with a perspective too ' rapid which will give rise to an uncomfortable sensation on the part of the spectator. Fig. 32 shows an example of the inconvenience of such a choice. The same diagram serves to illustrate another weakness of ' geometrical * perspective. Unless we choose our various distances with great care, the impression given by a picture is false. In this case the three lines of cloud vanish to one of the distance points, d. It follows that these lines of cloud must be at angles of 450 with the picture plane in the real landscape. In the diagram not one of the three gives us that impression, indeed, the right-hand one seems to be at right-angles with the picture plane, and so to recede straight away from us, instead of lying diagonally across our field of vision. So we see that ' geometrical' perspective is no truer than other systems, it
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