(12, Plxin of StaJwn pouiL~
Section of Diagram, p. 117, perpendicular to Picture Plane which must be at right angles with the plan of the P.P. Next we fix on plan the position of the S.P. from the P.P. at such a distance from the object that a cone of visual rays, having a vertical angle of about 40° at the S.P., will entirely envelop it (see Fig., page 1x6). The reason for limiting this angle is that if a much wider angle be adopted there will be an appearance of distortion in the resulting perspective view. The student should make a few experiments upon this point. Finally the vanishing points (F.P.) have to be located.
These are points to which parallel lines appear to converge. If the parallel lines are horizontal they will appear to con
riS THEORY OF PERSPECTIVE
verge to points upon the horizon —that is to say, in the ILL. If they are inclined to the horizontal they will converge to points above or below the H.L., as the case may be. If they are parallel to the picture plane they are represented as parallel in the perspective, though they only remain sensibly parallel within the limits of the cone of restricted angle above referred to. In the form of perspective here described, only horizontal and vertical lines are dealt with ; the latter, being of course parallel with the P.P., remain parallel in the perspective view. The V.P.'t for horizontal lines are found by means of the plan as explained below.
With the above outline of the theory as explanation, we can now proceed to describe in detail the construction of the several drawings (pages 117, 121, 123, 125), each involving different requirements. It will be seen that the examples given combine in each case a plan, which is set out in its required relation to the P.P.; an elevation, where necessary to give dimensions not shown by the plan, and the perspective view itself, which is partly projected directly from the plan and partly set up by means of dimensions taken from the elevations. One and the same line does duty for the plan of the P.P. and for the G.P. in the perspective view— that is, we, in effect, when making the perspective drawing, assume that the picture plane is rotated into the horizontal plane, as indicated by the dotted arc in the diagram, page 116; where G.I.., which is also the plan of the picture plane1, reaches g.l. in the horizontal plane. This is the position shown on the perspective part of the drawing on page 117.
The plan and the lines radiating therefrom to the S.P. could, of course, be set out on a separate piece of paper, and the horizontal dimensions on the P.P. could be transferred to the perspective view by " tieking-01'f" : this is the usual procedure in office work with more complicated drawings, but the beginner will find this method of drawing the plan adjacent the perspective view and the obtaining the dimensions by projection much clearer
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