The Picture Plane

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it were, a cone ot rays, the base of which contains the object, the apex of the cone being in the eye of the observer ; if, then, it is conceived that a transparent plane is interposed between the object and the person viewing it, and that the rays of light passing through the plane are connected by a series of lines on the plane, it will be obvious that we should thus get a representation of the object upon the plane, differing in size only, according to the distance the plane is placed from the eye.

This imaginary plane is the one dealt with in making a perspective drawing, and it is termed the picture plane, usually denoted by the symbol, P.P. Obviously, the nearer the picture plane is placed to the object the larger will be the view obtained upon it; also the farther the observer stands from the object the smaller will it appear to him. We know by experience that when a flag is placed at the top of a tall mast it looks very much smaller than when at our feet on the ground.

Also we know by experience that if we observe, an object obliquely we get a different view or impression upon the retina of the eye than we do when it is seen from directly in front. Now as the purpose of a perspective drawing is to represent the object as it appears to a person in a certain position and at a specified distance, it will be understood that the first thing to be done is to map out these various data upon the drawing, which we can then proceed to make In conformity with them.

The necessary data lor commencing the drawing are indicated by the diagram on page iiO, which is a section, or end view, at right angles to the P.P. of the drawing shown on page 117. As the object of this method is to simplify procedure, we make two arbitrary assumptions at the start: first, that the ground plane {(i.P.) is 5 feet below the horizontal plane (H.P.), the reason being simply that 5 feet is the height of the average person's eye above the ground. The second assumption is that the nearest angle of the object n6 THEORY OF PERSPECTIVE

touches the picture1 plane (P.P.). These assumptions enable us to set down at once on the paper the horizontal line [ILL.), which is an edge-view of the plane in which lie the station-point (S.P.,) and the central visual ray (C.V.R.), which is another name for the axis of the cone of rays before referred to. The elevation of the C.V.K. in the perspective view is called the centre of vision, C.V., see page 117, We can then draw the G.L. 5 ft. below and also lay down a plan of the object with a plan of the P.P. touching one angle, and we can draw a plan of the C.V.K.

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Hviiiwntal Plane

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