Fig. 225.
It is thus necessary to get a view of the block from another side; the direction in which it will be most instructive to obtain additional views is in the direction of the breadth and of the length and square to the lines of sight of the first view.
[Fig. 226] shows how the lines of sight would strike the object in the three directions. If these lines should be rays of light, some of them would pass by the body until they squarely strike the large plane surfaces, I, II, III; naturally the rays of light on the faces of the object will be retained, and cannot strike the plane surfaces, thus leaving dark shadows of exactly the same outlines as the block; these would be exact drawings of the faces, and if by some means they can be fixed and retained on the plane they can be completely measured and dimensioned.
This throwing forward of the outline of the object in different views on the planes is called projection of the object, and furnishes a highly important means of fixing the outlines and dimensions in the three main directions of height, width and depth; evidently, the light rays passing by the front face may not all reach the plane of projection, but they may be retained by protruding parts of the object behind the front face. These protruding parts naturally would also be projected on the plane in the same manner as the main body of the block.
These projections of the protruding part are plainly visible in plane II and plane III, while the part would not be drawn in outline in plane I. It may be imagined, however, that the greater thickness of the body in the direction of the protruding part would intensify also the shadow, thus outlining the face of the protruding part in plane I.
It is apparent that these three projections are all needed, but as drawing is all done in one single plane, the three projections will for the sake of convenience have to be brought into a single plane. This can be realized if plane II is swung around axis O Z and plane III around axis O Y, until all three surfaces are in one single plane, which would then appear as shown in [fig. 227].
It is also possible to assume transparent planes in front of the body and extend the parallel lines of sight forward instead of backward. Thus an outline picture will be created on each of the three planes I, II, III in [fig. 228], in a manner similar to [fig. 226]. For drawing purposes, all three views again have to be brought into a single plane, which is done by swinging II around O Z, and III around O Y in the same manner as [fig. 227] was evolved from [fig. 226].
It will be noted that in [fig. 229] plane III is now above I and plane II on the left-hand side of I, while in [fig. 227] they were below and at the right-hand side of plane I. As the swinging of the top and side planes takes place around the edges of the front plane I two systems may thus be distinguished, according to the position of plane I in regard to the object. [Fig. 226] thus represents the system of backward projection, while [fig. 228] represents the system of forward projection.
Either system can have, however, the plane II at the right- or left-hand side edge, while plane III may be attached to the top or bottom edge of plane I; it is readily understood that a number of combinations are possible for each system, as it is not necessary to adhere absolutely to one rule. The system of forward projection is the one generally practiced and further examples are all executed by this system, meaning that the planes are always between the observer and the object.
