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A.j)lane, strictly defined, is nothing more than a perfectly flat "sur- face," without any reference to substance ; but as it cannot be dealt with for explanatory purposes without being assumed to be material and inflexible, it will, when spoken of, or used for that purpose in this work, be considered as having such a thickness as would be repre- sented by a line. Assuming this, the edge view of a plane will, under any circumstance of position, be a perfectly straight line. If, then, two planes intersect or meet each other at an angle, as the " planes of pro- jection " we are about to deal with do, their meeting will be in a line, which forms a boundary or dividing line between them, and is called Fig. 70 the " intersecting line " of the planes. This line will throughout the subject have IL for its distinguishing letters. Knowing, then, the true relative position of the " planes of pro- jection " on which we wish to obtain the representations of an object, we will first proceed to find the projections of a " straight line " in different positions with respect to those planes. Let its position at first be perpendicular to the VP. Here, as the thing to be projected is a " line " having ends or points, before we can obtain its projections we must first know how to find those of a "point." Let, then, A on the left in the diagram Fig. 70 be a point in space, such as a small bead invisibly suspended, and let it be required to find its vertical and horizontal projections that is, its projections on the VP and HP. To obtain these, we have to find the points in the VP and HP where a visual ray or projector perpendicular to each of the planes, and drawn through A, would penetrate them. This, it will be seen in the