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or is intended, and the two adjacent sides are then drawn at a common oblique angle, most frequently 450, all <jf the sides which are parallel in the object being drawn parallel in the projection. The chief drawback of this method is that the oblique sides appear out of proportion to the adjacent side.

Two modifications of the method have been adopted to counteract this effect: one by drawing the oblique side to half the scale of the parallel side ; the other by making on auxiliary drawing from which the projection is made upon a special picture ยก)lane.

The first method sins against a cardinal rule of draughtsmanship. in using two scales or proportions upon one object, a fruitful source of error. The second is a complicated and involved process, with little to recommend it upon the score of economy of time. The three methods are explained in detail in Chapter VII., together with another method used by the author for some years, but now published for the first time.

Perspective or Radial Projection represents solids by means of diagrams in which each geometrical " point " is defined upon the plane of the drawing by a projector, which passes through the actual point represented and through a fixed point which is the same for all the points in the draw ing. The position of this fixed point in relation to the plane of the drawing and to the object represented must be selected within certain limits, or an appearance of distortion will result in the drawing.

This method of drawing shows objects in the positions in which they appear to be in relation to the eye of the observer, and not as they really exist. Thus a long, straight track of railway lines is made to appear as though converging in the distance, though of course they are in fact parallel, but by representing them in this manner we produce the impression of distance for the observer.

It is not necessary in this small treatise to explain fully the theory of linear perspective, for it is only intended to