Device on a Drafter for Perspective Projection

It is a well-known fact that an object can be projected on two different planes, which are symmetric with respect to one of the straight lines of projection, as mutually congruent images projected by a oblique parallel projection. However, we can not find yet the proof of this fact from a viewpoint of the axonometric projection. The author gives a geometric proof of the above fact, also shows new formulas related to the image plane, the line of projection and the axonometric axes and, moreover, a few of their applications.