SOME THEOREMS SERVING

FOR THE DEMONSTRATION OF PERSPECTIVE.

First Theorem.

THINGS ARE SEEN WHERE THE VISUAL RAYS ARRIVE.

Explanation.

A geometric diagram shows a point A (the eye) connected by straight lines to a vertical line segment BC (the visible object). A smaller segment DE is positioned behind BC, blocked from point A.

Even though this theorem is quite self-explanatory, it will not be bad to give some explanation and example: let the eye be A and the visible thing be BC. Because the visual rays arrive at each end of the visible thing without finding any impediment, thus it is seen in its entirety.

Second Theorem.

THINGS CANNOT BE SEEN WHERE THE VISUAL RAYS DO NOT ARRIVE.

Explanation.

This can be understood by the preceding one, for if there is some thing behind the visible thing BC, like DE, it is certain that because the visual rays cannot arrive there, that thing DE cannot be seen.

Third Theorem.

THAT WHICH IS ABOVE THE HORIZON IS SEEN FROM BELOW.

Explanation.

A diagram shows point A (the eye) at the right, with rays extending to an object BC located above a horizontal line labeled "horizontal line".

Let the eye be A and the visible thing be BC, which is above the horizon. Thus, as the visual rays arrive from below the visible thing, so the said thing will be seen from the same side.

Fourth Theorem.

THAT WHICH IS BELOW THE HORIZON IS SEEN FROM ABOVE.

Explanation.

A diagram shows point A (the eye) at the right, with rays extending to an object BC located below a horizontal line labeled "horizontal line". A lower line is labeled "ground line".

Let the eye be A and the visible thing be BC. As the visual rays arrive from above, so it will be seen from the same side.