VISUAL RAYS ARE STRAIGHT LINES

IMAGINED STARTING FROM THE VISIBLE THING AND COMING TO THE EYE.

Second Definition.

The thing seen, coming to represent itself to the eye, comes there by straight rays or visual rays. And one must understand that we only draw the said visual rays from each angle of the visible thing, because having all the angles of the figure, it will be easy afterwards to draw the lines from one angle to the other to complete the said figure.

THE VISIBLE THING IS WHAT THE EYE LOOKS AT.

Third Definition.

The figure that the eye looks at is named the visible thing. There are two kinds of figures: those that are composed of straight lines, and those that are of curved lines. Those that are of straight lines will be easy to put into foreshortening; those that are of curved lines are more difficult, for it is necessary to form angles on the planes of the said figures in order to draw visual rays from those angles. And after the report of the said angles has been made in the foreshortening, it will be necessary to draw the curved lines from one angle to the other with judgment, such that the more angles there are on the planes of the visible thing, the more perfect the figure will be in the foreshortening that will be made of it.

THE GROUND LINE IS A LINE REPRESENTING THE SURFACE OF THE EARTH.

Fourth Definition.

In ignography ground plan/floor plan, the surface of the paper where the said ignography is traced is imagined as the surface of the earth. In orthography elevation/side view, the bottom line upon which the visible thing is elevated is also imagined as the surface of the earth.