no longer a line but a surface, and we are investigating the
properties of a line, and not of a surface. And as the line, having
no centre of thickness cannot be divided, we must conclude that the
line can have no sides to intersect each other. This is proved by
the movement of the line _a f_ to _a b_ and of the line _e b_ to _e
f_, which are the sides of the surface _a f e b_. But if you move
the line _a b_ and the line _e f_, with the frontends _a e_, to the
spot _c_, you will have moved the opposite ends _f b_ towards each
other at the point _d_. And from the two lines you will have drawn
the straight line _c d_ which cuts the middle of the intersection of
these two lines at the point _n_ without any intersection. For, you
imagine these two lines as having breadth, it is evident that by
this motion the first will entirely cover the other--being equal
with it--without any intersection, in the position _c d_. And this
is sufficient to prove our proposition.

81.

HOW THE INNUMERABLE RAYS FROM INNUMERABLE IMAGES CAN CONVERGE TO A
POINT.

Just as all lines can meet at a point without interfering with each
other--being without breadth or thickness--in the same way all the
images of surfaces can meet there; and as each given point faces the
object opposite to it and each object faces an opposite point, the
converging rays of the image can pass through the point and diverge
again beyond it to reproduce and re-magnify the real size of that
image. But their impressions will appear reversed--as is shown in
the first, above; where it is said that every image intersects as it
enters the narrow openings made in a very thin substance.