How to measure the pyramid of vision.

As regards the point in the eye; it is made more intelligible by
this: If you look into the eye of another person you will see your
own image. Now imagine 2 lines starting from your ears and going to
the ears of that image which you see in the other man's eye; you
will understand that these lines converge in such a way that they
would meet in a point a little way beyond your own image mirrored in
the eye. And if you want to measure the diminution of the pyramid in
the air which occupies the space between the object seen and the
eye, you must do it according to the diagram figured below. Let _m
n_ be a tower, and _e f_ a, rod, which you must move backwards and
forwards till its ends correspond with those of the tower [Footnote
9: _I sua stremi .. della storre_ (its ends ... of the tower) this
is the case at _e f_.]; then bring it nearer to the eye, at _c d_
and you will see that the image of the tower seems smaller, as at _r
o_. Then [again] bring it closer to the eye and you will see the rod
project far beyond the image of the tower from _a_ to _b_ and from
_t_ to _b_, and so you will discern that, a little farther within,
the lines must converge in a point.

The Production of pyramid of Vision (58-60).