Objects in relief, when seen from a short distance with one eye,
look like a perfect picture. If you look with the eye _a_, _b_ at
the spot _c_, this point _c_ will appear to be at _d_, _f_, and if
you look at it with the eye _g_, _h_ will appear to be at _m_. A
picture can never contain in itself both aspects.
29.
Let the object in relief _t_ be seen by both eyes; if you will look
at the object with the right eye _m_, keeping the left eye _n_ shut,
the object will appear, or fill up the space, at _a_; and if you
shut the right eye and open the left, the object (will occupy the)
space _b_; and if you open both eyes, the object will no longer
appear at _a_ or _b_, but at _e_, _r_, _f_. Why will not a picture
seen by both eyes produce the effect of relief, as [real] relief
does when seen by both eyes; and why should a picture seen with one
eye give the same effect of relief as real relief would under the
same conditions of light and shade?
[Footnote: In the sketch, _m_ is the left eye and _n_ the right,
while the text reverses this lettering. We must therefore suppose
that the face in which the eyes _m_ and _n_ are placed is opposite
to the spectator.]
30.
The comparative size of the image depends on the amount of light
(30-39).
The eye will hold and retain in itself the image of a luminous body
better than that of a shaded object. The reason is that the eye is
in itself perfectly dark and since two things that are alike cannot
be distinguished, therefore the night, and other dark objects cannot
be seen or recognised by the eye. Light is totally contrary and
gives more distinctness, and counteracts and differs from the usual
darkness of the eye, hence it leaves the impression of its image.
31.
Every object we see will appear larger at midnight than at midday,
and larger in the morning than at midday.
This happens because the pupil of the eye is much smaller at midday
than at any other time.
32.
The pupil which is largest will see objects the largest. This is
evident when we look at luminous bodies, and particularly at those
in the sky. When the eye comes out of darkness and suddenly looks up
at these bodies, they at first appear larger and then diminish; and
if you were to look at those bodies through a small opening, you
would see them smaller still, because a smaller part of the pupil
would exercise its function.
[Footnote: 9. _buso_ in the Lomb. dialect is the same as _buco_.]
33.
When the eye, coming out of darkness suddenly sees a luminous body,
it will appear much larger at first sight than after long looking at
it. The illuminated object will look larger and more brilliant, when
seen with two eyes than with only one. A luminous object will appear
smaller in size, when the eye sees it through a smaller opening. A
luminous body of an oval form will appear rounder in proportion as
it is farther from the eye.
34.
Why when the eye has just seen the light, does the half light look
dark to it, and in the same way if it turns from the darkness the
half light look very bright?
35.