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