How the eye judges lines, angles and shadows

Bodies under one common contour appear joined

If a single line is the common encompasser of many bodies placed separately one after another, those bodies will appear joined and without any interval. The shared outline hides the gaps between them from the eye.

A body lit past a shadowing object seems lit by many lights

That body illuminated by the light that passes beyond an interposed shadowy body will be as if illuminated by several lights. The note is one of the canceled passages on the sheet.

A curved line covering a straight one looks straight

That curved line will appear to the eye to be straight, all of whose parts occupy all the parts of a straight line. Vision cannot separate the two when their extents coincide.

An angle is judged as the straight line it occupies

The angle will not be judged to be other than a straight line, which occupies as much straight line to the eye. It will be judged nothing other than a straight line that occupies an equal straight line to the eye.

Derivative shadow larger than the primitive shadow

That derivative shadow will be greater than the primitive one whose acute angles are more unlike the obtuse angles of that primitive shadow. The note stands among the canceled texts beside the diagrams.

In alto a destra studio di ottica. Dall’alto verso il basso, testi alternati a testi cassati, quest’ultimi non tutti forniti in trascrizione diplomatica nell’edizione qui trattata.

A fianco della crocetta, sotto una serie di testi cassati:

Sotto, cassato:

Segue altro testo cassato, non trascritto nell’edizione qui trattata, e sotto:

Sotto:

Segue altro testo cassato, non trascritto nell’edizione qui trattata, e sotto, a fianco dei diagrammi, altro testo cassato:

Upper right, an optical study. From top to bottom, texts alternating with canceled texts, the latter not all given in diplomatic transcription in the edition treated here.

Beside the little cross, under a series of canceled texts:

Below, canceled:

Another canceled text follows, not transcribed in the edition treated here, and below:

Below:

Another canceled text follows, not transcribed in the edition treated here, and below, beside the diagrams, further canceled text: