The Da Vinci Notebooks at sacred-texts.com
An experiment showing that though the pupil may not be moved from its position the objects seen by it may appear to move from their places.
If you look at an object at some distance from you and which is below the eye, and fix both your eyes upon it and with one hand firmly hold the upper lid open while
with the other you push up the under lid--still keeping your eyes fixed on the object gazed at--you will see that object double; one [image] remaining steady, and the other moving in a contrary direction to the pressure of your finger on the lower eyelid. How false the opinion is of those who say that this happens because the pupil of the eye is displaced from its position.
How the above mentioned facts prove that the pupil acts upside down in seeing.
53:45 : 82. 14--17. The subject indicated by these two headings is fully discussed in the two chapters that follow them in the original; but it did not seem to me appropriate to include them here.